I have just the fishfinder you need. I'll sell mine for $60.00. Fishin Buddy 1200 portable. Clips onto your gunnel and works fine on rivers. I've used it on some big salmon holes. See it here:

http://www.cabelas.com/cabelas/en/templa...rentId=cat21390 &parentType=index&rid=&cmCat=MainCatcat21276&hasJS=true