736 



Fishery Bulletin 93(4). 1995 



the forebay, and on the trashracks of the dams as 

 the Lower Snake River dams were constructed. Most 

 of the debris accumulated at the uppermost dam in 

 any given period. With periods of high spill, some 

 debris passed downstream through spillways, but as 

 the volume of spill decreased, the trash load at the 

 upper dam increased. By 1979 at Lower Granite Dam, 

 debris extended upstream from the dam approximately 

 1 km (Fig. 3). The debris that collected at Little Goose 

 and Lower Granite Dams after their construction pro- 

 vided a continual supply of woody material that clogged 

 trashracks, accumulated in the gatewells, and collected 

 throughout the fish facilities. Gatewell orifices and all 

 other components of the bypass systems were continu- 

 ally obstructed by debris. Although debris was con- 

 stantly in the forebay s, attempts were made to remove 

 it from the trashracks. However, the rakes that were 

 used to clear the trashracks were ineffective and in- 

 stead, large, heavy-steel beams were occasionally low- 

 ered down the trashracks in an attempt to push im- 

 pinged debris to the bottom. As judged by water levels 

 and turbulence in the gatewells, this procedure met 

 with limited success. 



To compound problems, when first constructed the 

 fish facilities at Lower Granite and Little Goose 

 Dams had undersized plumbing systems and other 

 poorly designed components through which fish 

 moved. Lower Granite Dam had only 20.3-cm dia- 



meter orifices to the bypass system which were of- 

 ten plugged and required continual efforts (usually 

 futile) by fish workers to remove debris to maintain 

 unobstructed flows. During peak collection periods 

 at the collection facilities, workers often required 1 

 hour, and at times up to 3 hours, to transfer fish from 

 one of the five raceways into a fish transport barge. 

 Occasionally, the 6-inch transfer lines would com- 

 pletely plug with debris and fish. The effect of debris 

 throughout the bypass and collection systems was 

 to increase fish injury, descaling, and ultimately 

 mortality from dam passage (Table 1). Total mor- 

 talities at Lower Granite Dam were often so high 

 (personal observations by the authors) that indi- 

 vidual dead fish could not be counted. Most often, 

 we kept volumetric estimates (buckets full) of dead 

 fish dipnetted from tail-screens in raceways. The fish 

 not collected at the uppermost dam passed through 

 either the spillway or through the trashracks and 

 then the turbines. Although not measured, the mor- 

 tality of fish that were not collected at the upper- 

 most dam, but which passed through the debris on 

 the trashracks and then the turbines, probably was 

 higher than that at downstream dams where debris 

 on the trashracks was much less of a problem. For 

 example, in 1979, fish sampled from the gatewells 

 at Little Goose Dam showed far less descaling (a re- 

 duction of 50%) after the trashracks at Lower Gran- 



Figure 3 



Debris in the forebay of Lower Granite Dam, 1979. 



