High-Water Channel. High-water channels flow only during high-water 

 periods. The hydraulic effects of removing gravel from high-water channels 

 are not as great as they are in the active channel where the disturbed area 

 is subjected to flow throughout the year. The changes that can be expected 

 are similar to those described for the active channel although they occur 

 only during the period when the site is subjected to flow. 



Effects on water quality are only evident during the high-flow period. 

 Localized widening or deepening of the high-water channel would slow the 

 water velocity and thus facilitate deposition of both bed load and suspended 

 materials. Depending on the degree of change to the channel this deposition 

 would reduce the time required to re-establish near-natural conditions in 

 the area. Also, any fines exposed during mining would be available for 

 suspension during high flows. 



Removing gravel from a high-water channel could trap fish and benthic 

 organisms in the depressions of the disturbed areas as flow recedes. Many 

 benthic organisms that are adapted to a riffle community and most fish 

 species would not be able to survive in such a habitat. 



Since high-water channels are subjected to less flow than active chan- 

 nels, they tend to be more stable and are usually bordered by established 

 terrestrial vegetation. Any disturbance to these channels causing lateral 

 instability during high flows could facilitate erosion of adjacent banks and 

 thus serve to reduce the areal extent of vegetated areas. Loss of habitat 

 would cause localized elimination of small mammals and displacement of birds 

 and larger mammals. Having water pooled in the high-water channel during 

 low-flow periods could attract shorebirds, particularly where a benthic 

 fauna has become established to serve as a potential food source. 



The most serious effect from a gravel removal operation in a high-water 

 channel is bank destruction which often occurs with this type of operation. 

 This aspect is discussed in a subsequent section on removing gravel from 

 banks . 



365 



