Well, in a fast river you would want to find somewhere that the fish could quickly and easily move in and out of current, ie:a big boulder breaking the current, just behind that bolder in the calm spot would be excelent. Then the fish can move intot he current to feed and whatnot, then jump back into their calm spot to rest, and if there's food there, eat that too. In a slow moving river you'll do good in the same scenario as above, but also anywhere with good cover, overhanging trees, undercut banks, logs, thick vegetation, and pretty much anything. If you're worried about getting snagged up, there's probably cats there. A common misconception with cats is that you have to cast far and deep, ehhh wrong! Occasionally that does work out, but the best place is somewhere with cover, not open water. Don't worry about fishing shallow either, I've caught a 6 1/2 pound cat in about 14 inches of water in a little opening in some reeds about five feet from the bank. Good luck with those cats, they're my fav, and taste great deep fried.
Hunter