Good call. I was going to say the TN River, but that is vague.

When driving over Ft Loudon Dam in Knoxville I always picture that is way to get to Chattanooga by water - Watts Bar.
Fort Loudon Lake > Watts Bar Lake > Chickamauga Lake > Nickajack Lake > etc.

Watts Bar Lake by boat feels like in goes on-and-on. I assume 75+ miles. I think the Chickamauga stretch is close to that also.