Though dams often provide flood control, hydroelectric power, a reservoir of surface water for irrigation and domestic needs, as well as a recreational lake, there is increasing sentiment to remove dams to reverse their impacts on the watershed and especially their impact on downstream reaches of the watershed. Common negative downstream impacts of dams include degradation of the stream channel and riparian zone, and risks associated with dam failure. Removal of dams is seen as a means of resurrecting natural downstream flows and eventually natural sediment transport, resulting in improved downstream riparian corridors, fishery habitats, sports fishing, recreational rafting, and reappearance of a more pastoral setting. Scientific proponents of dam removal, including fishery biologists, surface-water hydrologists, geomorphologists, as well as interested individuals ranging from recreational enthusiasts to renowned poets, rightfully point to controlled streamflows as a clear cause of degradation. The dam problem is viewed as a surface phenomenon, involving surface-water hydraulics, sediment transport, fishery, benthic, and riparian ecology, as well as a plethora of aesthetic issues. This vantage point possesses merit, but lacks the bottom half of the picture. Surface water and ground water are a single resource, requiring management as a continuum to ensure a wish has a hope of being realized. This commentary discusses the role of ground water on flows downstream of dam sites, before, during, and after the dam.
The effect of ground water on downstream flows is discussed here for semi-arid and arid environments, where dams have created large impacts on the downstream civilization. Figure 1 provides an idealized sequence of four scenarios depicting a hydrogeologic crosssection spanning hundreds of kilometres from mountainous terrain draining toward a ground-water basin, with a major stream system flowing out of the mountains. The upper panel in the figure depicts a natural setting, such that the water table often approaches ground surface as the stream discharges from the mountainous bedrock. The hydrograph inset in the panel gives an example of the natural streamflow over time for a stream in a climatic setting, with a distinct wet season and a dry season (the most common climatic setting in which large dams are found). Note that streamflows within the two reaches indicated by the arrows have similar flow characteristics for the natural case, i.e. high peak flows due to surface runoff and prolonged base flows due to ground-water discharge.
The second panel depicts the general changes in the hydrologic system after the erection of a dam on the flank of the mountainous This article is a US Government work and