River basin managers responsible for water allocation decisions are increasingly required to evaluate tradeoffs between environmental flow protections and human water security. However, the basin-scale effects of environmental flow regulations on water users are not well understood, in part because analyses are complicated by the spatial and temporal variation in water availability, human demands, and ecosystem needs. Here, we examine alternative regional environmental flow policies and their effects on a distributed network of water users in a small (182 km2) river basin in coastal California. We use a hydrologic model to simulate water diversion operations under three policy scenarios and quantify potential impacts to bypass flows for adult migrating salmon and agricultural water storage. The results indicate that there are inherent tradeoffs between environmental flows and agricultural water security, with the most restrictive environmental policy associated with the greatest impacts to water users. Surprisingly, the moderate environmental flow policy had larger impacts to bypass flows than the unregulated management scenario, suggesting that ecological benefits of the moderate policy are small relative to the adverse effects on agricultural water users. Conflicts between environmental and human water needs were greatest in upper catchments (<2.5 km2), where flow protections caused the greatest reduction in water storage. Although natural supplies were adequate for meeting water needs in most years regardless of policy restrictions, potential for conflict between environmental flow protections and water security was evident in dry years. Therefore, strategies are particularly needed for drought-year water management to ensure adequate environmental flows while reducing human water allocations in an equitable manner. Copyright © 2013 John Wiley & Sons, Ltd.