The 2026 Impaired Waters List

by Dan Haug | Mar 2, 2026

Cyanobacteria bloom in Lake Darling, July 2024

St. Patrick’s Day is the only day of the year it’s acceptable to have bright green water. It’s also the last day to make public comment on Iowa’s 2026 Impaired Waters List, which features some waters that sometimes turn green for other reasons. That evening, I’ll be giving a presentation about the Impaired Waters List as part of the Iowa Izaak Walton’s League’s series of Watershed Talks. If you can’t make it, I have a few resources below.

Update on nitrate and drinking water

The last time the Iowa DNR submitted a list of impaired waters to the EPA, there was a dispute about six stretches of river that supply drinking water to Des Moines, Cedar Rapids, Iowa City, and Ottumwa, and used to supply water to Oskaloosa. The DNR claimed that the drinking water standard for nitrate should be evaluated using a 10% threshold, but even with this interpretation, two of the rivers failed to meet the standard this time. This chart shows what all the fuss is about.

The Goose Excuse Bill

Ioway Creek in Ames and West Indian Creek in Nevada have very high levels of E. coli, but aren’t on the impaired list because of a state law which limits how the DNR can use our data. They would be joined on the Waters in Need of Further Investigation list by hundreds more rivers if the legislature passes House File 2530:

An Act requiring the Department of Natural Resources to identify specific animal sources of pollutants to a water of the state when determining the water’s inclusion on a list of impaired waters.

The Iowa Environmental Council has nicknamed it “the Goose Excuse Bill” and says it will will threaten Iowa’s compliance with the Clean Water Act. I joined IEC for Environmental Advocacy Day at the state house last week and passed out a few handouts with four more reasons to oppose the bill, which I encourage you to pass on to your representative. Microbial source tracking is very useful, but it’s not cheap, and this bill would force it to be used at the wrong point in the process.

Finding clean waters

I am sometimes asked where to go in Iowa to find clean water for paddling, swimming, floating in an inner tube, or just letting the kids splash and catch crayfish in the creek. A map or list of impaired waters is not very helpful for this, because the waters that aren’t included might be clean, or they might not have been assessed. So I made an interactive map, color-coded to show which lakes and rivers met or exceeded the primary and secondary contact recreation standards, in the last four recreational seasons.

The Fine Print

If you explore the Impaired Waters List and the rest of the assessment database, you will likely run across some things that don’t make sense. I share your frustration! This pair of short videos from our “Clean Water Act: 50 Years, 50 Facts” series contrasts how Section 305(b) and 303(d) of the Clean Water Act should work in theory, and how it can go wrong in practice. However, I continue to see improvements in the assessment database (ADBNet) and water quality database (AQuIA) and want to express my appreciation to IDNR for the data they collect and their efforts to be make it available to the public.

Lost Lakes and Found Lakes

by Dan Haug | Mar 2, 2026

March is Iowa history month and that’s a good opportunity to dust off some material from the archives to share the history of lakes in Iowa, some lost and some found.

Iowa has just 34 natural lakes remaining, most of them located in northern and central Iowa, which was covered by the last advance of ice sheets 12,000 to 14,000 years ago. The examples I will use are from Hamilton County, where naturalist Thomas MacBride had this to say:

“None of the lakes hereabout are very deep. They are all marsh-like, only distinguished from a thousand marshes by the courtesy of the pioneer who called them lakes to suit his fancy, recognizing their greater width and possibly, in some cases their bluffy shores.”
-Thomas H. MacBride, Geology of Hamilton and Wright Counties (1910)

1875 Andreas atlas, showing lakes in Hamilton County

Residents of Hamilton County should recognize one of the lakes on this 1875 map. Little Wall Lake is a popular spot for swimming, fishing, and motor-boating. At 249 acres, it’s plenty big enough to call a lake but only deeper than a marsh because of regular dredging. The lovely cabins for rent from Hamilton County Conservation were built from Iowa-sourced white oak logs as part of a Prairie Rivers of Iowa forestry and economic development project in 2013!

water sports on Little Wall lake in 2014

However, the 1,300 acre Lake Cairo and 870 acre Iowa Lake disappeared shortly after A.T. Andreas’s atlas was made. You can still see the shoreline of Lake Cairo on LiDAR, as well as the ditches (Rahto Branch and Ditch 71) built to drain it. Lost Lake Farm, a dairy located on the north “shore”, is named as a nod to that history. They employ rotationally managed grazing practices that build soils and protect water. I got to see this in action as part of Watershed Management Authority field trip in 2018; here’s a photo of the cows making a beeline for the tall grass after the fences are moved!

