Can we grow vegetables hydroponically on floating rafts on a river?

If you are considering an aquaponics sort of system where you float rafts on the river and dangle plant roots into the water, I would say no. The nutrient and oxygen levels in the river are likely to be a lot lower than the plants would need to grow well. My tanks have quite a few fish per cubic foot ( a lot denser than a river ) and they have to contribute to the "nutrient" supply for the plants.

If you are considering hydroponics where you have a relatively closed system with water ( no fish ) and added nutrients at concentrations that you control and adjust, sure.. But you could set that up in a parking lot as well.