Monday, March 10, 2008

On the Importance of Meandering

Streams love to meander for reasons that are simple to understand yet profound to ponder. When a stream meanders around a bend the outside water must travel faster than the water on the inside of the curve. This increased speed leads to increased erosion and the bend becomes wider. As the bends become wider the overall length of the stream increases. Also, because the stream is eroding the landform more evenly, the overall grade is reduced and the water slows down. This decreased speed combined with increased length vastly alters the hydrological characteristics of the watershed, contributing to aquifer replenishment and filtering.

Changes in the Mississippi river's meanders:Meanders are a good thing that happen naturally. Sometime humans want to help the process along, especially for streams that have downcut and linearized (straightened). Water has momentum and will bounce off an oblique line of in-stream rocks, known as a vane, toward the opposite bank, starting an oscillation that can become a full-fledged meander.
Vane closeup showing rocks, stabalizing posts, and planted willow:
Several parallel vanes create an induced meander:
The banks that form behind vanes are first accretions of large boulders, then smaller rocks, and then small particles, all stacked somewhat like oranges at the supermarket. Because of the gradient of particle dimension, these banks act to soak up moisture from the stream through capillary action and are therefore great places to plant trees.
The old channel can still be seen seeping out from under the bank on the right while the new induced meander flows around to the left.:
Another view of the old channel seep (now on the left) and the new channel (on the right) flowing into a pond:
A dam can create ponding, which also slows water. The ratio of riffles to still water is an important measure of hydrological health, and for streams of this size and location the ratio should be close to 1:1. Even large rivers should often have this ratio; for example, the Columbia river, before it was artificially dammed and its watershed logged, held vast log jams stretching from bank to bank creating huge lakes. The easiest and most natural way to create ponding is with beavers, a keystone species that create habitat for hundreds of other species.

Unfortunately, there is not enough vegetation to support a beaver colony on Las Huertas. Creating an artificial pond with a wicker weir backed by an infill dam is a good short-term solution.
Eventually the dam will fill in with sediment, raising the stream bed and creating good streamside habitat. At present the pond helps raise the water table as well as store surface and subsurface water. The weight of the water in the pond pushes water further into the alluvium around the dam, supporting our revegetation project. Eventually there will be enough trees to support beavers and a self-sustaining ecological-hydrological system.

Another way to create ponding (and slow water) is a Zuni Bowl, an in-stream depression lined with rocks. It is named in honor of the Pueblo Indians in the Southwest who invented many of these water-control structures hundreds of years ago.
Slowing water is one way of slowing erosion. Another is stabilizing slopes.

The slope pictured below is divided into two regimes. The lower pitch is an equilibrium arrangements of small particles. This slope is also known as the angle-of-repose. The upper pitch is a nearly vertical cliff where disturbances can cause catestrophic failures.
The slope pictured below is composed of alternating regimes of angle-of-repose and cliffs. The slope erodes from the bottom and then unstable slopes just above that collapse, and so on. The progression of erosion from bottom to top is an example of effects moving "upstream": erosion at the mouth of the river affect the headwaters even though water only flows downstream. Even though stones can only roll downhill, their downhill absence propagates uphill to affect the perch of still-higher stones. Thus , what happens in a valley can determine the shape and slope of the mountains around it. By slowing erosion here we determine the shape of mountains.

A plant's roots can often form a barrier to the progression of erosion by holding together soil in a steeper slope. However, unless the root causes of erosion are addressed (e.g. with vanes) eventually soil will be washed away from the roots and/or the entire plant will be undercut.
A combination of engineered solutions and revegetation can work together synergistically to slow erosion. Indeed, since water velocity is determined by slope, decreased erosion is itself responsible for slowing water and hence slowing further erosion. The key to influencing such feedback systems is knowing where to interfere. Often restorationists find themselves faced with the bemusing problem of wanting to work simultaneously both downstream and upstream of the target stretch, asking themselves the question, "does the water above push the water below or does the water below pull the water above"?

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