Source EPA. The 303(d) Listed Impaired Waters program system provides impaired water data and impaired water features reflecting river segments, lakes, and estuaries designated under Section 303(d) of the Clean Water Act. Each State will establish Total Maximum Daily Loads (TMDLs) for these waters. Note the CWA Section 303(d) list of impaired waters does not represent waters that are impaired but have an EPA-approved TMDL established, impaired waters for which other pollution control mechanisms are in place and expected to attain water quality standards, or waters impaired as a result of pollution and is not caused by a pollutant. Therefore, the "Impaired Waters" layers do not represent all impaired waters reported in a state's Integrated Report, but only the waters comprised of a state's approved 303(d) list. For more information regarding impaired waters refer to EPA's Integrated Reporting Guidance at: http://water.epa.gov/lawsregs/lawsguidance/cwa/tmdl/guidance.cfm. 303(d) waterbodies are coded onto NHDPlus v2.1 flowline and waterbody features to create line, area, and point events. In addition to NHDPlus reach indexed data there may also be custom event data (point, line, or polygon) that are not associated with NHDPlus and are in an EPA standard format that is compatible with EPA's Reach Address Database. These custom features are used to represent locations of 303(d) waterbodies that are not represented well in NHDPlus.