More on Tearing Down Mountains: Groundwater and Rivers

We have seen how landslides and other mass movements supply rocks and dissolved materials to rivers, which carry the material to the sea; from there, the material goes down subduction zones or gets squeezed in obduction zones to make new mountains that produce new landslides.  Here, we will look at the way rivers work, how they move their load of rocks along their beds and otherwise interact with this great cycle, and how they are fed by water in the ground.

Video Overview (2 minutes)

The following video is a parody of Proud Mary (Rollin' on the River) and helps introduce some of the topics we will see in Module 6.

Learning Objectives

• Recognize that rainwater soaks into the ground and flows through the ground to rivers
• Discuss how rivers must adjust to move both sediment and water supplied to them
• Explain how caves and related features, together called karst, form in rocks that dissolve easily
• Understand how these processes affect people, our cities, and our drinking water  

What to do for Module 6?

You will have one week to complete Module 6. See the course calendar for specific due dates.

• Review all of the course materials
• Take the RockOn #6 Quiz
• Take the StudentsSpeak #7 Survey
• Continue working on Exercise #3

Questions?

If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.

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Overview of the main topics you will encounter in Module 6.

"It was kind of solemn, drifting down the big, still river, laying on our backs, looking up at stars, and we didn't even feel like talking aloud..."

Water, Rivers, Floods, and Caves: Canyonlands, Delta, and Mammoth Cave 

• Most of the rain that falls then evaporates, primarily from plants; most of the water that is not used by plants soaks into the ground.
• Soil and shallow rock usually have air as well as water in spaces; deeper, below the water table, the spaces are all water-filled.
• The water table looks like a smoothed version of the ground surface, and reaches the surface at lakes and rivers.
• The ground acts something like a sponge, with spaces filling during rains, and draining to keep rivers running between rains.
• So, the water table rises in elevation during wet times and sinks during dry times. 

Rivers Move Rocks 

• Most of the water that reaches rivers flows through the ground first.
• Mass movement supplies most of the rocks that reach rivers.
• If more rocks arrive at a river than the water can move away, the rocks pile up, steepening the river flowing away from the pile so it can move more of the rocks.
• When the rocks are mostly small clay particles, which tend to stick together, the river often has a single, deep meandering channel, and many small rock particles are transported while suspended up in the water.
• When the rocks are mostly larger pieces such as sand, gravel, or even larger chunks that don’t stick together, the river often has many channels that split and rejoin, and is called a braided river; more of the rock transport occurs as bed load bouncing or rolling along the bottom. 

Dams Make a Big Difference 

• When a river carries rocks into a reservoir (the lake behind a dam), the rocks are dropped as a sediment deposit called a delta (deltas also often form where rivers carry sediment into other lakes or the ocean, although strong currents in a lake or the ocean may carry away the sediment, too).
• A delta builds out into the reservoir but also builds upward so it continues to slope downward into the reservoir, and this “backs up” sediment that can bury fields and houses for some distance upriver.
• Dams typically reduce or eliminate floods farther downriver.
• This makes a huge difference for what lives on a floodplain (in particular, it favors humans over nature).
• Without floods, big rocks are no longer moved by rivers.
• Clean water released by dams picks up small rocks such as sand, removing sand bars and affecting river ecosystems. 

Ignoring rivers can be dangerous 

• A delta is a big pile of sediment, which slowly compacts under its own weight.
• The Mississippi Delta is miles thick and compacts a lot.
• Naturally, the sinking was balanced by new mud from floods, which usually happened every spring.
• Humans hate mud in our houses and floods on our roads, so we have built levees, often on top of natural levees, to keep the rivers out of cities and towns.
• But the delta continues to compact and sink—much of New Orleans has subsided below river level, and some is beneath sea level.
• Wetlands south of New Orleans toward the Gulf of Mexico have been lost as levees and dredging for shipping and oil production have kept floods from depositing mud to balance sinking.
• The city now is low in elevation, beside a higher river and sea as human-caused warming raises sea level, with reduced wetlands that would have protected the city if they still existed, and all of this can lead to huge disasters when hurricanes hit.
• Before recent hurricane disasters happened, scientists, disaster planners, many journalists, and others repeatedly warned that the disasters were coming.
• The sinking of the city and rising of the sea are continuing; rebuilding without major changes will cause the next disaster to be even worse. 

Caves are Cool 

• Some rocks (esp. limestone) dissolve easily; if such rocks start with just a few cracks, the dissolution will be focused in just a few places.
• Such focused dissolution makes low spots called sinkholes at the surface, with caves beneath, commonly with springs and other features, giving a landscape called karst.
• Caves tend to form near or below the water table and to be filled with water initially, but the water may then drain out leaving the cave air-filled; water dripping in then may lose extra CO2 picked up from the soil, depositing dissolved limestone to make cave formations.
• Water goes through caves quickly; pollution discharged today may harm someone farther downriver tomorrow.
• Water usually moves through other types of rocks much more slowly so pollution released today may not “get” anyone for a while, but once it does, clean-up is usually very hard and very slow.
• Scientists and engineers have developed some clean-up options, but the best option is to keep poisons out of the ground. 

The muddy Colorado River, Canyonlands National Park, Utah.

Credit: Left: The Confluence, NPS/Emily Ogden, Public Domain
Right: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Southwest of Moab, Utah lies Canyonlands National Park. A “rough” park with few services available, little water (except for the rather muddy rivers), pot-holed roads, and awesome mountain biking, Canyonlands preserves the confluence of the Colorado and the Green Rivers. Many “visitors” to the park never actually enter it, choosing to gaze down on the Colorado from the vantage point at Dead Horse Point State Park. The two rivers of the park are incised a third of a mile (half a kilometer) or more into the red sedimentary rocks of the Colorado Plateau. Those rocks, mostly sandstones (made from sand) and shales (mud rocks, made from pieces smaller than sand), give the distinctive cliff-slope pattern of the canyons—resistant sandstones form cliffs and cap the flat-topped mesas, while softer shales form slopes. 

