I have been wondering this for a while and I am getting ready to look further into it. The question is does the river or creek channel on a reservoir flow on the bottom or the top of the water? For instance, you have an S-bend in the channel over 50 feet of water, does the actual current flow on the bottom or the top? And what keeps it following the original channel? Why doesn't it get lazy and just take the shortest route between two points ?

I can see your predicament. My 2 cents though... at what point does the mass of the new reservoir absorb any current that may be flowing in from creeks.

the bottom, I would guess.

The surface and relatively shallow water would be effected by wind current and things like that, I'd say.  The current of an old river channel would effect the water immediately around it.  I'd say that the current doesn't just take the shortest route because the deeper portion of the lake bottom that makes up the channel will kind of act as a funnel, pulling the water along its path.

All that said...I emphasize that its JUST A GUESS...I have no true idea how it works.  

depend on the type of dam that is pulling/pushing the water and how much flow the rivers/creeks are pushing into the lake.....The amount of current in this situation at the bottom would depend on the topography of the bottom and where you are looking/fishing it from that could contribute to the current at the bottom of the water column being faster or slower in areas...... Too much too really come to a good answer without some serious research. I always fish according to the visible current.

Something else to consider is what events(weather,power generation,water release) can you tell that stirs up stuff off the bottom more than normal..