Well the main channel would typically be the deepest trough running through the river. It is probably where the little creek that long ago started this river flowed. The thing to remember is that the are often back channels, side channels etc - all of them can hold fish.

Yes it will hold on smaller rivers the only thing to watch is depth. If the river is only 6 feet deep and fairly clear, then won't be able to vertical jig - might have to pitch so you aren't on top of the fish. I have vertical jigged as shallow as 5 feet in dirty water - set the hook and the fish are on (or above) the surface.

I fish the Rock in Beloit, depth is 5 ft to 10ft on average with sand muck bottom. I catch walleyes and smallies throughout year on crank baits.
Looking for ideas to catch more than one here and there.

Sometimes, especially on smaller rivers (and I am not familiar with the Rock so I do not know how big it is), catching one ot two fish in a spot is about all you can ask for. If it's a bigger river and you are sure an area is holding more fish, sometimes just changing lure colors will trigger a coupelmore fish to bite.

I believe channel edges and their relationship with fish staging on or near them, becomes more or less important given a vast amount of variables as well as seasons. They'll always be important to larger preditor species in low or clear water conditions as easy transition spots from shallow flats areas to hiding and ambush spots for baitfish. They also provide a cooler place to rest (and basically survive) when the river water temps get warmer and start to excede each species comfort zone.