DISCONTINUITIES IN WATER AND SEDIMENT FLUX OVER YEARLY TO DECADAL TIME SCALES CAUSED BY MULTIPLE STREAM FLOW DIVERSIONS

Stream flow diversions extract water from streams and may disrupt sediment flux by altering transport capacity and by extracting a portion of the sediment load. While impacts immediately above and below diversions have been described, the cumulative impact of many diversions in a channel network over many decades remain poorly understood. Network-scale impacts may include sediment deficit and/or surplus and associated channel adjustment. We have conducted a 2-yr program of stream flow, sediment transport, and channel morphology measurements in a southeastern Idaho watershed where 3 large diversions have been in operation for more than 100 yrs. The cumulative effects of variations in diversion operating style coupled with natural variation in the temporal and spatial distribution of snowmelt runoff are such that sediment transport rates are discontinuous in the watershed. In 2006, a high runoff year, high flows and transport rates in the lower part of the watershed preceded runoff in the upper watershed. During peak snowmelt, flow and sediment transport rates were greatest in the upper watershed and decreased downstream as flow was extracted by diversions. Thus, the flux of sediment from the upper to the lower watershed was not continuous. These findings suggest that sediment surplus conditions where the channel is affected by one or two diversions and tributaries provide sediment inputs. Deficit conditions might exist further downstream where the upstream supply is limited. The effects of diversions on sediment flux also depend on runoff magnitude. Water flux is affected most dramatically during snowmelt runoff in low-runoff years and during the irrigation season, when 90% or more of the water may be extracted from the stream. Sediment flux, however, is most affected by the operation of diversions during in high-runoff years when small percentages of water extraction can result in substantially reduced daily and annual sediment loads. These effects are most pronounced in the downstream parts of the drainage network where the cumulative volume of water extraction by multiple diversions is greatest. Preliminary analysis of channel morphologic data indicates subtle sediment accumulation in segments that have a sediment surplus and channel incision in the downstream segment where deficit conditions may exist.