The Big Sioux River starts from the eastern side of the Black Hills in South Dakota. Its headwaters are located near the small town of Deadwood, where it begins its journey as a small stream. From there, the river flows predominantly southeast through South Dakota, traversing cities like Sioux Falls, before eventually reaching the borders of Iowa.
As it continues its course, the Big Sioux River joins the Missouri River at the Iowa-South Dakota border near the town of Sioux City. Overall, the river spans approximately 380 miles, providing vital water resources to the areas it flows through and serving as a natural boundary between South Dakota and Iowa at various points.