Thousands of large reservoirs have been constructed throughout the United States, nearly all during the twentieth century. The rate of large-reservoir construction has since declined almost to a halt as suitable construction sites already have been developed and as society’s environmental sensitivities have shifted. While reservoirs are designed to address specific water-supply and water-control needs, they also provide habitat for fish, plants, and wildlife as well as extensive recreational opportunities for people. Reservoirs often are dismissed as unnatural, ephemeral, and ecologically disruptive, but they are a product of public policy and now prevalent features in our river basins. So long as society prizes the existence of reservoirs, they cannot be ignored if we are to conserve our aquatic biota effectively.
Much like natural lakes, reservoirs have traditionally been considered as standalone systems, separate from their surrounding watersheds and tributaries. Under this paradigm, fish habitat management approaches have focused on in-lake practices such as maintaining adequate water quality and enhancing structural habitat conditions. However, some of the properties of reservoirs are different from lakes. Reservoirs are usually not independent aquatic systems inasmuch as they have strong connections to the river upstream and downstream and, in serial reservoirs, to other reservoirs in the river basin. Reservoir systems exhibit longitudinal patterns both within and among reservoirs. Fish habitat management focused on the traditional in-lake scale may forego the potential benefits associated with considering reservoirs as part of the landscape. A scale broader than just the reservoir may provide the advantage of integrating abiotic, biotic, and socioeconomic characteristics active across the landscape.
Reservoirs are impounded rivers and, as such, have distinct habitat characteristics and aging patterns. Unlike natural lakes, reservoirs tend to have large watersheds and large tributaries because they were engineered to capture as much water as possible to serve diverse water storage purposes. This origin is manifested by relatively large inputs of inorganic and organic loads, nutrients, and contaminants. Depositional filling effectively has resulted in surface area, depth and volume reductions, backwater isolation, and habitat fragmentation. Artificial water-level fluctuations and wave action degrade riparian zones that were once uplands and are now unable to withstand continued flooding, resulting in erosion and ultimately homogenization of once-diverse nearshore habitats. Well-vegetated riparian zones and wetlands that provide key ecological services to natural lakes and the original river are missing in many reservoirs. Lack of woody habitat deposition, limited access to backwaters and wetlands, and unstable water levels that preclude establishment of native vegetation characterize barren littoral habitats in reservoirs.
Many fish habitat characteristics are expressed at the reservoir scale but are the upshot of broader scale factors operating outside the reservoir. Fish in reservoirs are shaped by conditions inside and outside the reservoir, and the relative importance of these internal and external factors often differs among reservoirs. Thus, fish in reservoirs may not respond to in-lake habitat improvements that fail to consider important elements of the entire watershed system. As a result, fish habitat managers may spend a large amount of resources with little benefit to fish.
Fish habitats show varying levels of degradation as reservoirs age. The intensity of habitat degradation differs among reservoirs depending on climate, physiography, land-use patterns, and a multiplicity of local conditions. The extent of degradation was investigated recently by the Reservoir Fisheries Habitat Partnership (RFHP). In a nationwide survey of about 1,300 large reservoirs, the RFHP study identified broad causes and levels of degradation.
Though the RFHP has quantified the prevalence of degradation, what is missing is practical guidance to address specific habitat degradation. Steered by the results of the nationwide survey, the RFHP developed this document to assist field biologists and administrators advance restoration and protection of fish habitats in reservoirs. The guidance provided in this document is deliberately general and nonprescriptive. Because this guidance is mostly unspecific it may not apply directly to local conditions. Instead, it is a starting point for development of site-specific prescriptions based on aspects such as goals, local site conditions, and socioeconomic circumstances. The aim of this document is to help identify “what to do,” but local circumstances will likely dictate “how to do it.”
Fish habitat management in reservoirs is in its early stages. Much of the methodology presented in this document has not been sufficiently tested, and its application is likely to have various amounts of uncertainty. The next stage in advancing reservoir habitat management is to organize a nationwide feedback system to assemble data on application successes, failures, and alternative actions. This system could involve standardized monitoring and reporting and use of the feedback to inform and refine the effectiveness of management activities.
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