This report presents a literature review of natural flow and a description of the potential effects of flow alteration on aquatic life, as well as examples of water quality criteria that some states have developed to support natural flow and maintain healthy aquatic life. The report also describes a flexible technical and scientific framework that state water managers can consider if they are interested in developing narrative or numeric targets for flow that are protective of aquatic life.
This scientific and technical report is non-regulatory and does not affect or constrain state or tribal discretion.
Hydrologic alteration can include an increase or decrease in water volume, seasonal flow disruption, and dramatic variation in water temperature. Hydrologic alteration can affect aquatic species’ ability to spawn, gather nutrients from the stream system, access high-quality habitat, and more. Hydrologic alteration may be further exacerbated through climate change. Recent climate trends have included the change in frequency and duration of extreme weather events, such as droughts and floods, which can have an impact on flow and affect aquatic life. Maintaining flow targets may help increase a stream’s resilience to climate change by reducing or avoiding intensification of existing stressors.