Dams store water flowing down rivers and streams in reservoirs, providing protection from floods. Dams also serve as sources of electrical power, and they provide water for domestic and irrigation uses and flat-water recreation. By design and default, most dams in the United States also store sediment, indefinitely.
Sediment accumulation behind U.S. dams has drastically reduced the total storage capacity of reservoirs. Sedimentation is estimated to have reduced the absolute water storage capacity of U.S. reservoirs by 10%–35%. Consequently, on a per capita basis, the water storage capacity of U.S. reservoirs today is about what it was in the 1940s–1950s, despite there being more dams [Randle et al., 2019]. This comes as no surprise: More than 40 years ago, D. C. Bondurant warned, “It must be recognized, that with few exceptions, ultimate filling of reservoirs is inevitable” [Vanoni, 1975].
At the same time, reaches downstream of dams have been deprived of sediment, resulting in declines in the health of downstream habitats and organisms [Ligon et al., 1995]. After rivers and streams deposit their sediments into reservoirs, the remaining clear water is more effective at moving sediment in the channel downstream [Kondolf, 1997]. High-energy “hungry water” releases erode downstream channel beds and banks, leading to incised rivers [Williams and Wolman, 1984], accelerated beach erosion [Dai et al., 2008], and oversimplified channels lacking critical habitat features such as backwaters, connected floodplains and wetlands, pools, riffles, and runs [Kondolf and Swanson, 1993].
Managing reservoir sediments in the United States has historically involved dredging, excavation, and removal of sediment to off-site locations. These approaches are expensive and do not restore sediment continuity with downstream river channels. Alternative management approaches have revealed that mobilizing and passing sediment through reservoirs to downstream reaches can maintain or restore both reservoir capacity and downstream ecosystems.
Here we present recommendations to address the escalating issue of sediment trapping in reservoirs. Without action, continuing accumulation of sediment in reservoirs will further reduce reservoir capacities, increase maintenance costs, reduce reservoir operational flexibility, and increase degradation of downstream environments. In rare cases that foretell what a future of sediment mismanagement might look like, failure to deal with captured sediments has led to catastrophic dam failure [Tullos and Wang, 2013].
The Mechanics of Reservoir Sedimentation
Streamflows entering reservoirs are released downstream by way of intakes that are generally located well above the bed of the reservoir, either at the water’s surface on towers or across a surface spillway along the length of the dam. Because sediment transported into a reservoir is heavier than water, it settles to the reservoir bed, reducing the storage space available for water. Many dams also have low-elevation outlets that allow for sediment flushing. These outlets are most common at diversion dams but also exist at many water storage dams, where they were constructed above their respective sediment storage pools. Continued sediment accumulation to reservoirs can either cover or compromise these low-level outlets.
To prolong reservoir life and recover lost storage volume, low-level outlets or bypass tunnels may need to be constructed to direct newly inflowing or already accumulated sediment through or around a dam. Even with the high costs of modifying existing dams with such features, passing sediment through a reservoir is still less expensive over the life span of the reservoir than dredging and off-site storage [Wang et al., 2018].
Efforts to mobilize and route sediment past dams are often delayed by regulatory requirements shaped by the long-standing—though misguided—belief that sediment always negatively affects water quality and increases risks to downstream communities. To increase the sustainability of reservoirs, sediment management regulations need modernizing, informed by the knowledge gained from years of scientific research and monitoring of reservoir and downstream river conditions.
Sediment management in the United States exists within a regulatory environment that is “ever changing and…[continuing] to grow in complexity,” according to a report by the International Commission on Large Dams . Today, discharging sediment downstream of a dam requires an individual, project-specific federal permit. The 2007 court case Greenfield Mills Inc. et al. versus Robert E. Carter Jr. et al. set the precedent for this requirement, establishing that the flushing of sediments is considered a “discharge of dredged material from a point source” and subjecting the practice to regulation under Section 404 of the Clean Water Act (CWA). In addition to federal regulations, sediment management operations often require additional authorization at the state or local level.
