The natural flow regime of a water body, defined as its characteristic pattern of flow magnitude, timing, duration, frequency, and rate of change, plays a critical role in supporting the chemical, physical, and biological integrity of streams and rivers and the services they provide.

Human-induced alteration of the natural flow regime can degrade a stream’s physical and chemical properties, leading to loss of aquatic life and reduced aquatic biodiversity. Protecting aquatic life from the effects of flow alteration involves maintaining multiple components of the flow regime within their typical range of variation.

This report was developed (1) to serve as a source of information for states, Tribes, and territories on the natural flow regime and potential effects of flow alteration on aquatic life and (2) to provide a flexible, nonprescriptive framework that can be used to quantify targets for flow regime components that are protective of aquatic life. As a supplementary resource, Appendix A was added to provide examples where states and Tribes have applied Clean Water Act (CWA) tools to protect aquatic life from altered flow.

Anthropogenic landscape change and water management activities are modifying flood flows, base flows, peak-flow timing, and other flow characteristics in streams and rivers throughout the United States. Under natural conditions, a stream’s flow regime is determined by hydrologic properties at two scales, the upstream drainage area (catchment) and the local, reach scale. At the catchment scale, climate determines patterns of water and energy input over time, whereas physical characteristics like soils, geology, and topography determine pathways, rates of runoff, and routing through the stream network. Reach-scale factors such as local groundwater dynamics further influence natural flow regime characteristics.

Human activities that alter the natural flow regime also occur at both the catchment and reach scales and include impoundments, channelization, diversions, groundwater pumping, wastewater discharges, urban development, thermoelectric power generation, and agricultural practices. Many of these activities alter hydrologic processes like infiltration, groundwater recharge, channel storage, or routing and lead to flow conditions outside the natural range of variation. Others directly add or remove water from a stream such that flows are uncommonly high or low over long periods of time.

Occurring in conjunction with these activities is climate change. Climate trends observed in recent decades and future projections (for example, rising ambient air temperatures, increasing frequency of heavy precipitation events, reductions in the thickness of snow pack and ice) may magnify the effects of other anthropogenic processes on the natural flow regime.

Alteration of the natural flow regime can have cascading effects on the physical, chemical, and biological properties of riverine ecosystems. Effects on physical properties include altered channel geomorphology (channel incision, widening, bed armoring, etc.), reduced (or augmented) riparian and flood-plain connectivity, and reduced (or augmented) longitudinal (upstream-downstream) and vertical (surface water/groundwater) connectivity.

Effects on water quality can also result from altered flow magnitudes. For example, salinity, sedimentation, and water temperature can increase when flow volumes are reduced, whereas erosion and sediment transport can increase with amplified flow volumes. These changes to a stream can in turn lead to the degradation of aquatic life as a result of the loss and disconnection of high-quality habitat. Furthermore, altered flows can fail to provide the cues needed for aquatic species to complete their life cycles and can encourage the invasion and establishment of non-native aquatic species. The ability of a water body to support aquatic life is tied to the maintenance of key flow-regime components.

Efforts to implement strategies to protect aquatic life from flow alteration will be most effective if numeric targets are identified for flow-regime components that equate to intact and healthy aquatic communities. This report presents a flexible framework that can be used to quantify flow targets that incorporate U.S. Environmental Protection Agency Guidelines for Ecological Risk Assessment (ERA) and concepts from contemporary environmental flow literature. The framework consists of eight steps that begin with identifying biological goals and assessment endpoints and end with an evaluation of effects on aquatic life under varying degrees of flow alteration.

The framework does not prescribe any particular analytical approach (for example, statistical or mechanistic modeling methodology), but rather focuses on the process and information needed to evaluate relations between flow and aquatic life and the development of narrative or numeric flow targets.