Focused aquifer regeneration produced outsized gains for a threatened fish in a populated, dryland watershed

Abstract A combination of groundwater pumping and shifting precipitation patterns is drawing down aquifer levels in many of the world's drylands and is predicted to generate widespread loss of perennial streamflows over the next few decades. This process broadly threatens freshwater biota, especially in arid and semiarid regions. Efforts to recharge aquifers and raise water tables could ease this threat, but usually focus on human needs. Here we argue that spatial patterns of groundwater recharge and pumping can be organized to improve river baseflows while sustaining human needs and examine a river where this strategy has been used for 30 years to restore perennial aquatic habitat and recover a valued native fish, the steelhead trout ( Oncorhynchus mykiss ). The way the strategy was implemented allows us to regard it as an experiment with positive and negative controls. Analysis of subsequent monitoring data shows that the strategy restored surface flows to a large portion of the river and that the steelhead trout successfully exploited this renewed habitat. Spawning activity in the experimental domain was 2.5× higher than in the negative control and matched the positive control. Average recruitment of various life stages of the fish was close to habitat capacity, despite considerable year‐to‐year variation. Recruitment by life stage was 3.2× to 8.5× higher in the experimental domain than in the negative control and reached 77%–91% of recruitment observed in the positive control. Recruitment of adult migratory steelhead, a primary goal of the pivot in aquifer management, nearly doubled relative to the historical scenario of complete aquifer overdraft. In this system, spatially focused aquifer regeneration produced outsized gains for a valued species and its aquatic ecosystem relative to constraints on human water use.