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

Authors: RANDLE, TIMOTHY - Bureau Of Reclamation; MORRIS, GREGORY - Ea Engineering; TULLOS, DESIREE - Oregon State University; WEIRICH, FRANK - University Of Iowa; KONDOLF, G - University Of California; Moriasi, Daniel; ANNANDALE, GEORGE - George W Annandale, Inc; FRIPP, JON - Natural Resources Conservation Service (NRCS, USDA); MINEAR, TOBY - Cooperative State Research, Education, And Extension Service (CSREES, USDA); WEGNER, DAVID - Woolpert

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/9/2021
Publication Date: 7/14/2021
Citation: Randle, T.J., Morris, G.L., Tullos, D.D., Weirich, F.H., Kondolf, G.M., Moriasi, D.N., Annandale, G.W., Fripp, J., Minear, T., Wegner, D.L. 2021. Sustaining United States reservoir storage capacity: Need for a new paradigm. Journal of Hydrology. 602:126686. https://doi.org/10.1016/j.jhydrol.2021.126686.
DOI: https://doi.org/10.1016/j.jhydrol.2021.126686

Interpretive Summary: The USA’s 90,000+ large dams and their reservoirs represent critical infrastructure required to support economic activity and social well-being. They provide reliable water supplies for municipal, agricultural, and industrial use as well as hydropower, flood risk reduction, navigation, and recreational benefits. However, their capacity is steadily being diminished by sedimentation. Sediment trapping also deprives downstream river channels and coastlines of their natural sediment loads, with resultant environmental impacts. The current practice of allowing reservoirs to continually fill with sediment is not sustainable. This paper discusses this sustainability issue and outlines the path towards a sustainable use paradigm for reservoirs. A sustainable-use paradigm for reservoir management to preserve long-term capacity represents a fundamental shift from the traditional design life approach under which reservoirs simply continue to fill with sediment until decommissioning. Achieving sustainable utilization of the nation’s water resources under this new paradigm, will generally require better monitoring data, changes in reservoir operations, structural modifications to dams, and modifications to the environmental regulatory framework. Three key actions are needed under a sustainable use paradigm. These include: 1) a need for a national scale survey program of all water storage reservoirs to screen and identify priority reservoirs for targeting sediment management, 2) development of long-term sediment management plans at critical sites including sediment yield reduction, bypass, or removal, and 3) modernizing environmental permitting processes, for example by considering restoration of fluvial sediment continuity along rivers an environmental benefit. The ultimate goal is to adapt sustainable use as an engineering design and operational criteria for storage reservoirs, and to gradually convert the nation’s inventory of critical reservoirs into sustainably managed infrastructure.

Technical 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 fulfil 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 1980’s to 810 Gm3 today. Yet, sustaining the nation’s long-term reservoir storage capacity has not been a priority for public or private dam owners. 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.