Fish Need to Breathe: Dissolved Oxygen Study Finds Different Results Across Souris River

kevin bunch
Kevin Bunch
northern pike souris

A five-year study looking at amounts of dissolved oxygen at three points in the Souris River shows dramatic differences based on the time of year and location.

Dissolved oxygen, or DO, is an important water-quality parameter. Fish and other aquatic species breathe it much like we breathe the air in the atmosphere. So when levels get too low in the water, many species will find it uninhabitable. 

Dissolved oxygen has been a water quality objective in the Souris River for years. The International Souris River Board has been observing how much DO is available in the stream and reporting to governments by way of the IJC when levels dip below the minimum amount agreed upon by Canada and the United States. For the Souris River basin, understanding flow and DO dynamics can help with control structure-related decisions for improving DO levels.

What set this five-year study apart from regular monitoring is that it was continuous. 

Normally, water samples are drawn from the river at periodic time intervals. Since dissolved-oxygen levels vary so much based on the time of day, ever-changing water flow conditions, the presence of algae and other short-term changes in the water, this approach can miss short-term changes in DO, said Joel Galloway, one of the lead researchers on the project with the US Geological Survey (USGS).

With IJC funding through the International Watersheds Initiative, USGS along with Environment and Climate Change Canada arranged for round-the-clock monitoring at three existing monitoring stations in North Dakota: Sherwood, Minot and Westhope. Sherwood and Westhope are located near where the winding river crosses the international boundary (from Canada into the United States and then later back into Canada, respectively), while Minot is the most populous community along its banks.

monitoring sites souris

The three monitoring sites on the Souris River. Credit: USGS

Purely looking at the data gathered, Galloway said it was clear that all three locations have unique conditions resulting in different dissolved oxygen conditions. Furthermore, the five-year study period had an usual range of weather and water flow conditions in the Souris River basin. 

Further analysis should involve developing statistical relationships between DO, flow and other collected parameters to develop a more robust understanding of their relationships and how flow may be managed, operationally, to improve DO levels in the Souris River. 

“We couldn’t have picked a better five-year period to monitor than we did,” Galloway said, noting that low-flow conditions are a factor that can result in low levels of dissolved oxygen, much like a fish tank without an aerator running. 

“In 2019, we came off of a higher year for flows, but we had low flow conditions in 2020 and 2021. In 2022 we had higher flows, and 2023 saw even higher flows, almost back to the average going back to 2012. If you were going to look at low DO, we captured those conditions well, and we got to include some higher flows too.”

At Sherwood, the datasets collected found a large difference in the amount of DO in the water between day and night, with concentrations dropping overnight and increasing during the day, most dramatically near Sherwood. Galloway said researchers suspect this could be because of organisms like algae photosynthesizing in the daytime and respiring at night. The amount of water flowing seems to play a role in how large this difference is, and he added that there are likely other factors at play too. Galloway noted there were some data gaps in the winters at Sherwood due to water levels being too low for the instruments to operate.

Downstream at Minot, DO levels remained well above the water quality objectives set by the governments until the winter of 2022, when they finally crashed. Galloway hypothesized that the high winter concentrations could be because of a large tributary entering the Souris River upstream, water temperature differences, or perhaps aeration from low-flow dams upriver as well but could not draw any firm conclusions. Further muddying the picture is the 2023 data, which saw higher flows but also the most instances of DO levels falling below the objectives—a situation that requires more research to determine what happened.

souris westhope

A graph shows how measured dissolved oxygen levels in the Souris River changed over time at Westhope, North Dakota, near the Canada-US border. Credit: US Geological Survey

By comparison, the Westhope site, where the river reenters Canada, saw more instances of DO levels dropping below the water quality objective and had completely different dynamics from the other two sites. 

While DO levels saw the same sort of day-night fluctuations from the other locations, as well as DO levels dropping in the winter, on the whole, the patterns in DO were fairly consistent year-to-year. This could be related to the water quality flowing out of the J. Clark Salyer National Wildlife Refuge just upstream, Galloway said, which has a series of pools operated and released by the US Fish and Wildlife Service.

In addition to DO levels, this project also recorded continuous water temperature and specific conductivity (another measure of water quality, as salts and chemicals can affect electric conductivity of the water). Galloway indicated that this data could be useful for understanding changes in the Souris River ecosystem as well. The data collected for this study can be found at the links below:

Souris River NR Sherwood, ND - USGS Water Data for the Nation

Souris River Above Minot, ND – USGS Water Data for the Nation

Souris River NR Westhope, ND - USGS Water Data for the Nation.


kevin bunch
Kevin Bunch

Kevin Bunch is a writer-communications specialist at the IJC’s US Section office in Washington, D.C.