The scale of the drainage work is impressive, especially given the technology available at the time, and was just one of many such alterations that built up Iowa’s agricultural economy. In this case it made farmable over 1000 acres of Blue Earth muck loams with a corn suitability rating of 63 to 66. However, even at the time, there were disputes about how to balance public and private interests. To learn more about the history of drainage, I recommend a presentation by Joe Otto, recorded on Iowa Learning Farms.

The balance shifted as cropland became abundant and natural areas became scarce. In 1920 (partly at the urging of Thomas MacBride, quoted above) Backbone was dedicated as Iowa’s first state park. The Civilian Conservation Corps built a low dam on the Maquoketa River to form Backbone Lake in the 1930s and since then over 100 other lakes have been created in Iowa for public use by damming streams or by digging quarries and borrow pits.

In 1919, Iowa’s first county park was established in Hamilton County, not far from Lake Cairo, and Briggs’ Woods Lake was created sometime in the late ’60s by damming Terwilliger Creek. There are some more log cabins at this park, also built with the help of my former colleague Mike Brandrup. The porch is a good place to watch the lake and reflect on the complicated history of Iowa’s land and water!

A boy catches a bass at Briggs Woods Lake.

Mike Brandrup at a wetland restoration field day

End the No Swim Era

by Dan Haug | Jan 28, 2026

Warning sign posted at beach for water quality advisory

The Driftless Water Defenders launched a social media campaign last summer calling for the end of Iowa’s #NoSwimEra. It’s a catchy slogan, and timely. 2025 was an especially bad year for recreational water quality. The Iowa DNR tests for E. coli (an indicator of poop in the water) and microcystin (a toxin produced by algae) at 41 beaches at state parks, every week between Memorial Day and Labor Day. If you had picked a beach and weekend at random for a family outing, there is a 1 in 4 chance you would have encountered a “Swimming Not Recommended” sign when you got there. Over the past 25 years, the number of microcystin advisories has fallen but the number of E. coli advisories and the number of beaches affected has increased.

Iowa’s water quality problems aren’t unique, but they are a lot worse than some of our neighbors. I analyzed 2025 E. coli data from Wisconsin beaches and found that 11% of the water samples had high enough E. coli levels to trigger an advisory (235 CFU/100mL), versus 24% in Iowa. 3% of Wisconsin beaches had average E. coli levels high enough to violate water quality standards (a seasonal geometric mean of at least 126 CFU/100mL), versus 25% of Iowa beaches.

Some of the advisories are posted at lakes where there has already been a lot of public funding invested in lake restoration and in voluntary soil conservation projects in the watershed, so don’t think we can solve this problem with more of the same. At Lake Darling, the work seems to have been undone by an expansion of the hog industry. At the Lake of Three Fires, the work seems to have been undone by conversion of pasture to cropland, motivated by the Renewable Fuel Standard. We need to take a hard look at how state and federal policies allow or even encourage farming practices that foul our lakes.

However, I follow the data where it leads, and it doesn’t always lead to a hog barn. Here is my latest summary of E. coli data collected by the water monitoring program in Story County. Hickory Grove Lake has been well-studied by the DNR, and both DNA markers and transects point to Canada geese on the beach, not livestock in the watershed. College Creek is usually our worst stream on days when it’s flowing, and it has an urban watershed. A large fraction of the bacteria in West Indian Creek came from an old sewage treatment plant that was just replaced this year. DNA testing of water from Ioway Creek showed that human waste is present. We’ll be doing more testing in 2026 to confirm this and narrow down the sources, including a volunteer event in early summer. Donations also help support our monitoring and education efforts!

