Quantifying Cumulative Entrainment Effects For Chinook Salmon In A Heavily Irrigated Watershed

Pacific salmon Oncorhynchus spp. experience multiple small-scale disturbances throughout their freshwater habitat, but the cumulative effect of these disturbances is often not known or not easily quantifiable. One such disturbance is water diversions, which can entrain fish and alter streamflow regimes. Threatened Lemhi River (Idaho) Chinook salmon O. tshawytscha smolts encounter 41-71 water diversions during their out-migration. We used passive integrated transponder tag data to model the entrainment rate of Chinook salmon smolts as a function of the proportion of water removed by an irrigation diversion. Under median-streamflow conditions with unscreened diversions, the estimated cumulative effect of the diversions was a loss of 71.1% of out-migrating smolts due to entrainment. This is a large potential source of mortality, but screening is an effective mitigation strategy, as estimated mortality was reduced to 1.9% when all diversions were screened. If resources are limited, targeting the diversions that remove a large amount of water and diversions in locations with high fish encounter rates is most effective. Our modeling approach could be used to quantify the entrainment effects of water diversions and set screening priorities for other watersheds.