Sustaining United States reservoir storage capacity: Need for a new paradigm

Abstract Although the hydrologic cycle is a continuously renewable resource, the natural rate of water delivery is highly variable. Water is made available to our society on a consistent and reliable basis largely due to flow regulation by storage reservoirs. However, under current management, the reservoir storage capacity needed for flow regulation is a non-renewable resource because this capacity is steadily being lost to sedimentation. Today’s reservoirs occupy unique sites and may be considered largely irreplaceable, making the nation dependent on a non-sustainable resource. Sedimentation is steadily depleting storage capacity and progressively degrading the ability of reservoirs to fulfill their designated purposes. Sedimentation is also causing environmental impacts upstream and downstream of reservoirs. In the United States, the combined impacts of sedimentation and population growth have resulted in an estimated 35% decline in storage capacity per capita since this value peaked around 1970. In absolute terms, the estimated total reservoir storage capacity in the U.S. has dropped from a peak of 850 Gm3 in the late 1980s to 810 Gm3 today. Yet, sustaining the nation’s long-term reservoir storage capacity has not been a priority for many public or private dam owners, especially when they lack a reservoir sedimentation monitoring policy. In many regions, future reservoir storage will have to serve an important role in the mitigation of climate change to help ensure water, food, and energy, and the reduction of flood risk. There is an imperative need to preserve existing reservoir storage capacity due to rising demands associated with population growth, and increasing hydrologic variability associated with climate change, and the challenges and costs associated with either expanding existing capacity or decommissioning and developing new storage capacity. The trapping of sediment behind dams has also contributed to the decline of freshwater and coastal environments downstream of dams. Reversing these dangerous trends in storage capacity and environmental integrity will require increased monitoring of reservoirs, application of both established and emerging sediment management technologies, and a new paradigm for sustainable reservoir design and management. It requires moving from the traditional design life (reservoir life expectancy) approach to the adoption of sustainable use as the appropriate criteria for reservoir design and operation, achieving a sediment balance across reservoirs to permit the indefinite operation of this critical infrastructure.