Channelization and Its Discontents

  • This forum is empty.
by Leland Searles

Rivers are central to our lives in ways we don’t often realize. Cities and towns across the Midwest were started along rivers, often at the confluence of a main river and a tributary. We are distant from the reasons for those choices. Still, no matter what the landform of our individual places of residence, flowing water shaped those places.

Figure 1: The Mississippi River Valley from Pikes Peak State Park

I write this in Marshalltown, along the Iowa River, which crosses three major glacial moraines and goes from the heavily farmed Wisconsin glacial surface to the hillier Southern Drift Plains, touching the Iowan Surface on its way.

Water in motion is very close to the essence of life, if such a thing exists. Our bodies contain mostly water in our cells and in the spaces between organs and tissues. Without water in the ventricles of the brain, we would exist in a vegetative state. What’s more, the water in these crucial “brain spaces” flows in and out, like a stream, as does the fluid in our ears and lymph systems. Blood is a more obvious example of this bodily flow. We are made of several different river systems that all go to one source, the liquids we drink.

The flows in our bodies and the flows in natural streams follow the same basic physical opportunities and limits provided by the physics of water molecules, with variations on that theme. Streams on the landscape find a compromise between stability and constant change.

Yet we’ve done much to make rivers do our bidding, rather than live with them as they are. In Germany years ago, I saw almost idyllic country landscapes in which the streams flowed in highly controlled, walled channels for long distances. The fields and roads and towns required protection from the tendency of streams to wander across the land. Two months’ stay in Rothenburg ob der Tauber allowed me to see a relatively wild segment of one German river, the Tauber, cascading down rock rapids, with the medieval town of Rothenburg on one high bluff and fields and woods on the other, but plenty of confining walls and structures elsewhere.

Figure 2: Rothenburg ob der Tauber in 1981, with the Tauber River Valley in the background.

Perhaps the most damaging thing we’ve done to any river is to channelize or “straighten” it. This labor-intensive effort occurred mostly in the 1800s and early 1900s in North America, when human labor was cheap, and the technology of the Industrial Revolution gave us seeming power over nature. It still happens around the world, and in the Midwest, sometimes illegally.

Straighter streams carry some advantages: a wider belt of fertile land on either side that could be planted with row crops, without having to turn the horse- or tractor-drawn implements inside the meanders. Farming in the bottoms became more efficient and more productive. When levees were added, cities could expand onto the valley floors that now flooded rarely, and riverside property became more valuable.

The physical science of fluid dynamics turned to streams in the early 20th century, giving rise to the twin specializations of stream geomorphology (the topography and shapes of streams) and stream hydrology (the action of flowing water in channels and valleys). One finding solves puzzle about many streams (from rivulets in beach sand to large rivers) in the Midwest: why do streams meander, that is, have so many S-curves or bends? Isn’t a straight-line flow more efficient?

Figure 3: The South Skunk River, along a channelized and leveed length in Polk County, Iowa

The meander pattern is a process for energy dissipation during floods and for sediment management. A rule of thumb is that moving water must carry some amount of dirt, so water on the landscape is usually erosional. The issue is the rate of erosion: millennia to carve out a valley and channel, a few years to cut a channel bed downward and blow out the banks, or a few days or hours for a new gully to appear and extend. Our land uses tend heavily toward the last two. Many streams have too much sediment for their channels and water to handle, and the flows become flash floods that rise quickly to a high peak, then drop rapidly, often causing damage and leaving some of that sediment behind.

When we channelize streams, we mess with the dynamic equilibrium of those natural processes. The altered flow begins a struggle between the physics of nature and the physics of human engineering. We win, at best, for a few decades. Nature wins, eventually, but often after much chaos. The rivers, it turns out, “knew” best how to make the adjustments they needed. We should have left them mostly as they were. In harmful ways, we have become the rivers. A shift in thinking would help us be those rivers in more mutually beneficial ways.

We need to let them to return to the flexible equilibrium they seek in much-altered landscapes, lending assistance from our understandings of geomorphology and hydrology. Those understandings, as good as they have become, will always be incomplete. Some functions of rivers simply cannot be measured precisely, because the attempt to measure interferes with what the stream is doing.

