US Army Corps of Engineers
Hydrologic Engineering Center

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Sustainable Rivers

The Environment

Healthy and sustainable ecosystems provide a wide array of services to human communities, including water quantity and quality, flood and storm protection, and provisions for the food and fiber that we rely on as part of our everyday lives. Over time, human influences have degraded ecosystems to the point where it is difficult to appreciate how productive they can be. Freshwater ecosystems, including rivers, floodplains and estuaries, are among the most altered in the United States today.

River ecology responds to the duration and amplitude of river flows, connectivity to the floodplain and water quality. River biota have adjusted over millennium to these seasonal and annual river patterns with key life cycle events prompted by these changes. Seasonal variations in flow and water levels cue fish and mussels to spawn, trigger invertebrates (i.e. dragonfly and mayfly) to emerge, spur annual and perennial wetland vegetation to grow and cause seed dispersal and seedling recruitment from floodplain trees. Storing water in reservoirs alters these seasonal changes and affects aquatic species.

Most dams in the US have been in place for decades. Environmental impacts of dams were not well understood until years after construction. Scientists and water managers continue to refine understanding of flow needs to sustain healthy ecosystems. With increased knowledge of impacts on river ecology, reservoir operations are increasingly recognized as an asset to reverse and minimize the effects on river habitat and biota. Across the country people are designing flows management strategies to balance the human benefits from the dams with ecological needs of the system.

Environmental flows were defined as the quantity, timing, and quality of water flows required to sustain ecosystems by scientists at the 2007 Brisbane Convention on Environmental Flows. In 2018, as part of the Brisbane Declaration and Global Action Agenda on Environmental Flows, scientists noted that “Aquatic species are declining more rapidly than terrestrial and marine species…[aquatic] ecosystems have evolved with, and depend upon, naturally variable flows of high-quality fresh water.” For the environment, establishing environmental flows in managed rivers is critical to restoring degraded ecosystems and improving resilience of increasingly threatened aquatic communities.

The SRP also recognizes a complementary definition from a resource management perspective: environmental flows are management decisions that manipulate water and land-water interactions to achieve ecological or environmental goals.

The SRP process for environmental flows has three phases: “advance, implement, and incorporate”. Advancing e-flows involves engaging stakeholders in a science-based process to define the flow needs of riverine ecosystems. Implementation involves testing the effectiveness and feasibility of the defined flows. Incorporation involves including environmental flow strategies in reservoir operations policy such as water control manuals. Environmental flows were the founding objective of SRP and remain the key focus. In recent years, the Program began exploring other reservoir-oriented actions with potential to produce environmental benefits.