Frequent Questions about Impaired Waters and TMDLs

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What is the TMDL Program?

The TMDL (Total Maximum Daily Load) Program, also called the 303(d) Program, is a part of the Federal Clean Water Act (CWA) that specifically concerns identifying and tracking surface waters impaired by pollutants and developing technical plans (TMDLs) for restoring them. Generally speaking, the TMDL Program is implemented by states, territories and tribes with EPA oversight. The two major parts of the program are the CWA Section 303(d) listing of impaired waters in each state and the development of TMDLs that concern how to restore those pollutant-impaired waters. A TMDL is a calculation of the maximum amount of a pollutant that a waterbody can receive and still safely meet water quality standards. Generally, a TMDL is in the form of a technical plan for reducing pollutants and allocating those necessary reductions among the different categories of all the contributing sources.

More details about the TMDL Program are available at EPA’s:

What is meant by TMDL Program results analysis?

TMDL Program results analysis involves assessing environmental outcomes, their probable causes and program progress in order to generate the insights needed to improve the TMDL Program. EPA’s national headquarters TMDL Program recognizes program results analysis and tracking as one of the main themes of its annual work plan.

What is meant by "outputs" and "outcomes"?

Program results are generally described as of two main types – outputs and outcomes – that are important indicators of the program’s effectiveness. Outputs are programmatic milestones or steps accomplished as a program progresses through a multi-step process essential to reaching its goals. Outcomes are environmental changes or improvements consistent with those program goals. An example of a TMDL Program output is a finalized and approved 303(d) list of impaired waters. An example of a TMDL Program outcome is the incremental improvement or full restoration of a formerly 303(d)-listed water that has had a TMDL developed and related pollution control and restoration actions put in place.

Why are TMDL Program results of interest?

The lakes, rivers, streams, wetlands and other waters of the US provide our country with irreplaceable goods and services worth tens of billions yearly. Threats to those goods and services can threaten human health and welfare, our economy and the sustainability of our environment. Further, restoring and protecting the nation’s waters requires public funds that must be used wisely and efficiently. The effectiveness and results from any program designed to protect the nation’s waters is of particular interest to EPA, to US citizens and to US decision-makers.

Who bears the responsibility for tracking TMDL Program results?

EPA is voluntarily tracking TMDL Program results in the interest of the overall Clean Water Act goal "to restore and maintain the chemical, physical and biological integrity of the nation’s waters," and to demonstrate accountability for the EPA resources dedicated to achieving that goal. Under the Government Performance and Results Act (GPRA), EPA is required to track and report at a more generalized level on the collective results from the several interrelated surface waters protection programs of the Clean Water Act, including but not specific to TMDL Program results.

EPA Water Program performance measures and tracking information is available at:

What is EPA’s role in the TMDL Program and its results?

EPA’s role involves much interaction between the Agency and states, territories and tribes (for brevity referred to here as states) because the TMDL Program is predominantly a state-delegated program.

States are charged with assessing the condition of their waters and compiling the 303(d) list of those waters impaired by pollutants every two years. EPA reviews, sometimes modifies, and approves state 303(d) lists, and tracks impaired waters nationally.

EPA may promulgate a state 303(d) list if the state fails to do so. For waters impaired by pollutants, states must develop TMDLs that calculate the pollutant loading reductions necessary to attain water quality standards and allocate these planned reductions to the contributing point- or non-point sources. EPA’s role is to review, modify and approve these TMDLs, as well as to develop and establish TMDLs where the state fails to do so.

The implementation of pollution controls identified in a TMDL is, again, a state role over which EPA’s statutorily-defined role has limited authority. TMDL Program outcomes (see above), therefore, are dependent on state action beyond the point at which EPA’s TMDL Program authority ends.

Why does the TMDL Program Results Analysis Project look beyond EPA’s own role in producing results?

Although EPA’s statutory role does not extend from start to finish of the entire process involving TMDLs and their implementation, all of its TMDL Program components consistently seek to support the "restore and maintain" goal of the Clean Water Act. It is essential to look beyond specific roles or programmatic milestones to eventual environmental outcomes in order to gain insights on how to achieve the most positive outcomes. These insights may relate to any stage in the impaired waters restoration process – whether or not involving EPA directly -- thereby revealing to EPA where its approaches are working or may benefit from modification.

Why are some kinds of TMDL Program results more difficult to document than others?

Outputs such as the number of impaired waters listed or TMDLs developed are unambiguous and easily documented program results. Outcomes such as waters restored to meeting water quality standards due to TMDLs are far more complex to document. For one, the act of impaired waters restoration after a TMDL usually coordinates many contributing actions – some TMDL-related, some not – to the common goal of recovery, and the exact effect of the TMDL alone is inseparable from other contributing factors.

Further, the wide variety of restoration actions are not centrally tracked in states and therefore it is exceedingly difficult to link specific actions to specific outcomes except on a case-by-case basis.

In addition, with tens of thousands of TMDLs completed, the expense of comprehensively tracking implementation of all pollution control actions and TMDL outcomes would be a severe financial burden on states and EPA. As an alternative, the TMDL Program is using case studies and sample-based approaches to analyze some forms of program results at a manageable cost.

How can results information be used to improve the effectiveness of environmental protection?

Program results document where program actions were or were not successful. However, merely knowing a program result without its cause leaves the program unable to improve. With information about relative successes as a starting point, program analysis can study the surrounding circumstances and patterns and build evidence for specific driving factors that underlie success. As evidence mounts on specific driving factors that underlie TMDL success or failure, EPA can apply those insights in program adjustments that favor greater success.

For example, evidence exists that having a technical plan such as a TMDL drives greater restoration efforts, and EPA’s promotion of widespread TMDL development is consistent with this driving factor.

In addition, evidence of greater success in collaborative, watershed-based TMDLs supports the recent EPA emphasis on funding more watershed plans and targeted watershed grants. Part of the TMDL Program results analysis project is dedicated to identifying new driving factors of TMDL success and developing tools to help EPA and state programs apply these factors to improve program effectiveness.

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Last updated on July 22, 2024