An old Dodge Ram advertisement from the 1990s asked, suggestively, “If we violate the laws of physics, will we be punished?” The answer to the ad’s suggestion is a resounding “No!” We may try to beat physics, and we may appear to win in very short time spans (0 to 60 in under 4 seconds, for example, or the 50-year life span of a bridge abutment), but the accident rates during urban rush hours indicate otherwise, and the problems with many old bridges tell us to change our thinking. No, the physics of motion and fluids like water are inviolable, and any appearance otherwise is an illusion.

Let’s look at a naturally “crooked,” meandering river and a straightened one. The natural river channel is a physical adjustment between many factors: the geology of its setting, whether glacial deposits or bedrock; the downhill slope of its valley; the amount of material coming to the stream from its bluffs, by way of sheet erosion, and from its tributaries; the amount of rainfall and snowmelt it receives; the amount and types of vegetation on its banks.

Figure 4: Meanders, also called bends or S-curves, in a small stream through pasture, where the flowing water has adjusted itself to its valley and the physical requirements of bed slope and sediment movement

Channeling changes all these: the geology, by adding concrete or high amounts of rip-rap or hardening the stream bed; the channel’s slope, by changing it from its response to the valley slope; the sediment materials entering or existing in the channel; the amount and timing of rain and snowmelt entering it; and the amount of vegetation encountered during a flood. The stream no longer is a naturally responsive, self-adjusting system.

What consequences follow? Channeling shortens the length of the stream between point A and point B. The shorter channel has

  • less flood-holding volume in the channel itself.
  • a steeper stream bed slope, meaning that
    • the flow runs faster down the slope.
  • increased speed and energy that usually leads to higher erosion in the channel, thus
    • the amount of sediment that the stream needs to carry increases.

If the flow is unable to handle more sediment, the sands, gravels, and large particles come to rest somewhere along the stream bed and on the land above the bank, changing how the flow works its way along the channel and further decreasing the storage of flood waters. In flood, fine sediment becomes the major pollutant, leading to the chocolate-brown appearance of Iowa’s rivers after a rain.

The steeper slope makes the stream capable of eroding its stream bed more deeply. This can cause scouring and headcuts, which are small waterfalls of a few inches to numerous feet in height. Even the smallest develop roller currents (a dam is a type of headcut) that undercut the base of the waterfall, and so the headcut travels upstream. A waterfall on the Mississippi River in Minneapolis, the St. Anthony falls, used to lie near the outlet of the Minnehaha River not far from the waterfall made famous in Henry Wadsworth Longfellow’s poem, “Song of Hiawatha.” Their current location is in downtown Minneapolis, several miles upstream. The same erosional process happens in every unrepaired gully and on steep bluff streams throughout Iowa. Headcuts and roller currents affect some channelized streams because of the steeper bed slope. Behold the power of water.

Figure 5: After channelizing, the Iowa River at Marshalltown floods its terrace, having no way to store the water in its channel and meanders. Much of it has levees to protect infrastructure, denying the Iowa even more natural floodplain.

With these alterations, a channelized stream sends its flood flows downstream, causing a “traffic jam” that covers low-lying farmland, bursts or overtops levees, encroaches on homes, towns, and cities, and causes much grief for we humans. We have not learned to live with the river, and we still think we can beat it with our smarts and our technology. Build the levees higher and wider. Install upstream gages so we can predict floods and evacuate if necessary. Pay higher insurance premiums for the losses. Place buildings on “pads” that are above a certain flood-risk elevation. Spend dollars on public education so that fewer motorists will drive into water running over streets.

Unfortunately, the wisdom from past floods is rarely consulted,[2] and the changes that are required are rarely implemented. We have too much at stake in the short run to be concerned about problems in a few decades or centuries.

We will not learn. We will pay ever higher costs of all kinds for our arrogance.

We cannot violate the laws of physics.


[1] My title purposefully recalls a book by Sigmund Freud, Civilization and Its Discontents, in which he explored the many ways in which we might express, without realizing it, our dissatisfaction with the restrictions of modern life. I think that his students were onto an important part of human nature, in that we suppress our discontent in the pursuit of our conscious goals, only to have that discontent surface in ways that are unrecognizable or deniable.

[2] The must-read description of flooding in Iowa is a book, A Watershed Year: Anatomy of the Iowa Floods of 2008, edited by Cornelia Mutel and published by the University of Iowa.

 

  • Oh, bother! No topics were found here.
Create New Topic in “Channelization and Its Discontents”
Your information: