Two intertwined but under-covered water issues threaten Brookings County: chronic pollution in the Big Sioux River and ongoing flood risks along Six-Mile Creek.
Big Sioux River Water Quality: A Slow Crisis
The Big Sioux River, which runs through Brookings County, suffers widespread pollution across multiple segments. According to the South Dakota Department of Agriculture and Natural Resources, as of 2024, roughly 75 percent of river segments exceed standards for total suspended solids and E. coli–jeopardizing warm-water fish and even limited recreational use.
Data from Friends of the Big Sioux River, which conducts independent water quality testing, confirms the department’s findings. The organization’s 2024 monitoring shows that 75% of Big Sioux River segments remain impaired for total suspended solids, with the same percentage failing E. coli standards for safe swimming and recreation, according to the group’s monitoring reports.
A draft Total Maximum Daily Load proposal calls for sediment levels to stay under 158 milligrams per liter, averaging no more than 90 milligrams per liter over 30 days, according to the department’s public notice. The proposal targets a 10-mile stretch of the river southwest of Brookings.
The department estimates about 73 percent of sediment in the segment originates from agricultural runoff, largely from erosion-prone fields, unbuffered ditches, and livestock access points along the stream banks.
This aligns with broader state findings. South Dakota’s 2024 Integrated Report for Surface Water Quality Assessment found that 78% of assessed river and stream miles are too polluted to support one or more assigned beneficial uses, primarily due to agricultural nonpoint sources, according to the department.
The Big Sioux River pollution problems reflect a statewide pattern. According to the department’s recent state reporting, about 80% of rivers, streams and lakes tested in South Dakota fail to meet standards for at least one of their intended uses, with agricultural runoff being the primary culprit.
Documents are available through the department’s website at danr.sd.gov.
Six-Mile Creek Flood Mitigation: Plans on Paper, Not in Headlines
Brookings city officials are simultaneously evaluating flood threats posed by Six-Mile Creek, a 28-mile tributary flowing through the western edge of the city.
A 2024 feasibility study—commissioned by the city in partnership with the South Dakota Department of Public Safety and FEMA—uses updated floodplain modeling to assess the flood risk corridor west of Medary Avenue, according to materials posted on the city’s website at cityofbrookings-sd.gov.
The study was conducted by RESPEC Company, LLC, with funding provided by the State of South Dakota Hazard Mitigation Grant Program, according to city documents. The city’s official documentation states that Six-Mile Creek “has the potential to impact more than 240 structures” during high-flow periods and flows through “20-plus bridge and culvert crossings.”
“Ultimately, we want to reduce future losses to structures and public infrastructure along the Six-Mile Creek Corridor,” Public Works Director John Thompson said in a March 4 presentation to the Brookings City Council, according to the city’s official presentation materials.
The study outlines mitigation options ranging from levee improvements to upstream retention structures. No formal construction has yet been approved. The city hosted two public meetings in 2024—March 4 and May 20—to present findings and gather community input on flood concerns and potential projects, according to city records.
Recent flooding events underscore the urgency of such planning. The Big Sioux River system, including tributaries like Six-Mile Creek, experienced historic flooding in June 2024 that resulted in at least one death and destroyed multiple homes, roads, and businesses, particularly in the McCook Lake area downstream, according to state emergency management reports.
Why This Matters
Pollution in the Big Sioux River threatens water quality, aquatic ecosystems, and downstream recreation in eastern South Dakota. The river serves as the primary watercourse for South Dakota’s most populated river basin, flowing through major cities including Watertown, Brookings, and Sioux Falls.
Flooding along Six-Mile Creek, meanwhile, poses a direct threat to homes, roads, and critical infrastructure. Climate experts say that more frequently severe flooding is due to shifts in climate and land use, while flood mitigation plans have not kept pace with changing conditions.
Both issues require public engagement and long-term investment, yet remain underreported and poorly understood. The state has allocated $3 million through its Riparian Buffer Initiative to help landowners create vegetated strips along waterways, according to the Department of Agriculture and Natural Resources, but broader solutions require sustained attention and funding.
More information is available at danr.sd.gov and cityofbrookings-sd.gov.
