What goes with the flow: exploring climate impacts on low-flows, their influence on water quality, and instream flow management responses across the United States

Across the United States (US), changes to low-flows in rivers are being driven by interactions between rising temperatures, changing precipitation patterns, and freshwater demand. Increasing frequency, duration, and extremity of low-flows profoundly impact water quality, and consequently aquatic ecosystem health and water supply. However, our understanding of how water quality constituents respond to low-flows regionally is limited by disparate knowledge across disciplines and space. This study leverages literature and expert knowledge to synthesize an interdisciplinary understanding of low-flows’ impact on water quality and aquatic ecosystems, and subsequent implications for national and regional-scale instream flow management. While regional hydrologic differences occur in drivers of low-flows across the US, consistent themes exist in water quality’s response. Low-flows generally result in higher water temperatures, elevated salinity, and lower dissolved oxygen in most studied rivers, with changes in turbidity and nutrients being inconsistent. Low-flow extremes are associated with declining aquatic ecosystem health, including reduced primary production and macro-invertebrate mortality, which can be exacerbated by climate change and other anthropogenic (stream hardening) and natural (high-biota density) factors. Some US regions are underrepresented in the literature regarding links between low-flows and water quality, particularly Hawai’i and US-affiliated Islands (regions with unique water availability and socio-ecological system challenges). Instream flow management prioritizes addressing such challenges, though flow management frameworks vary in both their scale and explicit representation of water quality constituents. Our review provides context about the US’ instream flow management challenges and the importance of water quality’s consideration in managing low-flow regimes.