TMDLs are to be developed for listed waters. Note that the regulation clarifies that effective nonpoint source management may exempt a waterbody from being listed. EPA considers a number of factors in this decision, such as whether the list includes the required components, the basis of listing decisions, and the process used to develop the list. Top of page. The number of waters listed by States varies greatly for several reasons, including variations in geographic size and the number and extent of surface waters.
Some States choose to list larger watersheds, others list smaller stream segments.
Differences in the availability of monitoring data and the scope and stringency of water quality standards are also factors, as are the types of sources present and the effectiveness of control actions to date. Top of page. Some have suggested that a strict policy of data discrimination may provide a disincentive to gather good data. Top of page. Historically, there have been considerable differences among States in listing decisions.