The Fabius River basin is easily divided into three main sub-basins (Figure Lo01). The North Fabius sub-basin originates in Davis County, Iowa. The Middle Fabius and South Fabius sub-basins originate in Schuyler County, Missouri. Approximately 6% of the watershed is in Iowa. The three principal streams flow in parallel relation southeasterly across northeastern Missouri, draining portions of eight counties (Schuyler, Scotland, Clark, Adair, Knox, Lewis, Shelby, and Marion). The Middle Fabius River joins the North Fabius in southeastern Lewis County. The North Fabius flows another 8.9 miles before merging with the South Fabius in northeastern Marion County to form the Fabius River. The Fabius River then flows only 3.5 miles before reaching its confluence with the Mississippi River in the Fabius Chute near River Mile 323. All three mainstem streams have upper tributaries named North Fork and South Fork. Bear Creek is the only other major tributary of the North Fabius River (besides the Middle Fabius River). Durgens Creek, once a North Fabius tributary, has been diverted and now drains directly into the Mississippi River. Bridge Creek is a major tributary of the Middle Fabius River, and the Little Fabius River and Troublesome Creek are major tributaries of the South Fabius River. The Fabius watershed is bounded by the Salt River basin to the west, the Wyaconda River basin to the northeast, and the North River basin to the south.
The Fabius watershed drains 1,543 square miles (988,900 acres) of land. The basin is about 80 miles long and up to 25 miles wide. The North Fabius, Middle Fabius, and South Fabius sub-basins compose 32%, 27%, and 40% of the Fabius basin, respectively (SCS 1992a, SCS unpublished). The only other fifth-order streams in the basin are the South Fork of the South Fabius River and the South Fork of the Middle Fabius River which drain about 53.5 square miles (34,311 acres) and 94.0 square miles (60,323 acres), respectively.
Streams were identified on USGS 7.5-minute topographic maps and ordered according to Strahler (1957). There are 57 third-order and larger streams in the basin (Table Lo01). The North Fabius River is the longest (105 miles) and largest (sixth order). The Middle Fabius River (75 miles long) and South Fabius River (81 miles long) are fifth-order streams, as are their respective South Forks. All other streams in the basin are fourth-order or smaller.