Oacoma, SD

Oacoma is a Lyman County town on the west bank of the Missouri River, directly across from Chamberlain. The 2020 census counted 386 residents. Lewis and Clark camped near American Island on September 17, 1804, and the river crossing remained a stopping point for explorers, fur traders, and steamboat crews through the nineteenth century. Oacoma was laid out in 1891 as the Lyman County seat; the county offices moved to Kennebec in 1922. American Crow Creek enters the Missouri at the town, giving the riverfront a second local water line.

The town's position makes the river both a boundary and a daily point of orientation. Chamberlain sits on the opposite bank, and Oacoma lies roughly halfway between Sioux Falls and Rapid City, placing it within a long east-west travel corridor across South Dakota. The river, creek mouth, and nearby American Island site keep the Lewis and Clark-era landscape present in the town's geography, even though the island itself was later connected to the mainland and then covered by Lake Francis Case. Oacoma's identity comes from that layered crossing place: a small town, a former county seat, and a Missouri River landing opposite Chamberlain.