CONTROL Stream Erosion

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In your walking or wading, you'll note that erosion isn't a constant force over the whole stream. Some points are badly damaged whenever the water rises, while others remain relatively stable. Your task is to ask why. Why does the water cut the bank here and not there? Why is the channel deep at one place and silted in at another? What changes have occurred, or might occur, in the stream's direction? What influence exerted on these processes would encourage stream stability and minimize damage to the land and waterway?

START WITH THE LAND

All stream erosion problems begin on the land. There are two main problem categories: excessive erosion of the bank, and sedimentation of the streambed. (Fig. 1 maps a stretch of stream containing sites with such problems.)

For the purpose of illustration, let's suppose that you have located one case of each kind of problem. At one point, your stream is destroying its bank. Pieces of turf hang out over the water, supported only by a few inches of soil, which periodically break off. Meanwhile, the stream-even at low-flow levelsis cutting away at the lower part of the bank. It has already undercut one mature cottonwood tree, which hangs precariously half in and half out of the water, and another tree is threatened.

This eroding bank is not only unsightly, but also dangerous to people or livestock who stray too near the edge. Furthermore, the stream is steadily eating away your land and depositing your good soil as harmful silt at downstream sites.

First the stream damages the bank. Then the bank damages the stream. Farther down from your collapsing bank is one of the first stops for your lost soil, as well as for other sediments originating above your property. Just upstream of a large clump of willows is a shallow, mud-bottomed reach that is neither attractive nor productive of fish. It used to be a gravel-bottomed riffle and a deeper pool. You can tell that by wading through there, because your feet encounter gravel under a thin layer of silt in the upper part of the reach. Then, although the water doesn't appear much deeper, you sink up to your knees in soft sediment.

UNDERSTANDING THE PROBLEM

There are two ways to attack both of these problems: in the stream, by redirecting the force of the water, and on the bank, by stabilizing the soil. Channelization, in its ultimate form, accomplishes both of these ends, albeit at the cost of all the stream's natural values. The channel is dredged perfectly straight, so that the force of the water against the bank is minimized, and the banks are converted to rock or even concrete walls, which resist erosive forces. However, we're looking for a more "natural" way of accomplishing these same ends.

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