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Abstract: River health can be defined as the degree to which riverine energy source, water quality, flow regime, habitat and biota match the natural conditions. In a healthy river, physical process and form remain actively connected and able to mutually adjust, and biological communities have natural levels of diversity and are resilient to environmental stress. Both physical diversity and biodiversity influence river health. Physical diversity is governed by hydrology, hydraulics, and substrate, as reflected in the geometry of the river channel and adjacent floodplain, which create habitat for aquatic and riparian organisms. Biodiversity is governed by biological processes such as competition and predation, but biodiversity also reflects the diversity, abundance and stability of habitat, as well as connectivity. Connectivity within a river corridor includes longitudinal, lateral, and vertical dimensions. River health declines as any of these interacting components is compromised by human activities. The cumulative effect of dams and other human alterations of rivers has been primarily to directly reduce physical diversity and connectivity, which indirectly reduces biodiversity. Restoration and maintenance of physical diversity and biodiversity on rivers affected by dams requires quantifying relations between the driver variables of flow and sediment supply, and the response variables of habitat, connectivity, and biological communities. These relations can take the form of thresholds (e.g., entrainment of streambed sediment) or response curves (e.g., fish biomass versus extent and duration of floodplain inundation). I use examples from Wyoming, Colorado, and Arizona in the western United States to illustrate how to quantify relations between driver and response variables on rivers affected by dams. [Copyright &y& Elsevier]

This paper was motivated by the 25th anniversary of the publication of Marc Reisner's book, Cadillac Desert: The American West and its Disappearing Water. Dams are ubiquitous on rivers in the United States, and large dams and storage reservoirs are the hallmark of western U.S. riverscapes. The effects of dams on downstream river ecosystems have attracted much attention and are encapsulated in the serial discontinuity concept (SDC). In the SDC, dams create abrupt shifts in continua of downstream changes in physical and biotic properties. In this paper, we develop a framework for understanding how channel geometry and network structure influence how the physical components of habitat and the biota rebound from discontinuities set up by large dams. We apply this framework to data describing the flow regime, temperature, sediment flux, and fish community composition below Garrison Dam on the Missouri River, Glen Canyon Dam on the Colorado River, and Flaming Gorge Dam on the Green River. Sediment flux in dam tailwaters is under strong control by channel geometry. By contrast, dam-related changes in temperature and flow variation are not significantly modulated by channel geometry or tributary inputs if flow volumes are small (Missouri and Colorado River tributaries). Instead, small tributaries provide near-native conditions (flow and temperature variation) and, as such, provide key refuges for biota from novel habitats in mainstem rivers below large dams. Unregulated tributaries that are large relative to their respective mainstem (e.g., Yampa River) provide refuges as well as significant amelioration of flow and temperature effects from upstream dams. Finally, the proportion of native fish increases with distance from dam and exhibits sharp increases near tributary junctions. These results suggest that tributaries-even minor ones in terms of relative discharge-act as key refugia for native species in regulated river networks. Moreover, large, unregulated tributaries are key to restoring continuity in physical habitat and the biota in large regulated rivers. [ABSTRACT FROM AUTHOR]