2 page handout summarizing E. coli in Story County

Some people are afraid to dip a toe in any lake or river in Iowa, and their No Swim Era goes back decades. That’s a shame. Even in a bad year, it’s possible to find clean water for recreation. For example, I took my daughter and her friend swimming and paddleboarding at Peterson Park, a Story County-owned beach where E. coli counts never exceeded the double digits. I joined Iowa Project A.W.A.R.E. to canoe and clean up trash on some scenic stretches of the Upper Cedar and Shell Rock Rivers, which averaged 69 and 45 CFU/100mL for the season–the standard is 126. I’ve updated this interactive map to help you find others.

The new Currents of Change report has some good tips for minimizing your risk of a waterborne illness when recreating in waters of poor or unknown quality. I regularly go in Ioway Creek, which I know to have poor water quality, but pack hand sanitizer and do my best to keep my head above water. I dunked my head in the Winnebago River (which was muddy and hadn’t been tested recently) multiple times during a whitewater kayaking class, but wore nose clips. I stay out of green water but otherwise am comfortable with a certain amount of risk to be able to enjoy the outdoors. But that’s just making the best of a bad situation. No Swim advisories have gotten way too common, and we need to work together to clean up our water and end that era!

Infographic from CISWRA report with recreational safety tips

How bad was nitrate in 2025?

by Dan Haug | Jan 6, 2026

When the ball dropped on New Year’s Eve, nitrate in the Raccoon River was once again above the drinking water standard, closing out a bad year for water quality in Iowa. Below, I’ve compared this year’s nitrate levels to long-term averages at sites in Iowa DNR’s Ambient Stream Network with at least 20 years of data to get a sense for where and when nitrate was highest and what was unusual about 2025.

To understand these patterns, it’s important to remember that 2025 was not a uniformly wet year. Heavy rains in July broke or came near to breaking some records and caused flash flooding in many communities. These rains also raised the water table enough to keep drainage tiles flowing into into the fall. However, most of the state received less than average rainfall from January through May and from August through December. In a band from Ames to Charles City, that works out to a much wetter year than normal. In other parts of the state, like Dubuque, Ottumwa, and Red Oak, that works out to a much drier year than normal.

Nitrate was much higher than normal in the Boone River, Skunk River, and many others in northern and central Iowa, especially in July and August. Nitrate was within 1 mg/L of the long-term average for the Nishnabotna and other rivers in southern Iowa and for the Rock and Floyd Rivers in northwest Iowa. In the table below, red indicates nitrate was higher than average, blue is lower than average, and white is close to the long-term average.

There are some unusually high readings (25 mg/L in the South Skunk River in January) and unusually low readings (3.3 mg/L at Beaver Creek In July) that may have been collected during a storm or other event that is not representative. This is a limitation of monthly grab samples and one reason why maintaining Iowa’s network of real-time nitrate sensors is important. The Ioway Creek sensor was removed at the end of 2024, but the pattern I documented here here (a quick flush of low nitrate surface runoff followed by a gradual release of high nitrate drainage water) happened over and over, in many places in 2025. These kinds of leaks can be avoided with changes in cropping systems and land use, reduced with better nutrient management and cover crops, or intercepted and treated with saturated buffers and wetlands, but we’ll need a lot more of them to prevent another bad year like 2025.

The economics of nitrogen have changed. Has our thinking?

by Dan Haug | Dec 4, 2025

Revised December 10

It sounds too good to be true, wrote Neil Hamilton in a 2021 opinion piece. Reducing nitrogen fertilizer application rates to the Maximum Return to Nitrogen (MRTN) recommended by Iowa State University promised to save farmers money while keeping nitrate out of the rivers and greenhouse gases out of the atmosphere. In retrospect, it was too good to be true.

