Flowing northeast to join with the Missouri River, the 8-digit Upper and Lower Gasconade River Hydrologic Units (HU) lie in the South Central portion of Missouri. The United States Geological Survey (USGS) 8-digit Upper and Lower Hydrologic Unit Code (HUC) #10290201 and HUC #10290203 are subdivisions of the Gasconade River watershed. These boundaries were adapted for water resources and soil conservation planning and inventory purposes by the USGS and the Natural Resources Conservation Service. The larger 8-digit hydrologic units are further subdivided to smaller 11-digit hydrologic units, representing tributaries to the Gasconade River watershed. Throughout this document the 8-digit Upper and Lower Gasconade River HUs will be called watersheds (although they are actually not complete watersheds) rather than HUs to eliminate confusion with similarly-named 11-digit HUs. These watersheds are drained by the Gasconade River and its tributaries except for the Big Piney River. While it is not part of this inventory and assessment, the Big Piney River flows into the Gasconade River and is part of the Gasconade River's natural drainage.
As the river meanders across the landscape, it travels through Webster, Wright, Texas, Laclede, Pulaski, Phelps, Dent, Maries, Osage, and Gasconade counties (Figure Lo01). The combined watersheds have a total surface area of approximately 2,806 square miles, which drain a wide upper watershed area of Webster and Wright counties and a more narrow lower watershed. A predominantly rural watershed, a significant portion of the upper watershed lies within the Mark Twain National Forest and Fort Leonard Wood U.S. Army Military Reservation. The Lower Gasconade River watershed has slightly less forest land, more pasture land, and more rural farm communities. A significant portion of the watershed's population is within the upper watershed area, particularly near Interstate 44.