Still in the nascent stages, if thinking of a recreational drone based approach. I am not a professional or well versed in this area, but have been researching the same from the bathymetry and riparian topography approaches. Literature that i have seen, suggest the use of LIDAR along with photogrammetry. If using lidar, decent resolutions and depth accuracy may be obtained with green and infra red used together. However laser does not work in whitewater or aerated sections .
Certain types of river features lend themselves to a photogrammetric approach given right light condition, up to the "secchi" depth for the water. However you need expensive software ( though an open source solution is also being developed) to extract the bathymetric profile. Sonar can be used in flat sections that are deeper than say a certain minimum depth. There is a video of a small toy Z boat dragging a sonar across a class 1 rapid in an Alaskan river. A kayak based approach on a lake or flat section can be seen by googling Paul Illsey.
I have also read of through the ground (ice) approaches involving the use of ground penetrating radar.
So prioritize your objectives, slice up the river into sections and apply technologies that are appropriate to each.
Stan
