The level of potential risk to human health and the environment from sediment chemical analysis, sediment toxicity data, and fish tissue residue data. For more information about agricultural pollution potential and what farmers are doing aabout it, see the Natural Resources Conservation Service's State of the Land . The process of impounding streams changes their characteristics and the reservoirs and lakes formed in the process can be more susceptible to pollution stress. This measures an estuary's susceptibility to pollution based on its physical characteristics and the propensity to concentrate pollutants. The Index of Watershed Indicators makes use of data collected at specific sites across the country.
More data are available in some watersheds than others.
In order to ensure that sufficient data exist to make a valid judgment of aquatic resource health, U.S. EPA set threshold levels for the number of indicators needed to categorize watersheds. For the condition assessment, watersheds must have information for at least four of the seven indicators. For the vulnerability assessment, at least six of the eight indicators are required. These appear gray on the national characterization map. It is important to note that this categorization does not necessarily mean that there are no data available for the watershed.