The great Colorado Plateau, flanked by the spreading regions of the Basin and Range to the West, and the Rio Grande Rift to the east, occupies large parts of Utah and Arizona plus some of Colorado and New Mexico, and includes Zion, Bryce, Capitol Reef, Arches, Grand Canyon, Petrified Forest and Mesa Verde National Parks as well as Canyonlands, and many national monuments and other public treasures. The Colorado Plateau is noted for reddish, flat-lying, rocks from the Paleozoic of a few hundred million years ago. 

Take a tour of Canyonlands National Park 

The details are not fully known of how the Colorado Plateau avoided extreme deformation for hundreds of millions of years, while the rocks in most of the West were being bent, broken, or pierced by volcanic eruptions. The silica-rich continental rocks of the Plateau may be a bit thicker than in its surroundings, so the spreading of Death Valley was unable to tear the Plateau apart and instead jumped across to the east side of the Plateau to continue as the Rio Grande Rift (the valley in which the Rio Grande flows). The spreading even seems to have nibbled away at the Plateau, with a few big pull-apart, Death-Valley-type faults visible in places such as Red Canyon just west of Bryce (see the picture below). However, our concern here is slightly different— the role of rivers.

Looking south along the Sevier Fault in Red Canyon, just west of Bryce Canyon National Park, Utah. The orange-red rocks on the left are much older than the blackish rocks on the right, which have been dropped along a pull-apart fault that runs almost directly under the camera, and slants down to the right below the surface.

You might be interested to go back and rewatch the Fault Hunters video from Module 2.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

The rivers of the Colorado Plateau are nearly as well known as their parks, for great rafting, incredible scenic views, and deep canyons. Rivers including the Colorado, the Green, the San Juan, the Fremont, the Virgin, and others have taken their place in history. But what are those rivers doing down there in the canyons?

Simply put, a stream or river is a conduit to take excess water, and sediment, from high places to low ones and usually to the ocean. Looking first at the water, rain or snow falls on the ground. (Streams are just smaller rivers, and streams are sometimes called runs or creeks or cricks or other names… and they all do more-or-less the same things, so you can use the terms interchangeably here.) Averaged around the world, rainfall (plus snowfall after it melts) is about 3 feet per year (1 m per year) (if you kept all the rain that fell in a year, and didn’t let any of the water evaporate or flow away or soak into the ground, it would make a layer about 3 feet deep). Pennsylvania's annual rainfall is also near the global average, as is the rainfall in much of the tree-covered eastern US. Some of this water evaporates directly, but most is used by plants and then transpires (evaporates) from their leaves. The evaporation from plants usually is lumped together with evaporation from other surfaces and called “evapotranspiration.” In a humid temperate climate such as central Pennsylvania, roughly two-thirds of the rainfall is returned directly to the sky by evapotranspiration; in dry climates, a larger fraction of the rainfall—often almost all of it—may be returned to the air by evapotranspiration.

Of the water that avoids evapotranspiration, a little actually falls on lakes or rivers, and some may fall on the land surface and then flow directly and rapidly over the surface into lakes or rivers, especially from the surfaces that humans have covered with buildings, roads, and parking lots, thus keeping the rain from soaking in. But most of the rain that avoids evapotranspiration soaks into the ground to form groundwater.

Soils and most rocks contain interconnected spaces, including gaps between grains of sand, cracks in the rock (which often are called joints), caves, and other openings. The ground acts a little like a sponge, with water soaking in and then slowly draining out to the rivers. We will discuss this groundwater flow a bit more when we visit Mammoth Cave, next; for now, simply note that because gravity pulls water down, rocks near the surface usually have some air in the spaces even where conditions are damp, and deeper rocks usually have all their spaces filled with water. The surface separating the deeper rocks with all spaces water-filled from those closer to the surface containing some air in spaces is called the water table, and where the water table intersects the surface of the Earth, a river or lake occurs.

Rivers flow even when it isn't raining because water is slowly draining through the ground from beneath hills to the rivers. The water table rises in elevation during wet times as the “sponge” of the Earth fills up with rain, and the water table falls during dry times as the sponge drains to keep the rivers flowing. And the water table is just below the surface in valleys and actually hits the surface at rivers, but you must drill deeper under ridges to penetrate the water table and complete a water well.

Video: Water Table (3:48 minutes)

The following video shows the movement of water in the atmosphere and land. Rainfall supplies evapotranspiration back to the air, runoff along the surface, and groundwater that flows through spaces in rocks to reach streams. The water table rises as rain fills the "sponge," and the water table falls between rains as the "sponge" drains to the stream.

Video: Grand Canyon Groundwater (1:43 minutes)

Water pumped out of the ground does not flow naturally to springs and rivers.  Sometimes, we use that water to flush our toilets or in other ways, and then dump the water back into the river farther downstream.  Other times, we use the water to grow crops or lawns and lose the water to evapotranspiration, so the water never reaches springs or rivers, which could dry up as a result. Here’s another vintage CAUSE video on how this sort of water use could impact nature at the Grand Canyon.  Such issues are increasingly important across the world.

Rearranging Rivers

As we saw back at the Badlands National Park, weather attacks rocks to produce loose pieces through the processes that cause weathering. And as we saw at the Gros Ventre slide near the Grand Tetons, processes on hillslopes including soil creep and landslides deliver the loose pieces (which we can call sediment) to rivers. A river is then faced with a balancing act; it must transport both the water and the sediment delivered to it.

