It flows from source tributaries in southwestern Iowa about 80 miles (130 km) to the Platte.
From that point, the Sullivan Line was surveyed east to the Des Moines River in 1816, and it was extended west in 1836 during the Platte Purchase, when Native American territory was purchased by the federal government and annexed to Missouri.
The three forks of the river cross the western extension of the Sullivan Line at points between 101 and 102 miles north of the Kansas-Missouri confluence.
Through the years, writers have speculated on etymologies other than the Sullivan Line coordinates: The three forks of the river rise in Iowa:[3] Each of the forks enters Nodaway County, Missouri, from Taylor County, Iowa, and converge near the town of Hopkins.
From this confluence the One Hundred and Two River flows generally southwardly, through Nodaway, Andrew and Buchanan Counties, past the towns of Arkoe, Barnard, Maryville, and Rosendale, in a heavily channelized streambed.