I have always had luck at Conawingo, see the eagles at least, the Friday and Saturday after Thanksgiving. I say "seeing the eagles" as they tend to be at quite a distance from the accessible viewing area. As long as there is open water there will be eagles there.
I have found that 400 mm on a Canon 40D (1.6x body) is only occasionally enough reach. With 700mm on a 1.6x body you should be able to do OK with flight shots.
We were up there on the 11th. A fair amount of fishing by eagles, but far out. Keep watching for flights over the parking lot. The eagles often go right over the pavement as that is a nice line between the trees. Be prepared because you can get some nice flight shots at that time. When we were there, there were some pretty interesting dog fights overhead. We didn't get any pictures, but be prepared for anything. Even though we came away with only a couple decent pictures, it was a lot of fun.
If the eagles aren't flying, you can practice flight pictures on the gulls as there are usually lots of them up by the dam.
Be aware that the rest rooms are closed up by the dam. There are portapotties down by the boat ramp. There is lots of construction going on up by the dam. I believe that this is a facility that will be for visitors, fishermen and photographers which was mandated to renew the hydroelectric facility permit. So don't complain too much. Next year should be much nicer.
Bob, this is not really current info , but I was at Conowingo on Jan. 17th and not much was happening eagle wise. I was there from just before sun-up to around 4pm. Counted 15-20 eagles on the far (very far) bank, but virtually no activity. Occasionally a bird would fly overhead, or perch in the trees nearby, but I did not see a single fishing run the entire day.
The regulars said that earlier in the season 100+ eagles were actively fishing at the dam but the dam had released alot of "fast water" the first couple weeks of Jan. that seemed to disperse the eagles.