You may be able to think of many ways for the river to adjust if it receives more or less water, more or less sediment, bigger or smaller sediment pieces, or “stickier” or “less sticky” sediment pieces (those more or less likely to clump together). For example, if more sediment is delivered to a river than it can remove, then the sediment will pile up, raising the elevation of the bed of the river where the sediment is being delivered. This steepens the river flowing away from the pile—the elevation of the ocean where the river ends has not changed, but the elevation of the riverbed is higher above the ocean—so the river flows faster and is better able to move the sediment. If nature delivers more water and less sediment, the river will tend to wash away all of the sediment supplied and have energy left over to carve into the rock of the riverbed. This cutting downward will make the river less steep from there to the sea—because the river ends at sea level and can’t lower sea level, lowering the upstream reaches of the river must make the slope to the ocean less steep. A less-steep river will carry less sediment and so reduce or eliminate erosion of its bed, and often will flow over loose sediment without removing that sediment and reaching bedrock. Such a river tends to reach a balance in which it just removes the water and sediment supplied to it. In the process of reaching this balance, rivers also may adjust the width, depth, and shape of their channels as well as the steepness.

That background, it should not surprise you that rivers are highly diverse. A white-water rafter braving the rapids of the Grand Canyon sees a very different setting than a cruise passenger on the Mississippi River going to New Orleans! The white-water guide and the cruise ship captain have jobs that require understanding rivers, and so do hydroelectric-plant managers, fisheries experts, city planners, designers and architects trying to protect people from floods, water-supply professionals, and many others. When Europeans settled in North America, they often moved inland from the coast along rivers to establish towns that became cities, and trade moved along rivers. Thus, today many people live near rivers, and all of us interact with rivers, if only by traveling over them on bridges and trusting that the engineers understood the way that the river interacts with bridge foundations.

Each river is unique, but we can find a few repeating patterns that will help you see the bigger picture. We will briefly discuss three of them: straight, meandering, and braided.  

Straight Rivers  

The only way to make a truly straight river is to dig a trench and line it with concrete… and that really isn’t a river. But, in many cases, a river flows in a single channel, often eroded into bedrock. If you look upstream or downstream along such a river, the canyon walls often look a little like the letter V with the river flowing in the bottom. Such rivers are often eroding slowly into the bedrock, with the water able to carry away all the sediment that is supplied plus the little extra rock that they break loose from their beds.

Video: Making a Sand Canyon / Grand Canyon National Park (1:30 minutes)

Downward erosion by a river into its bed produces steep river banks that then erode back, so you see the river banks making a sort of V if you look along the river, as in the picture of the Grand Canyon just above. In this vintage CAUSE video, Dr. Alley makes a very small-scale demonstration of how this happens, in sand near the Grand Canyon.

Meandering Rivers

In addition to these “straight” rivers, we often see rivers in patterns called meandering and braided (see the pictures). These patterns are common in rivers that have received more sediment than they could move, so the river flows across some of its own sediment. A meandering river generally has a single, deep channel, but that channel flows in great curves or meanders. A braided river, in contrast, tends to be wide and shallow, with the water splitting and rejoining around many sand bars or gravel bars scattered across the river.

Much of the difference between the braided and meandering rivers is related to the type of sediment they carry. If you have ever worked with clay in a pottery class, you know that it is made of very small pieces (you cannot see or even feel the individual pieces, the way you can see and feel larger grains of sand or gravel or boulders), and those tiny pieces stick together well (you can make clay pots, or you could make and throw balls of clay). When the sediment delivered to a river is rich in these small clay pieces, their stickiness allows the river to form a single deep channel with steep banks that don’t collapse. When the river does knock some sediment loose, that sediment tends to stay suspended up in the water (we call this “suspended load”), which flows rapidly because it is far from the river bed where friction with tree roots and the bed itself slows the water.

Such deep streams typically curve back and forth, or meander, along their paths. Put a tree’s roots, or a boulder, or almost anything else in the way of the river, and the water flow will be deflected away from the obstacle, hitting the other bank of the river and eroding it, so once started, a meander bend will grow. Meandering rivers usually occur in relatively flat, lowland regions towards the coast, such as the Mississippi heading for New Orleans, but you can find meandering streams elsewhere. (Meanders even develop without obvious causes, as in some streams flowing on top of the ice of melting glaciers—something disturbs the flow, and then the pattern of hitting the other bank and eroding it or melting it on a glacier leads to meanders.)

Braided Rivers 

a river receives lots of sand and gravel or even bigger chunks rather than clay, the large pieces do not stick together well enough to form a steep river bank, and instead tend to collapse into the water. The single deep channel of a meandering river would become wider and shallower if its river banks collapsed, and a wide, shallow river is good at rolling sediment along its bed. Sediment rolled along a river bed is called “bed load”. Often, the bed load will be piled into a lot of sandbars or gravel bars in the river during a flood, and the bars will be left sticking out of the water as the flood ends. Water then must flow around these sandbars. When viewed from above, the splitting and joining of parts of the river around the sandbars looks something like ropes of water that have been braided together, so these are called braided rivers. They are common in upland regions, where steep mountain slopes shed landslides of coarse sediment into the channels, but you can find braided rivers elsewhere.

Whether straight, meandering, or braided, rivers move water and sediment downhill. And, when people build dams to form reservoirs for flood control, recreation, or other purposes, we interrupt the sediment transport as well as the water flow, with consequences that we discuss next.

Video: How Dams Work (2:27 minutes)

Dams are generally built to influence water, but as noted on the prior page, dams always influence sediment, too. An important example of a dam influencing sediment as well as water, and sediment influencing many other things, occurs downstream of Canyonlands National Park and upstream of the Grand Canyon along the Colorado River. The Glen Canyon Dam was built on the river in the 1960s. The dam stopped floods coming down from the Rocky Mountains through Canyonlands. Water from floods that had raged through the Grand Canyon is now stored in the reservoir and then released gradually. Several things began happening to the Colorado River once the dam was completed. The dam traps the sediment carried by the river, and releases clean water, so the reservoir is filling with sediment, and in a few centuries or less will be full. Unique fish species that thrived in the muddy waters of the Grand Canyon suddenly are much more easily visible to predators and thus easier prey, often for introduced species, so many of the native species are endangered and disappearing. Evaporation from the large surface of the lake trapped by the dam removes water from the river, increasing the scarcity of water downstream.