Inconsistent interpretation of federal, state, and local permitting processes makes the application process complex and unpredictable. Factors include variations in how different U.S. Army Corps of Engineers (USACE) districts interpret existing permit frameworks and implement their regulatory programs, reflecting differences in regional conditions and in regulatory perspectives across states. The process is also hampered by a lack of consistent and adequate training and knowledge about sediment transport processes and about interactions between sediment movement, river morphology, and ecosystem response among staff at regulatory and resource agencies (e.g., EPA, U.S. Fish and Wildlife Service, NOAA), dam operators, nongovernmental organizations, and permittees.
In addition, examples of poorly designed and timed sediment discharges from reservoirs have caused legitimate concerns about negative effects on downstream ecosystems [Espa et al., 2016]. Taken together, these issues highlight why current regulatory tools are not favorable for sustainable management of reservoir sediments. However, they also reveal opportunities for modernizing those tools and ameliorating reservoir sediment management.
Achieving Sustainable Reservoirs for the Future
Broadly speaking, the following three key challenges characterize the current regulatory framework for authorizing sediment discharges from reservoirs:
- The definition of sediment as a pollutant and as fill under Section 404 of the CWA
- Traditional engineering practices that do not account for current knowledge of geomorphic and ecological processes, as well as a lack of training on and common understanding of these processes
- Regulations that are simultaneously inflexible (e.g., de minimis) and inconsistent
We identify four interrelated recommendations to address these challenges.
Recommendation 1: Broaden the Interpretation of De Minimis Sediment Release
Individual permits for sediment discharges are not needed when the amount of sediment released is below a de minimis standard. De minimis is a concept established by USACE as a sediment release that approximates the natural load of sediment entering the reservoir. However, it is difficult to flush stored sediment if the load released must be similar in magnitude, composition, and seasonal pattern to that entering a reservoir.
Releases of accumulated sediment loads could be authorized if de minimis standards were based on relevant geomorphic or ecologic criteria and had the goal of preventing degradation of aquatic resources (see recommendation 4). Such a framework would require robust and quantitative understanding of local hydrological, geological, and ecological processes. This type of framework would benefit from establishing a community of practice (COP), and implementing it would require training of permitting staff and practitioners on new processes and tools (recommendation 2).
Recommendation 2: Establish Reservoir Sediment Management Communities of Practice
USACE has previously established COPs to encourage collaboration and efficiency of applications and knowledge transfers across different technical fields (e.g., the Levee Safety COP, which engages a wide range of practitioners and regulators in collecting data, database building, and assessment of tools and policies for increasing levee safety). We recommend establishing a COP for reservoir sediment management. Key efforts of this COP could include the following:
- Developing a reference database of existing reservoir sediment management permits under Sections 401 and 404 of the CWA (Section 401 deals with permitting water discharge events with respect to their effects on water quality).
- Developing a screening tool with specific metrics (e.g., sediment types, management strategy, accumulation and expected release rates). The screening tool would help practitioners and regulators to identify high-risk sites where more careful design and monitoring are needed and to manage impacts to downstream infrastructure and ecosystems. It would also inform the permitting process.
- Convening and supporting experts to review best practices for defining de minimis criteria (recommendation 1) and for designing sediment discharge operations to minimize operational and economic burdens while maximizing ecological benefits.
- Convening permitting staff, resource agencies, and practitioners for instruction and training on the intersections of sediment and ecological processes and on evaluating, mitigating, and communicating potential risks of increasing sediment releases to downstream reaches.
Recommendation 3: Establish Regional General Permits for Regular Downstream Sediment Releases
We recommend that USACE districts issue regional general permits (RGPs) for sediment management that are specific to geographic areas. Current RGPs authorize desilting flood control channels, maintenance dredging of water bodies, beneficial reuse of dredged sediment, ecological restoration activities, and emergency activities. New RGPs that allow regular managed releases of sediment from reservoirs should be established. This update would streamline permitting for projects that cause minimal and predictable adverse environmental impacts to aquatic resources, allow for regular renewals, and motivate application of best practices for sediment management.
