Stream Channel Responses to Streamflow Diversion on Small Streams of the Snake River Drainage, Idaho

The effects on channels of small, low-head seasonal water diversions in the Snake River drainage were investigated. Channels below small diversions were compared to the channels immediately above the same diversions to determine if differences in flow conveyance, substrate sediment size distribution, or streamside vegetation density were present. Estimates of flow conveyance were greater above the diversions, as measured by the area between the edges of vegetation on both banks, and by frequent-flow indicators, which generally approximated bankfull stage. No significant difference in substrate particle size or in channel roughness was found between channels above and below diversions. Although use histories of the diversions were not available, limited observations and conversations with users suggest that many of the diversion structures did not substantially divert high springtime flows so that passage of channel-forming flows probably occurred. Some diversion structures apparently divert or trap a portion of the bedload. Stem diameters of vegetation 6 to 48 inches above the ground were significantly larger above the diversions. Tests of stem densities were not significant. Stratification by substrate, season and size of diversion, community type, and source of summer water may be necessary to properly evaluate the effects of small diversions on vegetation stem density and vigor. The elevation of the edge of vegetation appears to be a viable alternative to frequent flow or bankfull indicators for estimating flow conveyance. It also addresses the question of vegetative encroachment into the channel. Use of the edge of vegetation as a channel feature for flow estimates merits further testing. It appears that the operation of the small forest stream diversions studied has not substantially altered most of the parameters studied. Past hydrographs and historical hydrologic data, however, are needed to fully evaluate the channel and vegetation responses. Abstract —The effects on channels of small, low-head seasonal water diversions in the Snake River drainage were investigated. Channels below small diversions were compared to the channels immediately above the same diversions to determine if differences in flow conveyance, substrate sediment size distribution, or streamside vegetation density were present. Estimates of flow conveyance were greater above the diversions, as measured by the area between the edges of vegetation on both banks, and by frequent-flow indicators, which generally approximated bankfull stage. No significant difference in substrate particle size or in channel roughness was found between channels above and below diversions. Although use histories of the diversions were not available, limited observations and conversations with users suggest that many of the diversion structures did not substantially divert high springtime flows so that passage of channel-forming flows probably occurred. Some diversion structures apparently divert or trap a portion of the bedload. Stem diameters of vegetation 6 to 48 inches above the ground were significantly larger above the diversions. Tests of stem densities were not significant. Stratification by substrate, season and size of diversion, community type, and source of summer water may be necessary to properly evaluate the effects of small diversions on vegetation stem density and vigor. The elevation of the edge of vegetation appears to be a viable alternative to frequent flow or bankfull indicators for estimating flow conveyance. It also addresses the question of vegetative encroachment into the channel. Use of the edge of vegetation as a channel feature for flow estimates merits further testing. It appears that the operation of the small forest stream diversions studied has not substantially altered most of the parameters studied. Past hydrographs and historical hydrologic data, however, are needed to fully evaluate the channel and vegetation responses.