In this “straight” river mostly flowing over bedrock, floods in the past had washed sand into corners along the river to make sand bars. This stopped when the floods stopped. However, the clean water released by the dam still flowed fast enough to remove some sand, so the existing sand bars below the dam were slowly washed away. But, many types of wildlife depend on sand bars, and thus were harmed—cottonwood trees had grown in the sand, birds lived in the cottonwoods, and deer could drink from the river by standing on the sand bars but not from the rocky cliffs. Floods on un-dammed side streams continued to dump large rocks into the Colorado, but the Colorado lacked the high flows to move this material onward, so the rapids in the main river at the mouths of the side canyons began to steepen.

In the spring of 1996, efforts began to rebalance the system by releasing artificial floods from the dam. The first flood was a partly successful experiment, rolling some of the big rocks out of the rapids at the mouths of the side streams, freeing sand trapped beneath, and putting some of that sand into bars. But, those bars weren’t very big and didn’t last very long. Additional human-made floods have been released more recently, timed to occur when natural floods coming from side streams were delivering additional sediment, to help make bigger sand bars. Additional attempts may be made because these artificial floods really have helped. Such human-caused floods cost money (lost hydroelectric power when extra water is routed around the hydroelectric plant in the dam to get water into the river in a hurry) and require lots of planning (you need to warn people camping or hiking in the Canyon before you suddenly flood them out!), and may be stopped during times of drought when other uses for the water are considered to be more important.

Video: How Deltas Work (2:23 minutes)

Meanwhile, as sediment fills the reservoir, sediment will also accumulate along the river upstream of the reservoir. For the tributaries to Lake Powell behind the Glen Canyon Dam on the Colorado River, there aren’t many people living in places where this sediment accumulates, but if there were, their fields and houses would be buried by mud. A river slows down as it enters a reservoir, or any other lake or the ocean, and sediment is dropped from the slowing water. Unless strong waves and currents in the reservoir or ocean take that sediment away, a pile called a delta forms. But the delta cannot be perfectly flat on top. If it were, then the stream would drop its load when it hit the flat spot and slowed down, and that would raise the flat spot. So, as the delta grows into the lake, the upstream end of the delta must build up so that the river still flows downhill, and that, in turn, will cause sediment to build up for some distance upriver (see the figure below).

When two dams were built between 1910 and 1926 to supply hydroelectric power from the Elwha River, which flows north from Olympic National Park in Washington state, the dams caused major problems because of their effects on sediment, water, and wildlife. The dams were built without ways to allow passage of salmon farther upstream to spawn. Most of the salmon had spawned upstream, but some had spawned in sand and gravel downstream of the dams, and the river washed that sand and gravel away after the supply of new sediment was blocked by the dams. The annual “flood” of more than 300,000 salmon that returned to the river before the dams were built turned into a trickle of barely 3,000 salmon. (Instead of building fish ladders to allow salmon to move around the first dam and continue upstream, a fish hatchery was built, but the hatchery was quickly abandoned.)

After the river washed away the sand and gravel in which the salmon had spawned, the river quit delivering sediment to the beaches of the Strait of Juan de Fuca (an arm of the Pacific Ocean). Those beaches then washed away. Without the beaches, the native peoples were no longer able to carry on their traditional shellfishing because the shellfish were lost after their sand-and-gravel beaches were lost. The nearby harbor of Port Angeles had been guarded by sediment-fed sandbars that also washed away, requiring more money to be spent on human-constructed protection for the harbor. The little bit of hydroelectric power being produced did not come close to covering the costs of all these damages.

In an ambitious plan to help the port, the people, the beaches, the river, the salmon, and the park, the federal government purchased the dams to remove them.

Beginning in 2011, both dams were removed. Within a few months after removal, salmon were returning to the river. The river, beaches, harbor and more will take a while to get back to "normal," because so much was changed by the dams, but there is much optimism about the recovery, which is proceeding rapidly. You can see the amazing changes at the mouth of the river in the pictures below.

Conflicts such as these between dam-builders and those who prefer free-flowing streams are not new. You don’t need to know these details, but it might interest you to learn that In 1731, a mill dam on the Conestoga River near modern Lancaster, Pennsylvania was torn down because it was ruining the fishing industry, which, in a petition in 1763 to remove other dams on that river, was said to include shad as well as salmon, rock fish, and trout in tributary streams. In 2018, a dam on the Neuse River in Raleigh, North Carolina was removed to allow natural fish runs of shad as well as striped bass and Atlantic sturgeon.

Video: Dam It! (2:44 minutes)

Dams cause huge changes on rivers, both upstream and downstream. In this film clip, Drs. Anandakrishnan and Alley discuss the Glen Canyon Dam and Lake Powell on the Colorado River. Huge changes were caused by this project, including in the Grand Canyon far downstream. The CAUSE 2004 class used some clever editing to manufacture a disagreement between the professors, who are much closer to being on the same wavelength than you might imagine by watching this.

Delta National Wildlife Refuge on the map and an alligator.

Credit: Left, R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0
Right, © Deanna / stock.adobe.com

If you look back at the pictures of the delta of the Elwha River, you can see the regrowth of the delta after the dams were removed. The right-hand picture in that pair shows a large delta, which looks something like the delta from before the dams were built. Erosion of that delta from before the dams were built gave the no-delta situation shown in the picture on the left, taken while the dams were in place. Similar loss of deltas is happening in other places, including the delta of the Mississippi River, which we will visit next.

At the tip of the Mississippi Delta lies the Delta National Wildlife Refuge. This is one of several wildlife refuges along the US coast of the Gulf of Mexico. These refuges are homes for a great range of resident wildlife, and also draw migrants from the north. Ducks and geese, herons and cranes, gallinules and rails, the wetland birds of much of the North American continent fly in through the autumn, and then spread north again in the spring. (Yes, technically, a National Wildlife Refuge is not a National Park, but it is a national park, so we’ll cheat a little and use it—it makes a good story. And, it is just down the river from the wonderful bayous of the Barataria Reserve in the Jean Lafitte National Historical Park and Preserve—well worth a stop if you're in the area!)