For example, in watersheds that are highly erodible and that naturally experience regular, event-driven sediment pulses, an RGP could allow for multiple sediment discharge events spaced out over repeated episodes. Such multiple smaller releases would generally produce smaller downstream impacts than a single, large sediment release. Ultimately, an RGP will be most effective if it is based on a framework that recognizes local characteristics of catchments, reservoirs, and downstream areas.
Recommendation 4: Adapt a Flexible and Collaborative Approach Based on Local Conditions
Because sediment management at reservoirs is uncommon in the United States, the permitting process for such projects is particularly convoluted and protracted. A collaborative approach to permitting thus would be beneficial [Ulibarri et al., 2017]. Early and frequent communication among regulators, stakeholders, and permittees could facilitate common understanding of reservoir dynamics on the timelines of geomorphic and ecological processes as well as risk identification.
A collaborative approach would streamline permitting and reduce delays in obtaining CWA Section 401 and 404 permits. It would also assist in identifying points of flexibility in the design and permitting processes that best serve the project and the environment. Recent dam removals in the United States have demonstrated regulators’ flexibility to permit activities that allow moderate short-term degradation while maintaining and protecting existing uses of a waterway (e.g., for aquatic habitat, drinking water supply, channel maintenance, recreation). For example, dam removals in the Pacific Northwest have been timed to avoid negative effects on salmon eggs and to take advantage of seasonal weather conditions.
Dam removals are very similar to operations aimed at maintaining reservoir capacity in the type and duration of sediment releases; both generate sediment pulses that peak at the end of drawdown and during subsequent storm events, for example. These pulses ultimately benefit ecosystems by reestablishing natural sediment flows and improving environmental conditions [Bellmore et al., 2019].
Acknowledgment of long-term benefits of sediment releases by regulators can boost flexibility in setting timetables for dam managers to achieve regulatory compliance and for establishing water quality criteria aligned with the principle of antidegradation of aquatic resources (i.e., weighing the pros and cons of a proposed activity that could degrade water quality). For example, the CWA Section 401 permit allowing for removal of the J. C. Boyle Dam on Oregon’s portion of the Klamath River—currently scheduled for initiation in 2023—established a compliance period (the deadline by which standards must be met) of 24 months. This provision will allow dam removal operations to avoid liability for water quality violations during sediment releases immediately following the removal.
Another accommodation that recognizes real-world conditions involves transitioning from using water quality criteria simply based on changes from background concentrations to biologically based criteria, such as suspended sediment concentrations that damage fish gills or lead to lethally low dissolved oxygen. Such criteria balance the short-term degradation of water quality that aquatic organisms and resources can tolerate before suffering permanent degradation with the long-term benefits of restored sediment continuity.
Blending knowledge of local ecosystems, weather, geomorphic factors, and trapped sediment allows dam and natural resource managers to design sediment removal programs that minimize negative impacts to the environment and downstream users.
A Shift in Strategy
Many of the tools that U.S. regulators and managers need for implementing improvements in sediment management already exist. Applying advanced knowledge gained from physical, biological, and environmental sciences will help improve the sustainability of the nation’s constructed reservoirs and its ecosystems. And implementing policy recommendations based on science and practical experience would put the United States more in line with approaches currently being used in Europe and Asia. The shift in regulation strategy proposed here would provide a means to manage sediment that is better able to maintain reservoir integrity in the future under demands associated with climate change, aging infrastructure, and public safety.
Fundamentally, achieving sustainable reservoir management requires acknowledging that sediment is not a pollutant, but is, instead, like water, often a beneficial resource that must be wisely managed. The failure to acknowledge and account for this truth has led to devastating consequences for people and ecosystems in the past, and such consequences will occur more often and become more severe in the future unless we change our approach.