Minutes - Council - 06/09/1998 - Public Hearing

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Processes of Stream Erosion and Deposition 169 :s be- ari'ty osite :uses river side ated /end · the £UFS. ater out- . As ttes the side :des der ~rll- --ess .: tO :ely · < a .'_he :he ::ed 'A lq (A) Patterns of stream flow are deflected by any irregularity and move to the opposite bank, where erosion begins. begins, the flow of water continues to ¢mpinge upon the outside of the bend, developing a meander loop. At the same time, deposition occurs on the inside of the bend as a result ol the lower stream velocities in that area. Figure 11.24 Flood-plain features seen from an altilude of approximately 6000 m. Note meanders, cutoffs, oxbow lakes, and old meander scars (see also plate 14). (C) The meander is enlarged and migrates laterally with contemporaneous growth of the point bar. There is a general downslope migration of the meanders as they grow larger and ultimately cut themselves off lo lorm an oxbow lak~ Figure 11.23 Development of meander bends and bbrs. The process of meandering involves erosion on lhe (~utside of a curve in the stream channel, where velocity is ,) ¢4eatest, and dePosition on the inside of the curve, where v e"/'Oeffy-Ls x!. La. minimum. levees grow higher with each flood, they soon build up high embankments and the river actually can build its channel higher than the surrounding area (figure 1t.25). Backswamps. As a result ofthe growth and develop- mcnt of natural levees, a considerable part of the flood plain is below the level or the river. This area, known as thc backswamp, is poorly drained and commonly is the site of marshes and swamps. Tributary streams are un- able to flow up the slope of the natural levees, so they arc Forced either to empty into the backswamp or to flow Figure 11.25 Natural levees are wedge-shaped deposits of silt that taper away from the stream banks toward backswamp. They form during flood stages because, as the water overflows its banks, the ve/ocily ts reduced, causing the sill lo be deposited. As the levees grow hig,her, the stream channel a/so rises. So t,'~e r,'ver can be h;gher lhan the surrounding area. parallel to the main stream for many kilometers before connecting with the main stream. Strangely enough, then, the highest surface on the flood plain ma*,' be along the natural levees immediateh adjacent to the r'iver.