I don't think they're coming up to spawn, because they're almost always there so long as the water is warm. I"ll get a few more pics next time I'm there, but the water comes off from a shallow marshy pond and dumps into a pretty major water system. I think the nutrient-rich water attracts them.
There's sort of a ledge that they shouldn't be able to jump over, so that keeps them out. You can't fish from shore there, but if you navigate up the seneca river you can fish from the river side in a boat. I was up there two weeks ago, no carp yet but more Northern Shovelers than I've ever seen in my life. Beautiful ducks!
Taxidermist (Flagstealer)