The Big Sioux River, at the USGS station in Sioux City, Iowa, has a mean annual discharge of approximately 3,793 cubic feet per second.[9]
Tributaries
The Big Sioux River collects the Rock River from the northeast in Sioux County, Iowa.[7] A minor headwaters tributary of the Big Sioux in Grant County, South Dakota, is known as the Indian River.[8][10]Broken Kettle Creek has its confluence with the Big Sioux in Plymouth County, Iowa.
The Big Sioux is South Dakota's most populated river basin. Agriculture is the primary use of land along most of the river's course. To comply with the Clean Water Act, the state monitors water quality of its rivers. Most of the Big Sioux north of Sioux Falls was scored well in 2012. Portions near Lake Kampeska and between Willow and Stray Horse Creeks (Codington and Hamlin counties) exceeded federally allowable levels of E. coli and fecal coliform bacteria. However, the Big Sioux south of Sioux Falls is much more polluted with E. coli, fecal coliform, and suspended solids. Several portions heavily restrict fishing or human contact, and swimming is banned.[12]
Flood control
Between 1955 and 1961, an extensive flood control system was constructed by the U.S. Army Corps of Engineers along the Big Sioux and some of its tributaries in Sioux Falls to protect the city from a 100-year flood event.[13] Features of the system include 29 miles (47 km) of levees, a floodwall in downtown, and a 15,000 feet (4,600 m) diversion channel with a dam at one end and a 118-foot (36 m) spillway at the other.[13] The diversion channel connects two ends of the Big Sioux's natural loop around central Sioux Falls in an effort to channel floodwater away from the city.[13] The levees then act to contain any floodwater either remaining in the natural channel or originating from Skunk Creek (whose mouth is downriver of the diversion dam).[13][14] Additionally, a greenway covers much of the river's floodplain in southern and eastern Sioux Falls, further mitigating any property damage from high water.
The Big Sioux River Flood Information System was used to model flooding during the March 2019 bomb cyclone event.[16]
The river overflowed its banks between September 12–15, 2019, flooding three blocks of Dell Rapids, South Dakota, and damaging up to a dozen homes.[17]Interstate 90 was shut down between Mitchell and Sioux Falls.[15]
The river once again flooded in 2024, resulting in at least one death and multiple destroyed homes, roads, and businesses. Most notably in the McCook Lake area.[18]
^U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. The National MapArchived 2012-03-29 at the Wayback Machine, accessed March 30, 2011
^Jorgensen, Don. Sioux Falls diverts water to ease flooding[1]Archived 2011-09-27 at the Wayback MachineKELO-TV. March 15, 2010. (accessed April 19, 2010).