2004-2022 Integrated Report Big Sioux River Analysis

Every two years, the South Dakota Department of Agriculture and Natural Resources (DANR) produces an Integrated Report for Surface Water Quality Assessment for all waterbodies in the state. DANR has split the Big Sioux River into 16 segments, each with a list of beneficial uses. A waterbody is considered impaired when it does not meet its beneficial use criteria.

Before 2010, DANR tested for total coliform and fecal coliform. E. coli, a sub-group of fecal coliform with stricter standards, was added to the report in 2010. As a result, we have only included E. coli data in our analyses.

2004-2022 Big Sioux River Impairment Status

We analyzed the Integrated Reports from 2004 to 2022 for the percentage of impaired river segments for three contaminants in the Big Sioux River—Total Suspended Solids (TSS), E. coli, and nitrates. The graph indicates a spike in TSS and E. coli in the mid-2010s, with a slight decline beginning in 2018. Nitrate impairment has been at 0% since 2010. Overall, these data show an improvement in water quality in the Big Sioux River over the last five years. The 2024 Integrated Report will be crucial in determining whether these numbers continue to trend downward.

2004-2022 Big Sioux River E. coli Beneficial Use Status

SD DANR lists E. coli as having two beneficial uses: Limited Contact Recreation and Immersion Recreation. Of the 16 segments of the Big Sioux River, nine are designated for Immersion Recreation (235 CFU/100mL) and 16 for Limited Contact Recreation (1,178 CFU/100 mL). Immersion Recreation includes swimming and submersion, while Limited Contact Recreation includes fishing, boating, etc. Due to the disease risk posed by E. coli, we believe it is important to inform the public of the river’s E. coli impairment status.

Since 2012, all nine Immersion Recreation segments have been considered impaired or unsafe for swimming. Limited Contact Recreation impairment peaked in 2016, but it has been steadily declining ever since. These data indicate that E. coli levels in the river are improving year over year. As the less strict Limited Contact Recreation numbers continue to decline, we hope to see the more stringent Immersion Recreation numbers follow, with the eventual goal of the Big Sioux River being safe for swimming.