The Big Sandy River of Tennessee rises about five miles (eight km) northwest of Lexington, Tennessee, county seat of Henderson County, Tennessee, in the same vicinity as two other important rivers of West Tennessee, the Beech River and the Middle Fork of the Forked Deer River. Like the Beech (and unlike the Forked Deer, which flows to the Mississippi River), the Big Sandy flows to the Tennessee River.

From near its source downstream, much of the Big Sandy River has been channelized. It crosses into Carroll County near the community of Yuma. In Carrol County, it passes slightly east of the town of Bruceton. Turning somewhat northeast, it crosses into Benton County. The former channel (prior to channelization) of the Big Sandy forms several miles of the boundary between Benton County and Henry County. The lower Big Sandy is impounded by the Kentucky Dam project of the Tennessee Valley Authority; it forms the biggest single embayment on Kentucky Lake . The head of the embayment is the site of the town of Big Sandy and nearby is a major "dewatering area". This is an almost flat area (superficially resembling a tidal flat) which is flooded or not, depending on TVA's assessment of a proper level for Kentucky Lake, taking into consideration flood control, navigation, electrical power needs, and recreation (in that order, according to the 1933 Act establishing the Authority). Another, even larger, dewatering area is maintained by the help of an auxiliary dam on a tributary, West Sandy Creek. Despite the extensive channelization activity, much of what the river must have been like prior to this can be seen in the wetlands surrounding it near the Interstate 40 bridge and also along Tennessee Highway 69 between Camden and Paris.