How to Understand Iowa's Section 303(d) Impaired Waters Listings

What is the 303(d) Impaired Water Listing?

The federal Clean Water Act, specifically Section 303(d), requires that states compile a list of all waterbodies within their borders and their water quality status by April 1 on even-numbered years. This means that the Iowa Department of Natural Resources (DNR), which is responsible for compiling the listing, has the goal of completing the report by April 1 of 2020, 2022, 2024, etc.

How are Iowa's waterbodies assessed?

The water quality status is based on the primary use of that water body. There are ten possible uses, but the most common uses are for recreation, aquatic life, human health, or a combination of all three. Recreation refers to waterbodies where the primary use is swimming or water sports where a high degree of bodily contact with the water, or ingestion is likely. Aquatic life refers to waterbodies where the primary use is protection and propagation of fish, shellfish, and wildlife. Human health refers to waterbodies where the primary use is ingestion of water or aquatic organisms (fish, shellfish). For instance, waterbodies that have a human health designation must maintain a higher water quality standard than waters designated for recreational use. 

Based on the designated uses of the water, the Environmental Protection Agency (EPA) recommends water quality standards for safe use. States have the option to develop their own standards, based on scientifically defensible information, or to adopt the federal standard. Iowa uses the federal standards.  

Why and how are waterbodies listed as impaired?

Waterbodies can be listed as impaired if they do not meet water quality standards for just one of their designated uses. Some of the most common impairments in Iowa are indicator bacteria (E. coli), biological (a negative assessment of the fish or macroinvertebrate health), fish kill (this can have multiple sources, but the top cause in 2020 and 2022 was animal waste), algal growth, turbidity, and pH. 

Every two years the Iowa Department of Natural Resources works to assess and update the water quality status of Iowa’s waterbodies. Data is usually drawn from a three to five-year period depending on the water source; three calendar years of data are used for stream/river assessments and beach indicator bacteria assessments, and five years of data are used for lakes. For instance, the 2022 303(d) impaired water body listing will include data from 2016 to 2020. The data to make the water quality determination must meet robust scientific standards for collection and can come from multiple sources, but the vast majority comes from the Iowa Department of Natural Resources, the U.S. Geological Survey, the U.S. Army Corps of Engineers, major water utilities in the state, and surrounding states’ environmental and natural resource departments.  

In order to assess the waterbodies, water quality data from these various sources is compared to the water quality standard set by the EPA based on its main use. For example, Big Spirit Lake in Dickinson County was listed as impaired in 2018 because it exceeded the water quality standard for bacteria (E. coli) in a recreational lake. It was delisted in 2020 because the samples collected had a geometric mean that showed less than 125 E. coli organism per 100 ml of water, and no samples that contained more than 235 E. coli per 100 ml of water, therefore falling within Iowa’s water quality standards. 

What happens after a waterbody is considered impaired?

Waterbodies that do not meet water quality standards are included in the 303(d) impaired water listing and must create a watershed improvement plan, technically referred to as a total maximum daily load (TMDL). A TMDL identifies the maximum amount of a pollutant that a waterbody can receive while continuing to meet water quality standards and includes a plan for reducing the pollutants. 

What does the 303(d) Listing of Impaired Waterbodies tell us about Iowa's water quality?

In 2022, 597 impaired waterbody segments were included in the 303(d) listing. There are an additional 154 impaired waterbody segments, but these are not included in the 303(d) listing because they have completed a TMDL or the source of the impairment was not a pollutant. The greatest number of waterbody segments, 1,851 (Category 2 and 3), have insufficient data to be assessed. This means that due to staff capacity, they were not able to be fully assessed, although some data gathered indicated that there was an impairment (Category 3) or that water quality standards are being met (Category 2). Lastly, 15 waterbody segments in the state of Iowa are considered to have achieved all water quality standards for their designated uses.

Category Number of waterbody segments Explanation
2018 2020 2022


622 585 597 Impaired and included in the 303(d) listing 
4 145 167 154 Impaired, but a TMDL has been created 
3 1,433 1,542 1,530 Insufficient data overall, but potentially impaired 
2 352 271 321 Insufficient data overall, but potentially no impairment 
1 11 15 15 All water quality standards met 

How are waterbodies removed from 303(d) listing?

According to the EPA, impaired waterbodies can be removed from the 303(d) listing for several reasons including establishing (but not necessarily achieving) an EPA-approved watershed improvement plan or TMDL, new data showing the waterbody is no longer impaired, or flaws in the original analysis. Waterbodies that have created a watershed improvement plan but have not reached the approved water quality standard are still considered impaired, but can be removed from the 303(d) listing.  

From the 2020 to 2022 303(d) listing, 50 impairments were removed. Recall that a waterbody segment can have multiple uses, like recreation and human health, and therefore multiple impairments. Of those 50 impairments that were delisted, the majority (29) were delisted because data indicated an improvement in water quality. The second highest number (9) were delisted because they established a TMDL, or watershed improvement plan.

Is Iowa's water quality getting better or worse?

From 2020 to 2022 the number of waterbody segments that are considered impaired and included in the 303(d) listing went up, or declined in water quality, from 585 to 597. Perhaps a better indicator is to include both 303(d) listed waterbodies (Category 5) as well as those that are impaired, but a TMDL has been created (Category 4). From 2020 to 2022 the number of impaired waterbodies (Category 4 and 5) went down marginally, or improved, from 752 to 751. In terms of positive indicators of water quality, the number of waterbodies where all water quality standards are met (Category 1) stayed the same at 15 water body segments.