Total Maximum Daily Load

From Salish Sea Wiki

TMDL is a term defined under the Clean Water Act which describes a planning and implementation process for capping and reducing water quality indicators in streams. A TMDL is mandated after 303(d) listing of a stream for a violation of water quality standards. Washington State Department of Ecology has limited resources to conduct a TMDL, which requires an evidence-based evaluation of the factors causing the water quality standard violation.


  • Browse TMDLs by WRIAs -
  • Because of the rigor of a TMDL, it becomes a body of "best available science" which can then be used to inform Growth Management Act and Shoreline Managment Act planning, which requires the use of best available science.
  • TMDL analysis provides a basis for action, but there is not consequence for not following up on a TMDL analysis. The NPDES permit process is where a local jurisdiction can face penalties for failing to work toward correcting a violation.
  • I have not found any evidence for how many TMDL/NPDES processes actually lead to de-listing of a water body. It seems like concerted action for solving water quality issues often require a mixture of different forces. (See Shellfish Protection District or Drayton Harbor for examples driven by shellfish production impacts).