I've used maxbotix sonic rangers for water height measurement on flumes. They are pretty good. There are water-tight versions but I don't know about submersible ones in case the river is flooded. Maybe keep the main electronics on a pole way above the bridge but only lower the sonic ranger into the right height for monitoring. Most you can lose is one sonic ranger. I guess something around $200, with arduino and mobile hotspot, battery and solar charger. Yes, professional devices could cost $2,000 but for reliability consideration, I won't go down a $20 route.