CONTROL Stream Erosion

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Let's begin with the eroding bank. Why has nature decided to "pick on" this particular point instead of so many others? First, note that the problem spot is located on the outside of a bend, where the force of the current is strongest. Still, not every bend is as eroded as this one. Now take a look at the large, barely submerged outcropping of rock just upstream. A rule of thumb in stream engineering is that water spills off a solid object at an angle to the last surface it touches, and that's just what has happened here. The rock pushes the main force of the current (the thalweg, in fishery biologist jargon) directy into the bank, rather than parallel to it.

The downed tree is part of the problem, too. Cottonwoods like to grow along watercourses, and in an undisturbed, natural environment, that's fine. But in a degraded environment, cottonwoods may exacerbate bank erosion. These trees are easily undercut, and even when that doesn't happen, old, dying cottonwoods may just fall over, pulling out their own roots.

In our example, the gap left by the roots forms a secondary channel during high water, enabling the stream to cut still farther into the bank. In addition, the trunk of the tree

Stream Erosion

acts as a partial dam. Some water does spill over the top of it after a heavy rain or snow-melt butat all times-water is also forced wider the frunk, further undercutting the bank. Meanwhile, near the opposite bank, the damming effect of the top portion of the tree pushes still more water from the left bank toward the right.

WHAT TO DO

The first and easiest step toward restoring this problem spot is to get rid of the downed cottonwood. You could, for example, simply cut it up into logs, then float or carry them out. Whatever you do, try to avoid going into the streambed with heavy machinery. (Note: I'm not advocating removal of all deadwood as a general practice. In many instances, downed timber contributes substantially to stream aesthetics and to fish and wildlife habitats. The point is that this particular tree is clearly contributing to the erosion problem.)

Now let's see if we can redirect the thalweg so it doesn't collide abruptly with the bank. Removing the rock outcropping is out of the question; besides, it's attractive and a good fishing spot. One solution is to set up a counterforce, and the easiest way to do that is to place large rocks along the eroding bank-a technique known as riprap. The obvious purpose of riprap is to help the shoreline resist the force of the water, but it also creates turbulence-and turbulence tends to throw the current back toward the center of the stream, further reducing bank erosion and deepening the center so that it can accommodate more water.

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