Iowa DNR

Water Quality Assessments

Impaired Waters List

ADBNet Documentation

TMDL Prioritization


For the 2016 integrated reporting cycle and beyond, States review, systematically prioritize, and report priority watersheds or waters for restoration and protection in their biennial integrated reports to facilitate State strategic planning for achieving water quality goals


Iowa DNR prioritizes TMDLs that are able to address impairments on waterbodies with a high potential for social impact. An overwhelming focus of the state of Iowa has been nutrients and nutrient related issues.Additionally, the State of Iowa and its citizens place great value on their lake systems for recreation. As a result, the Iowa DNR will focus first and foremost on lake systems impaired for eutrophic conditions (algae,turbidity, pH). The Iowa DNR will also pursue a state-wide TMDL for bacteria impaired lake beaches. These swimming beaches are an important element in the recreational aspect of Iowa lakes. Finally, we will prioritize the Skunk River Nitrate TMDL. Three other river basin Nitrate TMDLs are already completed in Iowa and this impairment remains the sole nitrate impairment on the Impaired Waters List. As a human health concern, this also ranked as a top priority.

To understand priorities, we must first look at the Impaired Waters List. The TMDL Program’s candidate pool for development is restricted to impaired waters on Category 5 of the Integrated Report and,potentially, high quality waters for protection. Biological impairments are listed in Category 5b of the Impaired Waters List, stated as “Cause Unknown.” By definition, these impairments cannot have a TMDL written until a pollutant is identified as the cause of the impairment. Therefore, these impairments may or may not require a TMDL. Traditional methods of determining cause are prohibitively expensive for the TMDL Program. Ideally, these streams would be considered as “requires further investigation” rather than requires a TMDL. A statewide mussel survey is updating the existence of mussel impairments while a Fishkill Follow-up program is doing the same for fish kill impairments. A systematic verification sampling to confirm IBI impairments has been an ongoing effort for the past few years, but also carries a substantial cost. Going forward, impairments verified during these monitoring efforts will undergo a new investigative initiative led by the TMDL Program’s two staff biologists.Wetland impairments are relatively new to the Impaired Waters List and the DNR is currently investigating the usefulness of the TMDL process for impaired wetlands. Oxbow systems are essentially infant wetlands and are, geologically speaking, filling in as nature intended and therefore may not be a good fit for TMDL development.

Each of these impairment types carries a level of complexity and cost in time and money for the DNR to develop a TMDL. For example, multiple stream bacteria TMDLs in the same river basin could efficiently be developed using a load duration curve approach with a minimal amount of data required. On the other hand, a large complex lake system using advanced modeling techniques would take more time and cost more in terms of data requirements. A river basin bacteria project may produce, say, 15 TMDLs, whereas the same amount of work effort may only produce 1 larger, more complex lake system TMDL. Additionally, each type of system holds various levels of social impact. Multiple efforts reveal the importance of lake watersheds to the Iowa people, including Iowa State University’s research on the local economic impact of lake systems (CARD, 2009 – On the flip side, there is relatively little evidence in the potential social impact of reducing bacteria in streams.




Social Impact


Priority Group I

Impairments with relatively high social impact and a relatively low complexity and/or cost.

  • Smaller Eutrophic Lake Systems
  • River Nitrate

Priority Group II

Impairments with relatively high social impact and a relatively high complexity and/or cost.

  • Larger/Complex Lake Systems
  • Protection TMDLs
  • Statewide TMDLs


Priority Group III

Impairments with relatively low social impact and a relatively low complexity and/or cost.

  • Stream Bacteria

Priority Group IV

Impairments with relatively low social impact and a relatively high complexity and/or cost.

  • Biological Impairments
  • Lake Mercury Impairments
  • Metals Impairments

Plotting each impairment type on a simple 2x2 plot reveals a path toward prioritization. The upper left quadrant of the chart includes projects that are relatively high in social impact and relatively low in complexity / cost for development. Projects that clearly fit that description include the smaller lake systems impaired for eutrophic conditions and the Skunk River Nitrate impairment.

The upper right quadrant contains projects that hold a relatively high social impact but are more complex and may have greater data needs for TMDL development. These projects include larger and more complex lake systems, protection TMDLs for some of our high quality resources, or a statewide TMDL for something like beach bacteria impairments. Staffing and funding limitations would limit the DNR's ability to complete a lot of these types of projects.

Quadrant 3 contains stream bacteria projects where there is a low social impact but the investment in development is relatively low.

Finally, quadrant 4 includes projects with a relatively low social impact but high in complexity. These are projects that would represent low priorities at this time. Using this approach, the TMDL Program can more easily decide what projects to select for development that will 1) have a greater potential to be of value to the local users of the resource, and 2) provide a tool that leads to measurable water quality improvement.

This page was created 12/16/2016 2:53:16 PM and was last updated 11/14/2019 12:39:28 PM