Remember from the "Power Equation" that the two most important factors are flowrate (Q) and head (h). The flowrate is the amount of water that passes a point in a given length of time, specifically cubic feet of water per second (cfs). There are two main parameters which determine Q at a particular point on a stream. First is the amount of rain an area experiences. Second is the size of the drainage basin above that point. One other significant variable is the "land features", specifically the slope and the nature of the land (porosity, etc).
Northeast Georgia is the wettest area in Georgia (average annual rainfall) with a total of 69 inches per year in Clayton. Compare this to an average annual rainfall of 53.2 inches/year in Gainesville; 48.7 in Atlanta; 46.8 in Columbus, and 48.0 in Savannah. So, the area which contributes to flow in the Tallulah River has ample rainfall.
The area which contributes to flow at a particular point is called the "drainage basin". Much like the Continental Divide separates water which flows into the Pacific from that which flows to the Atlantic/Gulf, there is a "topography defined" area which separates water which flows to a certain point as compared to water that flows somewhere else. Again, the land area which contributes rainfall runoff to a particular point is its drainage basin, which are typically measured in square miles in the US.
On a particular stream, as one moves downstream there is more area which contributes to flow. And consequently the drainage basin increases as one moves downstream. For example, the drainage basin of the Tallulah River just upstream of Lake Burton is 58 square miles. At Burton Dam it is about 118 square miles. And at its confluence with the Chattooga River it is about 184 square miles (near the Tallulah Powerhouse).
The Tallulah River (as described above) is a very small river with a small drainage basin. Although rainfall is plentiful, the size of the drainage basin is small. By comparison to nearby projects, Hartwell has a drainage basin area of about 2088 square miles and Lanier's drainage basin is about 1034 square miles.