Take a tour of the Mississippi River Delta… and some other deltas

Here are a few pictures of deltas in Greenland, and some from the Mississippi River.

Take a tour of the Bayou, Barataria Preserve, Jean Lafitte National Historical Park and Preserve, Louisiana

Here are a few scenic pictures of a wonderful park set aside to preserve nature on the Mississippi Delta below New Orleans, with a little information about fossil-fuel formation in the last two slides.

Controlling Rivers

Unfortunately, the wetlands of Delta National Wildlife Refuge and Barataria Reserve in the Jean Lafitte National Historical Park and Preserve are disappearing at an astonishing rate, because of the indirect effects of human activities. Estimates are that every year Louisiana is losing over 100 square kilometers of wetlands (equal to loss of a square with sides more than six miles long). Whether the wetland birds will continue to stream north for generations to come may depend on how humans respond to the challenge.

The Mississippi Delta is a massive pile of mud and sand from the Rockies and Appalachians, transported by the river and dumped into the Gulf of Mexico. Long ago, the Gulf of Mexico extended much farther north into the heartland of what is now the U.S.; over the last 70 million years, the delta has grown southward from near Cairo, Illinois (up by St. Louis), until now the former embayment has been turned into a projection from the end of Louisiana out into the Gulf. There, the delta is as much as seven miles thick. If you have ever watched mud settle in a bottle of water, or if you have observed how your boot packs mud down if you step in it, then you know that, over time, mud will compact under its own weight or under the weight of anything placed on it.

As the delta grew into the Gulf over the ages, a natural balance was reached. The compaction that occurred during a year would leave a little space at the top, but the springtime floods would bring new mud to fill that space. Trees and other vegetation would grow up through the new sediment, or re-seed on top, and the system would continue, wildly productive and vibrantly green.

Wrestling with Mud

Humans don’t interact well with this natural system. Many people have settled near the river. Plants can grow through the mud of floods, but people don’t enjoy having their houses slowly fill up with mud. So, humans have built control structures. We built dams upriver, which trap sediment behind them, and which hold some floodwaters in check. Because large floods do happen even with those dams upriver, we also built levees along the river in its downstream reaches, great walls that hold the river in and attempt to channel the floods to the Gulf rather than letting the floods cover fields and towns and roads upriver. We also dredge the river, deepening it to carry the water—and shipping. Other channels have been cut through the delta for oil and gas prospecting and production. The great floods that shoot down the river then do not spread over the floodplain and the delta, and thus do not deposit fertile sediment to fill the space left by compaction of mud, but instead are piped to the Gulf through these human-made or human-deepened channels, carrying the sediment far offshore to settle in mile-deep water.

Way back in 1996, when the very first edition of this course was taught, we wrote:

"Today, much of New Orleans, which does lie on the delta, is well below sea level. A tanker in the river between its levees is higher than the playing field of the Superdome. Rainfall, and water seeping from the river, must be pumped out so that the city doesn’t fill with water. If the pumps were to fail, the city would become a lake. The city steadily sinks deeper, and the levees are steadily raised by the Army Corps of Engineers, as instructed by Congress, to keep the river caged. Meanwhile, the wetlands of the delta, unnourished by new sediment, are sinking beneath the Gulf..."

Students in Geosc010, and in many other classes at Penn State, learned what elected officials and coastal planners and students at other schools also learned, that New Orleans was a disaster waiting to happen.

In 2005, this sadly came true, when the powerful hurricane Katrina came ashore near New Orleans. Almost 1400 people lost their lives, damages exceeded $100 billion (that is more than $300 for each person in the USA), more than a million people were displaced from their homes (and more than half a million were still displaced a month later, often because their homes were gone). Where natural wetlands should have slowed the waves from Hurricane Katrina (which was not a really huge storm by the time it got to New Orleans!), the high waters of the storm surge roared almost unimpeded from the Gulf. Parts of the levees failed. The pumps failed. The city filled with water, as much as 20 feet deep.

A slideshow of the aftermath of Katrina

But the city has been (mostly) rebuilt where it was, the sinking will continue, the loss of wetlands will continue unless many things are changed, and the levees that have already been raised will need to be raised more. With the likelihood that the strongest storms will get stronger and sea level will rise in the future (we’ll revisit this later in the semester), the scene will be set for an even more horrific disaster at some future date. Many options are available, including restoring wetlands, filling parts of the city with debris or other materials, moving construction to higher parts of the city, moving out entirely, and more; it will be interesting to see how much of this will be done. But primarily, the donations and tax dollars from the rest of the country after the 2005 disaster were used to rebuild the city directly in harm’s way, with the knowledge that the rest of the country will once again foot the bill when disaster strikes.

Another story is being played out in this region as well. The river wants to leave New Orleans. The city has a love-hate relationship with the river, fearing the floods but needing the drinking water and the shipping channel. The river can harm the city rapidly by flooding, or slowly by leaving.

To understand this tendency of the river to leave New Orleans, note that especially large, muddy, flood-prone rivers normally have natural levees (which are much lower than the human-made ones). When a flood happens, the water spreads out of the main channel onto the flood plain, which is the flattish region of river-deposited mud next to the main channel. As the water spreads out into the trees or houses of the flood plain, the flow of the water slows, and the water drops some of its muddy load. Although some mud is deposited wherever the floodwaters flow, more of the sediment is deposited very near the river where the water first slows. Hence, the mud layer from a flood is thicker next to the river than farther away, forming a natural levee. Humans have raised these natural levees in many places.

