Reservoir Sedimentation and Storage Capacity in the United States: Management Needs for the 21st Century

The United States federal government invested significant resources to build dams in the mid-twentieth century to increase water storage capacity nationwide (Fig. 1); while only 5% of the dams in the United States are federally owned, they account for 61% of the total national storage capacity (U.S. Army Corps of Engineers National Inventory of Dams, http://geo.usace.army .mil/nid/). Reservoir storage capacity has been valuable for a variety of national purposes—when some of the reservoir capacity is empty, it is valuable to store flood waters, whereas when it is full of water, it is valuable for purposes including water supply, hydropower, or flow stabilization for navigation. This is an important yet subtle consideration for water management and hydraulic engineering: with the exception of turbines for generating power, the dam is not the resource, the reservoir storage capacity is the resource, and that resource is finite. Society is increasingly dependent on this storage capacity due to increased water demand, increased population growth on floodplains protected by flood control dams, or increased demand on hydropower as a critical part of the electricity grid. Simultaneously, reservoir sedimentation diminishes storage capacity. Thus, there is a persistent chronic loss of the very resource upon which many aspects of modern society depend. Any diminishing resource for which there is increasing demand merits being regularly measured, assessed, and efficiently managed relative to society’s expectations and dependence. Reservoir storage capacity is no different. In some ways, reservoir storage capacity is similar to a retirement nest egg. Like a retirement nest egg, reservoir storage capacity represents a large investment over decades that provides real longterm benefits, even as it decreases over time; reservoir storage capacity is essentially consumed by sedimentation. It would be irresponsible not to periodically monitor the value of the nest egg as it is drawn down and thereby assess whether it is sufficient to support the expected rate of spending. If the nest egg is declining faster than expected or not nearly as fast as expected, the prudent action would be to reallocate some of the assets from one type to another in recognition of the actual rate of decline. It is similarly irresponsible to not regularly measure, assess, and adaptively manage the enormous investments made over the past century in federal reservoir storage capacity. While reservoir sedimentation can be reversed via active sediment management (e.g., dredging, sediment bypassing), such actions are often extremely expensive. Even then, estimating the quantity and timing of active sediment management requires similar appreciation for rates of capacity loss. We propose here that (1) our ongoing measurements of reservoir storage capacity are insufficient to adequately quantify the rate of storage capacity loss, (2) some reservoirs are filling at rates far different from what was expected, and (3) storage space should be managed, potentially via reallocation, to better utilize the existing resource.