Interesting question. Had never thought about it in those terms. I would imagine a rule of thumb could be where the river backs up to. Poorly worded. Let me try another way.
If the dam built is say 50 feet tall and raises the river elevation from say 700 ft to 750 feet, I would guess that if you went upriver to the point where the surface was above 750. Logic would say above that point water was flowing into the lake (ie a river). Below that point you would say water was being backed by the dam.
Look at Chickamauga lake. Presently the above the dam elevation is just under 677 feet. The down stream side of Watts Bat is 685 feet. So the drop is 8 feet over 59 river miles. Around Soddy it "feels" like a lake, around Dayton it "feels" like a river. I'd say that some where between those two points the surface elevation crosses that 677 foot mark. So around Dayton would be where the river / lake "evolution" takes place. This would mean that most of the 8 feet of drop occurs between Watts Bar and Dayton.
Or it is possible I need more coffee this morning.
Last edited by dwardmba; 02-10-2013 at 09:15 AM..