The Sabine River rises in three main branches-the Cowleech Fork, the Caddo Fork, and the South Fork. A fourth branch known as the Lake Fork of the Sabine or Lake Fork Creek, joins the main stream forty miles downstream from the junction of the other three branches. The Cowleech Branch rises in northwestern Hunt County and flows southeast for thirty-five miles to the extreme southwestern corner of the county, where its confluence with the Caddo Fork and the South Fork forms the Sabine River proper (at 32°48' N, 95°55' W). The former juncture is now inundated by Lake Tawakoni, constructed in 1958. The Caddo Fork rises in two tributary forks, the East Caddo Fork and the West Caddo Fork, in northwestern Hunt County. These streams unite in the southern part of the county to form the Caddo Fork, which flows southeast to its junction with the South Fork and the Cowleech Fork. The South Fork rises in the southwestern corner of Hunt County and flows east for eighteen miles to join the Caddo Fork and Cowleech Fork. From this point the Sabine River runs southeast, forming the boundary lines between Rains and Van Zandt, Van Zandt and Wood, Wood and Smith, and Smith and Upshur counties. After crossing most of Gregg County, the river forms portions of the county lines between Gregg and Harrison, Harrison and Rusk, and Harrison and Panola counties before it bends more sharply across Panola County. At the thirty-second parallel in the southeastern corner of Panola County the Sabine becomes the state boundary between Texas and Louisiana, and thus the eastern boundary of Shelby, Sabine, Newton, Orange, and Jefferson counties. The river empties into Sabine Lake (at 29°59' N, 93°47' W), which is formed by the confluence of the Nechesqv and the Sabine rivers; the lake is drained by Sabine Pass into the Gulf of Mexico.

The Sabine flows for 555 miles. Its total drainage basin area is 9,756 square miles, of which 7,426 is in Texas and the remainder in Louisiana. Unlike most Texas rivers, the Sabine is entirely in an area of abundant rainfall. Average annual precipitation is between thirty-seven inches at its source and fifty inches at its mouth. Also it flows through forested sandy country adaptable to the conservation of runoff and is fed by many flowing tributaries and springs. It has, therefore, a remarkably strong flow for its length, and it discharges the largest volume of water at its mouth of all Texas rivers. Average runoff within 97 percent of the Sabine River basin during the 1941-67 period was about 640 acre-feet per square mile. Two large reservoirs have been constructed on the Sabine: Lake Tawakoni, at the junction of the South and Cowleech forks, now in Hunt, Rains, and Van Zandt counties; and Toledo Bend Reservoir, on the Texas and Louisiana border. The Sabine River basin is characterized by flat slopes and wide, timbered floodplains. High rainfall rates produce frequent flooding of low-lying areas, and large floods occur, on the average, every five years. Floods generally rise and fall slowly, although flash floods occasionally occur in the basin. During flood the lowest part of the basin usually remains inundated for many days, and sometimes for several weeks. The extreme southern portion of the river is subject to hurricane flooding. In its upper reaches the river traverses rolling terrain with soils of deep sandy loams, loamy sands, and sand. Loblolly, longleaf, and shortleaf pine, post, southern, red and white oak, and flowering dogwoods grow throughout the region. Cottonwood, cypress, hackberry, pecan, blackgum, hickory, and blackjack oak are scattered throughout the area. A variety of native grasses is also found, including little and big bluestem, Indian grass, switch grass, grama grass, and Virginia wild rye. In its lower reaches, the Sabine flows through generally flat terrain with a substrate composed of sand, gravel, and mud. Vegetation in this region consists largely of water-tolerant hardwoods, conifers, and grasses.

The Sabine River basin has long been the site of human habitation. Archeological excavations have discovered evidence of all stages of southeastern Indian development, beginning with the 12,000-year-old Clovis culture. Indian development reached its peak after the arrival of the Caddos about A.D. 780. The early Caddoan Period, which lasted until about 1260, saw the construction of large mounds, the southwesternmost example of the Mississippian mound-building culture. In the Late Caddo Period, many of the mound sites were abandoned, but numerous sites show a continuing Caddoan presence in the area until the beginning of the historical era.