For Clean Water Act Status reports, a county or state is considered to "contain" a portion of a watershed if at least 5% of the watershed's land area is located within the administrative unit's border. A watershed is considered to contain a portion of a county if at least 5% of the county's land area is located within the watershed's border. County and state water quality data are calculated by summing or averaging over all watersheds the county or state contains.

Note: Scorecard's Watershed reports generally identify more watersheds as contained within a county or state than Scorecard's Clean Water Act reports. This is because Scorecard uses a stricter definition of "contain" when it is analyzing Clean Water Act data for Water Quality reports. If 5% or less of a watershed's total area is not within a county or state, that watershed does contribute data for aggregation into Clean Water Act reports (although it is identified as a watershed within a county or state by a Scorecard Watershed Indicator Report). Scorecard uses this stricter definition in Clean Water Act reports because it prevents watersheds that represent only a small fraction of a county from distorting county summary values for percent and number of impaired waterbodies.

The percentage of surface waters with impaired or threatened uses is the approximate percentage of surface waters in an area with impaired or threatened uses due to a loss in water quality. Some waterbody types (such as estuaries, wetlands and lakes) are typically measured in square miles or acres. For the sake of comparison, Scorecard measures all these waterbodies in linear miles. Impaired and threatened water mileage for lakes and estuaries was calculated using the distance for a line running through the middle of the waterbody. The exceptions to this method include the Great Lakes, Long Island Sound, Delaware Bay, and Chesapeake Bay. Coastline mileage was used for these larger waterbodies.

An impaired or threatened waterbody is any waterbody that is listed according to section 303(d) of the Clean Water Act. A waterbody is considered impaired if it does not attain water quality standards. Standards may be violated due to an individual pollutant, multiple pollutants, thermal pollution, or an unknown cause of impairment. A waterbody is considered threatened if it currently attains water quality standards but is predicted to violate standards by the time the next 303(d) list is submitted to EPA. The 303(d) list is a comprehensive public accounting of all impaired or threatened waterbodies, regardless of the cause or source of the impairment or threat.

Designated beneficial uses are the desirable uses that water quality should support. Beneficial uses include drinking water supply, primary contact recreation (such as swimming), and aquatic life support. Each designated use has a unique set of water quality requirements or criteria that must be met for the use to be supported. A waterbody may have multiple beneficial uses. A waterbody is considered impaired when it does not attain the water quality standards needed to support its designated uses.

Priority for regulation indicates how EPA has prioritized a waterbody for regulatory controls under the Clean Water Act. To bring waterbodies into compliance with water quality standards, EPA calculates the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards. This Total Maximum Daily Load (TMDL) calculation provides the basis for permitting decisions under the CWA. A TMDL specifies the reductions needed to meet water quality standards and allocates those reductions among the pollution sources in the watershed. The objective of the TMDL process is to systematically identify impaired or threatened waterbodies and the pollutant(s) causing the impairment and ultimately establish a scientifically-based strategy for correcting the impairment or eliminating the threat and restoring the waterbody. Scorecard reports six categories of priorities; targeted (a TMDL is expected to be developed within two years), high, medium, low, not assessed (the state agency responsible for TMDL development has not determined a waterbody's TMDL development priority), and not reported (no information was submitted to EPA regarding a waterbody's TMDL development priority).

Scorecard reports four waterbody type classifications: Rivers, Streams, and Creeks; Lakes, Reservoirs, and Ponds; and Estuaries, Bays, and Coasts; Fresh Water Wetlands. There are also additional waterbodies listed without the data needed to determine their appropriate category. These are identified as Body Type Not Designated.