Written by Derek Schook, a 2016-2017 Sustainability Leadership Fellow and Postdoc in the Department of Forest and Rangeland Stewardship, CSU and Water Resources Division of the National Park Service.
Tumbling down from the mountains, rivers cut across the land on voyages to the sea. Although we primarily think of rivers as transporters of water, an equally significant role they play lurks beneath the water’s surface. Under natural conditions, rivers scour the earth to create landforms and habitats, both in the channel and on the land. However, the ability of rivers to do this is directly affected by human activities. Recent human-induced modifications to rivers are leading the waterways and their dependent species into uncharted territory.
In vast networks that sprawl across the landscape, river systems erode, transport, and deposit rocks of all sizes. The amount of material transported by rivers is massive. For example, every year the Mississippi River deposits 150 million tons of sand, silt and clay (which, when combined with rocks, are collectively termed “sediment”) into the Gulf of Mexico. To picture that quantity, imagine a sediment pile neatly stacked within the confines of a football field. By the end of the year, the pile would ascend 13 miles into the sky, or twice as high as you were on your last flight across the country.
Zooming in from the landscape scale down to the channel itself, the rocks found along river beds provide habitat that supports aquatic-terrestrial food webs, an ecological system well-described in a previous post by Alisha Shah. At the base of the food web are invertebrates that rely on the right size, placement, and stability (or lack there-of) of rocks on the river bed. These invertebrates are eaten by fish, which are eaten by other fish, and these tasty relationships eventually provide food for a variety of terrestrial animals ranging from spiders and warblers to grizzlies and eagles. The river bed itself appears to be comprised of a chaotic assemblage of rocks randomly strewn about. However, the rocks’ positions are determined by physical processes governed by hydraulics (the physics of fluids in motion). Gravity provides the underlying engine that moves both water and rocks downhill, and the burden of potential energy is reduced along each inch of the descent. As water flows with velocity and momentum, it contains kinetic energy capable of destroying natural and manmade structures. Destruction of natural things, such as river banks, can be beneficial to the river and its watershed, but destruction of human infrastructure is problematic.
This destruction of infrastructure, coupled with our desire to harness water for human uses, has led to widespread changes to water and sediment flows. These changes have damaged river health. One change is that flow regulation often allows rocks on the river bed to stagnate. Through time they “armor”, or become increasingly difficult to mobilize because of long intervals between high scouring flows. A locked-down river bed is a worse substrate for bugs and food webs, and also for fish like spawning salmon who form redds among river cobbles.
Along the sides of rivers, people have built walls made out of concrete, metal, and even old cars. These barriers prevent rivers from gathering sediment as they attempt to go about their natural meandering ways. The walls protect human infrastructure in the short term. However, they deprive the river of sediment it would naturally collect from its banks. This, in turn, leaves more energy in the channel and increases the potential for destruction downstream.
Even more disruptive than preventing a river from eroding its banks, damming creates a pronounced barrier to water and sediment transport downstream. Instead of sediment crashing into the dam before it is halted, sediment transport is indirectly prevented. This happens because as water backs up behind a dam, its velocity decreases so the water loses the energy required to transport it. Another way to think about velocity affecting transport capacity is to consider the ease with which fast winds in a tornado can pick up and transport a cow, but a gentle breeze may just make the grounded cow smile.
In addition to the severe alterations dams impose on the timing and magnitude of flows, by denying sediment passage, dams modify the downstream river in two main ways. The first is that reduced sediment transport means the river will retain more erosive energy downstream, which often results in bed incision and channel lowering. Channel lowering separates the river from the floodplain it created. Many floodplain plants need either flooding or river-supported high water tables for their reproduction and growth, so some plants can disappear after sediment transport is modified. As a result, animals living among the vegetation can be forced out too as their habitat is lost.
The second result of dam-induced sediment deprivation is that downstream sediment deposition is reduced in and around the river channel. The consequences of this seemingly mundane process become evident after decades have passed. Like the barren landscape left behind after a volcanic eruption, channel movement followed by sediment deposition creates a clean slate of new land. This land is readily colonized by floodplain plants. For example, sand that deposits on the inside of a river bend provides ideal habitat for regeneration of some floodplain plants such as willows and cottonwoods. These plants eventually mature and become the backbone of biodiverse floodplain ecosystems. Episodic, even cyclic, sediment deposition and forest regeneration sustains floodplain forests that are a mosaic of habitats young and old, big and small.
Sediment transport is one of many issues worth knowing something about when it comes to the complexity of natural systems. We are living in a time of changes that are occurring faster than the natural world has ever seen. Species, ecosystems, and watersheds develop slowly, but they generally have evolved to find a delicate balance of interactions. You might already have your favorite environmental issues to think about, to talk with others about, and even to take action on. To bolster this list, I encourage you to head down to a local river and think about processes occurring beneath the surface. Maybe you’ll be inspired to learn more about the rocks that roll down the river, and the tricky solutions we must find to balance the needs of people and nature. Even if you’re not hooked on the cryptic topic of sediment transport, at the very least you’ll likely enjoy your time along the river and strengthen your bond with nature. And that in itself is a win, both for people and for rivers.