during its upstream migration and gradually reaches an equilibrium con- 

 dition. However, the degradation may extend over a long reach before equili- 

 brium is ach i eved. 



Sediment Transport. Sediment transport is the movement of sediments 

 past a specific cross section of a river. The sediment may be transported 

 as suspended load or bed load. Suspended load is sediment that is trans- 

 ported long distances suspended in the water column. Bed load is sediment 

 that is transported by saltation (bouncing), or by rolling or sliding along 

 the river bed. The sediment size distinction between bed load and suspended 

 load varies with variations in discharge. At low flows, assuming the sedi- 

 ments were available, silts and clays may be transported in suspension 

 and sands and gravels transported as bed load. During floods, suspended 

 load may include clays, silts, sands, and gravels, with cobbles and boulders 

 transported as bed load. Often, the suspended load is assumed to include 

 clays, silts, and sands and the bed load includes gravels, cobbles, and 

 bou I ders. 



Changes Due to Gravel Removal 



Very little sediment data were collected at the study sites. Direct 

 measurements or observations of bed or suspended transport were not made 

 because site visits were scheduled during periods of low flow when the sites 

 would be most workable. Because the sedimentation characteristics prior to 

 gravel removal were also unknown, the upstream cross section was usually 

 used as the undisturbed cross section. The effects of gravel removal were 

 evaluated by comparing sedimentation features in the gravel removal area to 

 those in the undisturbed upstream area. 



At six sites, a decrease in the median size of the surface layer, 

 or armor layer, was observed in the mined area as compared with the undis- 

 turbed area. Similarly, an increase or decrease was observed in the median 

 diameter of the material underlying the armor layer at eight sites. In many 

 cases it was difficult to evaluate whether the variation in median diameter 

 was a result of the gravel removal operation or simply a result of the 



I 17 



