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40 widen. In general, the braided stream :La shallow, has degraded 8treambanks, an increased slope, carries exe:essive sediment, and generally provides poor wildlife habitat. Meandering Pattern A meandering channel is a term applied to streams to describe alternating S-shaped bends, curves or loops and the characteristic sinuosity of some streams (Figure 7). Meanders commonly form in alluvium (water-deposited material, usually unconsoli,dated) wherever the stream traverses a gentle slope. They are invariably formed by a process of erosion, transportation and deposition of the material of the valley floor. The overall meander pattern of most streams may be irregular and its details may change from year to year. Local changes may occur wherever or whenever stream flow exceeds the thresholds for erosion of materials composing the banks and channel bed. In the context of an entire watershed, a meander will occur where the material constituting the streambank 11 comparatively uniform. Therefore, meandering occurs in floodplain areas more readily than in upstream headwater or confined areas. It is not unusual for individual me,a,nder bends to become distorted and alter their relative positions. Meanders may meet and actually shorten the path of the stream. Meander Morphology A meander is a stream reach that includes one complete bend, curve, or loop (Figure 9). The upstream and downstream boundaries of a meander extend across the stream at the center of the bends immediately upstream and immediately downstream. This distance between the upstream and downstream bends, which includes one complete bend, is called the wavelength. A concave (outer) bank and ~~ (inner) bank is located at the apex of each bend. Likewise, a pool is usually located adjacent to the concave (outer) bank and a point bar adjacel1t to the convex (inner) bank. Two riffle areas are located within each wavelength, one upstream ,and one downstream of the pool and point bar areas. The water depth becomes more uniform and often decreases as the water pal;ses over a riffle. In naturally meandering streams the force of the flowing '",ater produces a sloping channel with the deepest part of th.! channel near the concave bank. The line that follows the deepest part of the stream channel or the valley floodplain is called the thalweg. High water flows tend to maintain the thalweg by scouring the sediment that ac:cumulates during low flow periods. In semiarid and arid regions, and during droughts, the thalweg channel tends to function as a low flow channel. Meander Flow Pattern , The flow pattern within each meander is a little n~re complex than the simple pattern of the S-curve. The primary current eXE,rts an erosIonal force a little downstream of the IIlid-point of the bend (loop, curve,) on the concave bank, and the main point of depositicm is silllilarly downstnam of