When we discussed reservoirs, we saw that the delta of sediment formed when a river enters a lake must build up as well as out, so that the river still flows downhill into the lake. The same is true for a river entering an ocean. The Mississippi River, with its levees, naturally dumps mud into the Gulf of Mexico, slowly building out and up, lengthening and raising the riverbed. After a while, the river is a bit like a log flume in an amusement park, following a long path to the Gulf; a break in the levee wall would allow a much steeper, shorter, and more exciting downhill trip. The recent history of the Mississippi Delta is that, roughly every 1,500 years, the main outlet of the river has broken through the natural levee, like a log full of park-goers breaking through a curve in the ride, and the river has then followed that new shortcut. But, as mud is deposited along that new shortcut, it builds out and up, lengthening until it is like a long log flume, and then the river breaks through its side again, someplace else. This break-build-break-build helps create the classic shape of a delta.

During the 1940s and 1950s, the Mississippi started to break out, into a side stream called the Atchafalaya River. To save the shipping channel and the water supply for New Orleans, the Army Corps of Engineers has used levees and dams, especially the Old River Control Structure, to allow some water to go down the Atchafalaya while keeping a vigorous flow in the main log-flume channel past New Orleans. During a flood in 1973, the Corps very nearly lost the Control Structure, and the river, when a giant whirlpool undercutting the dam came close to causing it to collapse. The task of the Corps is very difficult, taming immense natural forces as the system becomes more and more out of balance.

Video: How New Orleans Doesn't Work (2:26 minutes)

An excellent account of this is given in John McPhee’s book The Control of Nature, 1989, Farrar, Straus and Giroux, New York, which may be a little out of date but is still fascinating, and shows that policy-makers and others were warned about the dangers in the area long before the disaster of the 2005 hurricane.

Cave formations in the “Violet City” section of Mammoth Cave, Kentucky.

Credit: Left, R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0
Right, Violet City. Jackie Wheet, National Park Service (NPS) (Public Domain).

Near the start of this Module, we discussed how rainwater soaks into the ground and moves toward rivers through spaces in the rocks. This process is always important, but it usually is not very dramatic. Occasionally, though, it makes spectacular features worthy of being national parks… caves. The cave with the greatest total known length of passages is Mammoth Cave, deep beneath the rolling hills of Kentucky. Mammoth Cave has more than 425 miles of surveyed passageways (almost 700 km), and the Park Service estimates that there may be 600 more miles to be mapped (almost 1000 km). And, Mammoth Cave National Park includes more than 200 other smaller caves that are not known to connect with the big cave.

Mammoth Cave has been used by Native Americans for at least 5000 years. After European settlement, the cave was mined for saltpeter (containing nitrates) for use in gunpowder, especially during the war of 1812. The source was bat guano (the polite name for it) deposited over the ages by great flocks of bats. There are over 1000 archaeological sites in the cave.

Take a tour of Mammoth Cave

Caves typically are found in special landscapes, usually called “karst”, that have certain special features. These karst landscapes give us beautiful parks, but cause major challenges for construction and drinking water.

Dissolving a cave

Mammoth Cave and its surrounding karst landscape, like the great majority of large caves, was dissolved in a rock type called limestone. The limestone was deposited in shallow seas during the Paleozoic Era (a few hundred million years ago), and comes from shells and other materials deposited by sea creatures. Mammoth Cave is so big in part because the limestone lies beneath a strong sandstone layer from old beaches, which provides a “roof” that does not collapse easily as the cave is dissolved into the limestone.

As we saw in discussing rock weathering to make muds for the Badlands back in Module 5, rainwater and soil water are weak acids. Chemically, the calcium carbonate that makes up the limestone is especially prone to attack by acid. (In fact, the usual test for limestone is to drip a little weak hydrochloric acid on a sample; limestone fizzes vigorously as the rock decomposes to free carbon dioxide gas, but most other rocks react much more slowly and do not fizz.) Where soil waters move through limestone, the rock dissolves and washes away. You wouldn’t see much change from year to year while a cave is forming, but the rock dissolves very rapidly compared to many geologic processes.

Not all limestones make big caves when they dissolve. If the limestone has lots and lots of cracks, the water may spread out into so many different paths through those cracks that not enough rock dissolves along any one crack to make a cave. But if the limestone has just a few cracks for water to flow through, all of the dissolution will be concentrated in those few places, and cave passageways may form. Caves usually form while filled with water, but nearby rivers then may cut downward, draining water from surrounding rocks and lowering the water table, as Kentucky’s Green River has done near Mammoth Cave. This can empty the water from all or part of a cave, letting it fill with air.

Forming cave formations

The beautiful stalaCtites (from the Ceiling), stalaGmites (on the Ground), and other cave formations that tourists love to see in caves can then develop. You might be surprised that nature first hollows out the cave and then starts to fill the cave again, but this really does make sense.

Here are some hollow stalactites, called soda straws, growing in a human-made cave in the Arboretum at Penn State’s University Park campus. Cave formations may take a long time to form, but these needed just a few years.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Many processes in soil, such as dead things decomposing and worms exhaling, release carbon dioxide, and some of that carbon dioxide is picked up by rainwater as it soaks in and becomes groundwater. Thus, groundwater is more effective than rainwater at dissolving limestone. Occasionally, a cave may be so isolated from the surface that dangerous levels of carbon dioxide build up in the cave’s air, but caves usually exchange enough air with the outside world to have near-normal levels of carbon dioxide so that you can go into them safely (presuming you have lights and warm clothing and watch out for floods and don’t fall into giant pits...).

When groundwater drips into such an air-filled cave that has a near-normal carbon-dioxide level, the groundwater loses some of its extra carbon dioxide to the air. The water then cannot hold all of the limestone it has dissolved, and some of that limestone is deposited to form the beautiful stone features we see.

Forming other karst features

Almost all rocks have cracks, called joints. The next time you can safely examine a cliff or road cut, you should be able to see these joints. They occur in many orientations, but some are generally near-vertical, often in intersecting sets as shown in the picture.

Well-developed joint sets at St Mary's Chapel, Caithness, Scotland. Rainwater often soaks into the ground along such intersecting cracks.

