The South Sulphur River originates near Leonard in southern Fannin County, Texas, flowing eastward and southeastward through Hunt, Hopkins, and Delta counties before joining the North Sulphur River to form the main Sulphur River at the convergence of Lamar, Delta, and Hopkins counties. This approximately 50-mile waterway drains a significant portion of northeast Texas's blackland prairie and post oak savannah ecosystems.

The river's character varies considerably along its course. In its upper reaches through Fannin and Hunt counties, the South Sulphur maintains a relatively narrow channel with moderate current, flowing through agricultural bottomlands interspersed with hardwood forests. As it progresses eastward through Hopkins County, the river widens and slows, particularly where it passes through Cooper Lake, a major reservoir that dramatically altered the river's natural flow regime. Below the lake, the river resumes a more natural character through Delta County, meandering through forested bottomlands before its confluence with the North Sulphur.

Cooper Lake serves as the dominant feature along the South Sulphur River system, created by damming the river for flood control and water supply. The reservoir spans portions of Hopkins and Delta counties, offering flatwater paddling opportunities across its expansive surface, though wind can create challenging conditions on open water sections.

The river corridor supports typical northeast Texas riparian vegetation, including bald cypress, water oak, willow, and sycamore along the banks, with understory species like buttonbush and various vines. Wildlife includes white-tailed deer, beaver, raccoon, and numerous bird species such as great blue herons, wood ducks, and various songbirds. The waters hold catfish, bass, and sunfish species common to East Texas streams.

Access to the South Sulphur River is primarily through Cooper Lake State Park facilities and various county road crossings, though many sections flow through private property requiring permission for access. The communities of Cooper, Sulphur Springs, and Commerce serve as the nearest towns with services for paddlers exploring this watershed.

Seasonal flow patterns follow typical Texas precipitation cycles, with higher water levels during spring months and potential low-flow conditions during summer and fall. Spring flooding can render the river dangerous or impassable, while late summer may expose extensive gravel bars and sandbars in free-flowing sections.

The river's name derives from sulphur springs historically found in the watershed, though these features are less prominent today. The region's agricultural heritage remains evident in the surrounding landscape, with the river serving as an important drainage system for farmland throughout its basin.