Tennessee’s own explanation says:
What Is the 303(d) List and Why Is It Important?
Section 303(d) of the federal Clean Water Act requires that states develop a compilation of the streams and lakes that are “water quality limited” or are expected to exceed water quality standards in the next two years and need additional pollution controls (i.e. a Total Maximum Daily Load). Water quality limited streams are those that have one or more properties that violate water quality standards. They are considered impaired by pollution and not fully meeting designated uses. …
If a stream is impaired, regardless of whether or not it appears on the 303(d) List, the Division [of Water Resources of TDEC] cannot authorize additional loadings of the same pollutant(s). It may mean that dischargers will not be allowed to expand or locate on 303(d) listed streams until the sources of pollution have been controlled.
When a river is on a 303(d) list, the State is required to conduct a total maximum daily load (or “TMDL” for short), which is supposed be a comprehensive, thorough and objective study of how much pollution the river can handle, how to apportion who gets the ability to discharge how much into the river, and most importantly, how the river is to be restored so it can removed from the 303(d) list. Thus, a TMDL is a pollution reduction study & plan, since its ultimate objective is to restore the river and get it off the 303(d) list. For more information on TMDLs, click here.
The Harpeth River has been on Tennessee’s 303(d) as impaired by phosphorus pollution since 2004 – for fifteen years. Before that, since 1996, it was considered impaired by “nutrient” pollution. Unfortunately, very little progress has been made toward cleaning up and restoring the river. A new TMDL was announced in 2015, but very little progress has been made under it. Critical first steps, such as preparing work and sampling plans, remain undone even though the new TMDL was announced four (4) years ago. For more information click here.