Credit: Wikimedia Commons is licensed under CC BY 3.0

The rocks in the picture are not limestone, but limestones have similar joints, and where water soaks downward along intersecting cracks in limestone, the rock is often dissolved, leaving space that may make a low spot in the surface, or may partially or completely fill with mud. Such a hole, whether mud-filled or air-filled, is called a sinkhole. Sinkholes also form when the roof of a cave collapses, leaving a low spot in the surface. Somewhat confusingly, when a pipe breaks beneath a city street and a road falls into the space, people also call that a sinkhole. For this course, we will focus on the sinkholes that are especially formed by dissolution of limestone, and leave the collapsing sewer pipes in cities for a different course.

A “karst window”, where the roof of a sinkhole has caved in to reveal an underground river. The river flows into the sinkhole along the blue arrow, and leaves along the yellow arrow. A second sink is very close to this one, after the water flows under a driveway, and then the water flows under a road (which the photographer is standing on, behind the blue safety cable in the lower right of the photo) and emerges as a spring in the bank of Elk Creek. This site is well-known to geologists as the Smullton Sinks in Smullton, Pennsylvania, north of Millheim.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Sinkholes formed by downgoing waters are very common near Penn State’s University Park campus. The Geosciences Department is housed in the Deike Building, which required extra funding for special strengthening because the building has sinkholes beneath—a building can rest firmly on bedrock, but tends to fall into air-filled or mud-filled holes. Extra funds were similarly expended to strengthen the nearby Mt. Nittany Middle School, the runway extension at the airport, and other construction projects in the area. A newly constructed storm-water catch basin at the airport filled with water during its first big rainstorm, and the weight of the water blasted mud out of a buried cave passageway somewhere beneath, suddenly clogging nearby Spring Creek with trout-choking red mud.

Where sinkholes and caves are common, streams often disappear underground into swallow holes, only to re-emerge at springs. Spring Creek is aptly named—it is fed by a lot of Springs!—and many other similar features occur around central Pennsylvania, around Mammoth Cave, and in other such regions.

Swallow hole along Tadpole Road west of State College, PA.  Water flows along the blue arrow and goes down beneath the yellow arrow—notice that the stream does not continue beneath the snow bank beyond the yellow arrow.  

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Corn cobs once were dumped in a sinkhole behind a cannery at Old Fort east of Penn State’s University Park campus, and after a rain would pop out of a spring in Spring Mills, a few miles away. A stream flowing off nearby Mount Nittany goes down a swallow hole in the town of Pleasant Gap and then comes back out in a spring a mile or so away… and once, a basketball that had washed down the swallow hole came out in the spring! Regions with sinkholes, caves, springs, swallow holes, etc., are referred to as karst, after a region in Slovenia with many such features. Karst features are present across 20% of the Earth’s surface, and roughly 40% of the US population obtain at least some of their drinking water from karst, according to the National Park Service.

The spring at Spring Mills, Pennsylvania. In the past, debris such as corn cobs that were dumped in a sinkhole several miles away appeared here after heavy rains, and the corn cobs were not yet rotten, so must have traveled rapidly.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

In the past, people often threw trash into sinkholes. Big pieces would sink into the mud or fall into cave passages beneath, “disappearing.” When Dr. Alley was in high school and went to Sloan’s Valley, Kentucky to go wild caving (spelunking), one of the cave entrances was known as the Garbage Pit, which led into the Tetanus Tunnel. A commercial cave near Mammoth Cave was forced to close in the 1940s because of the stench from sewage draining in.

Slowly, we are learning just how stupid it is to dump things in sinkholes. A test conducted by the great Penn State hydrogeologist Richard Parizek during the building of the Nittany Mall east of Penn State’s University Park campus showed that a little harmless dye dumped in a sinkhole near the mall came out in a nearby trout stream in a day or two. It should be evident that anything else dumped in a sinkhole near the Nittany Mall (or many, many other sinkholes in the region and in other karst regions) would show up very quickly in the water used by people and wildlife.

A small stream flowing out of a small cave (yellow arrow) into Elk Creek (blue arrow) south of Millheim, Pennsylvania. A little harmless dye placed in a sinkhole a few miles east of here came out along the yellow arrow very rapidly, in the same way that dye placed in a sinkhole near the Nittany Mall came out in Spring Creek rapidly in the State College area. Notice that the limestone here has fairly thick beds and widely spaced vertical joints, so dissolution is localized in a few places, favoring cave formation.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Dr. Alley lives in a house served by a local water company well-known for its fine water from deep wells (with locations that were identified by Penn Staters Richard and Byron Parizek). But many years ago, before a reorganization of the water company and before the help from the Penn Staters, the intestinal parasite Giardia showed up in the local well water. Giardia causes intense and possibly dangerous stomach problems. Giardia usually is restricted to surface water; the spaces in most rocks are small enough to filter out the Giardia cysts before they reach a water well, or the water takes so long to go from the surface to the well through the small spaces that the cysts die of old age on the way. At the long-ago community meeting to discuss the water contamination, company officials noted that they had installed well filters to remove sticks, leaves, etc., that came out of the wells with the water. In karst country, surface water can become groundwater and return to the surface in hours or days. Whole streams go down and up, and if sticks can go through, microscopic cysts can, too. Clearly, contaminants dumped somewhere today can be poisoning someone tomorrow.

In some other regions, the groundwater-contamination problems are quite different. In sandstones, for example, the water moves slowly, pore-by-pore, through the rocks. In some places, the water can be shown to have first entered the ground during the ice age, more than 20,000 years ago, or even earlier. Contaminants dumped in such rocks may not bother people for a while. But, when the contaminants do start to bother people, clean-up can be very difficult.

Try this experiment. Squirt a little food coloring dye on a sponge, and squeeze the sponge a few times to distribute the dye well. The sponge is our rock, and the dye is the contaminant. Now, wet the sponge, hold it up, and squeeze it. Colored water will come out. Wet the sponge again, squeeze it again, and more dye comes out. Repeat, and repeat, and repeat. You may need ten or more times to remove enough dye that you no longer see it, and sensitive instruments would detect the dye through dozens or even hundreds of additional washings. Now, suppose that instead of edible food-coloring dye we had used a chemical that causes cancer in humans. If the water in the rocks naturally is hundreds or thousands or more years old, then nature takes a long time to wash out the rocks once, and washing them out ten or one hundred times will take much longer than all of human history.