Reducing fertilizer rates will cut into profits (for most farmers)

Early this year, ISU researchers published a study in Nature Communications showing that the amount of nitrogen fertilizer required to maximize yield (the agronomic optimum) and maximize profits (the economic optimum) have been steadily increasing, driven partly by corn genetics and partly by weather. The economic optimum is always lower than the agronomic optimum (the revenue from those last few bushels isn’t enough to pay for the fertilizer) but the difference between the two is getting smaller. I wrote about this in July, but since then I’ve had a chance to download and explore the data used in the study. Here are the trends in optimal nitrogen application rates for just the sites in Iowa, compared to actual nitrogen application rates, which I estimated using a combination on IDALS fertilizer sales data and INREC survey data. For more details on the data I used to estimate actual nitrogen application, read this attachment.

The scenarios in the Iowa Nutrient Reduction Strategy were based on data from 2006-2010. At that time, it would have been possible for farmers to reduce nitrogen application rates on corn following soybeans from 151 lbs/acre to 133 lbs/acre while increasing profits, on average. However, those figures were already out of date when the Iowa Nutrient Reduction Strategy was released in 2013, and in the decade since, fertilizer application rates have levelled off while the amount of nitrogen needed to maximize yields or profit has continued to increase. A minority of farmers may still find opportunities to boost profits by reducing nitrogen application, but average rates are now below the economic optimum.

This part of the study looks solid and matches what I’ve seen from other sources. Practical Farmers of Iowa have done their own trials and found that a majority of their participants were able to save money by reducing nitrogen rates in an especially dry year, but in a more typical year only 41% of farmers saw potential for savings.

Reducing fertilizer rates could still have significant water quality benefits

Farmers no longer have an economic incentive to reduce rates (on average) but it’s not hard to imagine policies that could shift the incentives by making it cheaper or less risky to apply at low rates, or more expensive to apply at high rates.

The ISU study includes an “environmental optimum nitrogen rate” that hints at that. The authors used a crop systems model to estimate nitrous oxide emissions and nitrate leaching for different scenarios, assigned a price to the pollution, and calculated the nitrogen application rate that would be economically optimal if those costs were reflected in the marketplace. Instead of evaluating the environmental benefits of reducing rates from current levels, they estimate the impacts of reducing rates (for corn after soybeans) from an economic optimum of 143 lbs/acre to an environmental optimum of 116 lbs/acre. Those are averages for the entire 20 year period and not at all relevant today. Oops! Because of this mistake, they conclude that “a reduction in N fertilizer rate towards improving sustainability will not have the anticipated reduction in environmental N losses because of the nonlinear relationship between N rate and N loss.”

Actually, the non-linear (curved) relationship between nitrogen application rates and nitrate pollution implies that rate reduction will have bigger benefits now than it did when rates were lower. The figures below contrast some outdated assumptions with new reality.

The increase in fertilizer rates has been bad for water quality

In most presentations and interviews about the Iowa Nutrient Reduction Strategy, ISU faculty correctly point out that fertilizer management alone is not enough to meet our water quality goals and emphasize the need for a variety of conservation practices. However, every scenario in the INRS assumed that fertilizer application rates would go down. It may not be possible to meet our goals now that fertilizer rates have gone up.

Based on the increase in fertilizer rates for corn after soybeans (from 151 lbs/acre in 2007 to 173 lbs/acre since 2017), we would expect a 16% increase in nitrate concentration in drainage water. The 3.8 million acres of cover crops reported in recent INREC aren’t enough to undo the damage. Nitrate in streams is also affected by weather, changes in land use, and other practices not modeled here, but fertilizer rates and cover crops do help to explain why nitrate concentrations in many streams peaked between 2013 and 2015 and have fallen since.

What about continuous corn?

Nitrogen application rates for corn after soybeans have gone up, but application rates for continuous corn may actually have gone down. I say “may” because we didn’t have good baseline data. Because corn stover ties up a lot of nitrogen as it decomposes, growing corn after corn requires higher nitrogen application rates to achieve the same yield. The confusing Figure 5 in the Nature Communications paper looks at the yield penalty for reducing nitrogen rates from the economic optimum to the “environmental optimum,” which is what it would make economic sense for farmers to apply if the societal costs of pollution were reflected in the marketplace. The authors concluded that reducing nitrogen application rates past the economic optimum would have unacceptable consequences for grain markets and food security, especially for continuous corn. I looked at the same figure and concluded that growing corn after corn would not be commercially viable in a society that valued clean water, a stable climate, and public health.

