Not sure if it would help being a river or not, not sure how strong the current is... but I might have the answer if you wanted to try. It is called a Humminbird Smartcast Fish Finder. It is a Remote Sensor that you can toss out with a fishing pole and it reads Temperature, Depth and any Fish in the area back to a Fish Finder that you strap on your wrist like a watch, pretty nifty. I bouth one the other day, still waiting for it to arrive and can't wait to try it out... $72 @ Amazon.com. When I receive it sometime next week, I will deffinately let you know the results. But... I am fishing a lake/resevoir? But the neat thing is, is it also acts like a bobber and you can fish right from under it... so I bet if you knew the depth you could just rig a sinker to the depth of the fishing area and the Remote Sensor would hold tight while you fish the area. Hope this helped and best of luck fishing sir!
It sounds like a wonderful solution... but to other fishing spots I go to. Unfortunately, the current is to swift in that river for most if not all fish finders I have looked at unless I use a really heavy sinker to anchor it in place. But for the two lakes I also fish this might work well. I hope you have success with yours. Thanks for the info.