For rivers in the lower 48 states, habitat condition was estimated by analyzing how strongly a range of human disturbances to habitat affects river fish in all parts of the country, using the logic that fish reflect the quality of the habitat where they live. For each disturbance type, we identified the disturbance level at which fish with a strong reliance on high quality habitats showed marked declines in abundance, and where these fish disappeared from the ecosystem altogether. This information was used to score streams according to their most likely condition given the values of disturbances in each location.
In the maps, streams that are expected to be in good condition have a low or very low risk of current habitat degradation, and streams in poor condition have a high risk of current habitat degradation. The national datasets used for this assessment included information about the amount of urban, agriculture, and pasture lands in watersheds, major point-sources of water pollution, frequency of dams and road crossings, and the locations of mines. Some important threats to fish and fish habitat could not be incorporated into the analysis due to data limitations. These include historical land use pressures, ground and surface water extraction, animal feed lots, forestry practices, and regional habitat stresses (e.g., oil drilling), all of which will be addressed in future revisions of this assessment. Disturbance scores in streams affected by unmeasured factors may underestimate the true amount of disturbance. The following disturbance variables were analyzed as part of the river assessments:
It is important to recognize that these broadly defined disturbance variables may act together with other measured or unmeasured threats to degrade habitat. Thus, while we may identify “urbanization” as a major threat to some regions, “urbanization” represents an umbrella term that describes the many facets of urban development that cause degradation to habitats, such as pavement, nutrient runoff from lawns, road salt, trash and detergents getting into the river, etc. Rarely does only one disturbance type act alone.
Data on human disturbances, fish populations, and habitat condition were limited in Alaska and Hawaii, so a simplified variation of the basic methodology used for rivers was employed for these two states. Disturbance variables were assigned to categories (e.g., land cover, point source pollution, infrastructure, barriers to fish movement, and industrial activity), and then a single score was calculated using a statistical approach called Principal Components Analysis. Because this methodology differs from the methodology used for the lower 48 states, the results cannot be directly compared—i.e., an area at high risk of current habitat degradation in the 48 conterminous states is not equivalent to an area at high risk of current habitat degradation in Alaska or Hawaii.