Your mileage may vary

My biggest takeaways from both the Iowa State University research and the Practical Farmers of Iowa research are how much the optimum nitrogen rate varies from year to year and place to place. One farmer in the PFI study saved money by reducing rates from 150 to 100 lbs/acre, while another lost money by reducing rates to 246 to 200 lbs/acre.

The ISU study includes nitrogen rate trials for seven sites in Iowa and six years since the Iowa Nutrient Reduction Strategy was released. If you had followed the recommendations from the old nitrogen rate calculator and applied 140 lbs/acre to corn after soybeans, 62% of the trials would been at least 10 lbs/acre below the economic optimum. But even in the most recent year, there were 2 sites where that would have been at least 10 lbs/acre above the economic optimum!

The Iowa Nitrogen Initiative addresses this problem through an expanded program of nitrogen rate trials and a decision support tool that can provide customized recommendations by county given assumptions about rainfall, planting date, and residual soil nitrate. Using the new information, some farmers will find an opportunity to increase profits while reducing nitrogen rates. A larger group of farmers will find opportunities to increase profits by increasing nitrogen rates. Dr. Castellano has made a complicated argument for how the water quality benefits of bringing down the high rates can be greater than the water quality penalties of bringing up the low rates. Great. Please apply that logic to manure.

What about manure?

Since 2017, the INREC survey report has asked farmers what percent of fields receive manure application (about 20%), how much commercial fertilizer is applied to fields that do not (174 lbs/acre for corn in rotation and 199 lbs/acre for continuous corn), and what proportion of cropland is planted to continuous corn (about 12%). Manure expert Dan Anderson recently did some algebra to see what that implies about nitrogen application rates for fields that do receive manure, and came up with 342 lb N/acre on corn-after-soybean and 391 lb N/acre on continuous corn. I used slightly different assumptions and came up with lower numbers, but they’re still much higher than needed to maximize yield. If you’ve read anything by Chris Jones, this won’t come as a surprise.

I’m showing the agronomic optimum rate rather than economic optimum because the economics of manure aren’t the same as commercial fertilizer. Manure has much lower nutrient content and is much more expensive to haul. Manure pits fill up and there’s often a time and labor crunch to get it applied. Manure has highly variable nutrient content, which adds to the uncertainty and makes a supplemental application of commercial fertilizer seem like cheap insurance. If farmers had a strong economic incentive to make the most of manure nitrogen, nobody would be applying it in early fall and we wouldn’t have a cloud of ammonia hanging over the Midwest. There are also some farmers who are doing an exceptional job of conserving soil and water by feeding cover crops, small grains, or forage to livestock, and we should figure out how to level the playing field to make it easier to replicate their model.

Are these changes in nitrogen management good or bad?

It’s a mixed bag. I had to puzzle over this for quite a while!

The increase in the economic optimum nitrogen rate is partly due to good things (improved corn yield response) and partly due to bad things (increasing nitrogen losses to the air and water).

It’s good that nitrogen fertilizer use has gotten more efficient. Farmers can grow more bushels per pound of nitrogen than they used to. It’s bad that manure management plans still allow nitrogen to be applied at a rate of 1.2 lbs per bushel of potential yield.

It’s good that fertilizer rates for corn following soybeans have levelled off recently. It’s bad that nitrogen fertilizer rates went up in the early 2000s.

It’s good that nitrogen application rates for continuous corn have fallen. It’s bad that farmers are planting corn after corn.

It’s good that farmers are now applying less commercial fertilizer (on average) than required to maximize yield. It’s bad that farmers are over-applying manure.

It’s bad that we don’t have a plan to reach the goals of the Iowa Nutrient Reduction Strategy without rate reduction, and it’s bad that the price tag of reducing rates (either to farmers, the public, or both) is higher than we previously assumed. However, it might still be a better deal than other conservation practices. It’s bad than more people aren’t talking about this.

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