The median sedimenl" size in the floodplain generally decreases in 

 the downstream direction along a river. Thus, the median size may be cobbles 

 in the headwaters and fine gravel near the mouth. However, the median size 

 can significantly vary around this general average within a sma I I area 

 at a specified point along the river. This variation is a consequence of 

 the variation in hydraulic forces from one point in the floodplain to an- 

 other. 



Channel Erosion. Channel erosion in rivers is generally considered 

 to be either local erosion (scour) or degradation. Both result from an 

 increase in the sediment transport capacity, or a decrease in the sedi- 

 ment load entering the area, or both. 



Local scour is most commonly a result of local increases in velocity 

 due to flow obstructions or contractions. The increased velocity increases 

 sediment transport capacity. Degradation can result if the channel bed 

 is steepened in a short reach by, for example, a meander cutoff. The sedi- 

 ment transport capacity would be increased through this reach causing ero- 

 sion and a general upstream progression of the steepened slope (Figure 35). 



Upstream Progression of Steepened Slope 



Steepened Slope 



Figure 35. Schematic diagram showing degradation process. 



The progressive erosion continues upstream until equilibrium is reached. In 

 theory, equilibrium is reached when the slope is equal to the slope prior to 

 the occurrence of the cutoff, which would require the steepened slope to 

 migrate to the headwaters. In practice, the steepened slope is reduced 



116 



