Instream water rights are (or should be) based on ecological flows. Several western states (including Oregon, Alaska, California, Colorado, and Montana) have statutory requirements for these flows, and the US Geological Survey’s Fort Collins Science Center hosts an ecological flows research center. Incorporating effects of changes in climate, human population size and distribution patterns, and other variables makes it difficult for policy makers to allocate a scarce and varying resource among competing vested interests. Each user interest has legitimate economic and societal justification for their claims. Policy makers and lawyers are challenged to optimally adjudicate competing claims. There is a valuable role for environmental science and data analyses to inform these decisions.