Identifying and mitigating dam-induced declines in river health: Three case studies from the western United States

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.