There are things that can be done about groundwater pollution. You can pump clean water in and dirty water out to speed up the washing, or pump steam or hot water in and out to wash even faster (and then try to figure out how to clean the dirty water or steam once you have them back on the surface). People are experimenting with installing filters so that polluted water will flow through them, sometimes using large masses of iron filings to react with and break down some organic chemicals in groundwater. Geomicrobiologists are searching in heavily polluted sites for microbes that “like” to eat pollutants, and then trying to introduce those microorganisms into other polluted sites to break down harmful chemicals, while other biologists are trying to design pollutant-eating microbes. But, such techniques usually are very expensive and not very effective. Most people who have thought about it agree very strongly that the best way to handle groundwater pollution is to keep the chemicals out of the ground in the first place. A whole lot of money has been spent on clean-up because we did not learn that lesson soon enough—and there are days when it appears that we have not yet learned that lesson.

Incised Meanders and Geologic History

Canyonlands poses a special puzzle. The rivers in Canyonlands meander, making big, sweeping curves through the red rocks. Earlier in this module, we discussed how meandering rivers typically occur in flat, lowland regions lacking supply of big rocks to the river. This is decidedly not the situation in Canyonlands. The rivers are somewhat steep, and in places (although not just where the picture below was taken) are laden with rapids, challenging for whitewater rafters. Landslides and rockfalls from the canyon walls deliver large blocks to the streams.

The likely story is that the streams once meandered across nearly flat lowlands. Then, uplift of the rocks began, raising the Earth’s surface to a higher elevation in what is now the US West, giving the streams a steeper slope to the sea and so speeding their flow and causing them to erode. But, the uplift was gradual enough that the streams held their old courses. The streams cut downward without a change in pattern, which is called incision. Canyonlands contains clear examples of incised meanders. Similar features are preserved throughout the Colorado Plateau, documenting widespread uplift of the Plateau. The Goosenecks of the San Juan River, shown below, are also beautiful incised meanders.

Much of geology involves study such as this; the history of a region produced the modern features. One can often learn much about that history by understanding the modern features and how they were formed. Clearly, if we did not know what conditions produce meandering streams today, we would not know what conditions likely occurred in the past when the meandering streams developed. The geological saying that captures this idea is “The present is the key to the past.”

More About Sinking Mud

Down at the Mississippi Delta, we saw that the mud under New Orleans is compacting under its own weight, contributing to sinking of the city. A couple of other things also contribute to the sinking.

As the delta grows, the weight of the mud pushes down the rock beneath, with soft, hot rock much farther below flowing away laterally, like air flowing out from beneath you if you sit on an air mattress. The sinking of the Earth under the weight of more mud can take thousands of years. And, just as the strength of the air-mattress cover spreads the dimple around you when you sit down, a fairly large region around the delta is pushed down by the weight of the delta. So, adding some mud anywhere near New Orleans causes a little sinking in New Orleans for a while.

Perhaps more importantly, as we saw with mass movements at the Grand Tetons (and as you can see in the picture of Canyonlands just above, by the cliff on the inside of the meander), rocks tend to fall off steep slopes. Sometimes a single rock falls, or a thin layer of rocks slides. Other times, large thicknesses of material move. The Mississippi Delta is a giant pile of mud towering above the deep waters of the Gulf of Mexico, and that “cliff” is subject to downward motion of its materials. Some of this occurs along faults that are something like pull-apart Death-Valley-type faults—the fault intersects the Earth’s surface well inland, sloping down toward the Gulf, and the mud on top slides down and toward the Gulf.

After Hurricane Katrina devastated New Orleans, geologists renewed their efforts to understand what is going on geologically—such understanding should help in planning how to slow down the next disaster. An argument has erupted about the relative importance of the various reasons for sinking in and near New Orleans, with faulting probably more important (and mud compaction less important) than previously believed, and with sinking of the rock beneath the delta small but not zero. All of these are almost certainly contributing, but with a little work remaining to figure out just how much to blame on each one. Naturally, all of them contributed to lowering of the surface, and sediment deposited from the river's floods filled that space—the river doesn't really care why the surface dropped, and deposits sediment in low places formed by any process.

Video: Erosion at Bryce Canyon National Park (1:04 minutes)

We can measure the uplift of mountains, which may occur slowly, or suddenly in earthquakes, and we can watch volcanoes erupt. But overall, nature tears down mountains about as rapidly as they form, and we can watch and measure the tearing-down, too. The slow disappearance of names from old tombstones, the hubcap-rattling holes in late-winter city streets, and the maintenance budget for university buildings all attest to the effects of nature on human-made things. Here, Dave Witmer takes you to Bryce Canyon, one of the many, many places where you can see nature removing natural things.

Video: Chain of Events / Zion National Park (1:18 minutes)

Geologists observe the wear-and-tear of nature on human-made and natural things, gaining clues to help understand how mountains are torn down. When climbing the sheer cliffs of Zion National Park into the mysterious crevice of Hidden Canyon, the intrepid hiker clings to a rather precarious-looking chain to avoid falling into the stream-carved potholes just beside the trail, and on down to the Virgin River, in the Canyon a hair-raising drop below. In these two clips, Dave Witmer and Dr. Anandakrishnan show how rocks are worn away, a little at a time, and what this has to do with south-Indian cuisine. You might begin thinking about what this wearing-away of rocks has to do with the Virgin River in the Canyon far below.

Video: Pothole Grinding / Zion National Park (1:27 minutes)

Review Module Requirements

You have reached the end of Module 6! Double-check the list of requirements on the Welcome to Module 6 page and the Course Calendar to be sure you have completed all the activities required for this module.

Comments or Questions?

If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.