I am thinking of using surface imaging for supplementing traditional velocity-meter based flow measurements at high river stage. It appears that a research group has developed an app to do this in IOS. I haven’t been able to test output from the app against velocity-meter based observations, but we were able to get plausible results the first time we tried it. This seems like a promising technology, particularly in settings where high quality velocity measurements are impractical to collect.
More background is provided in this 2015 IAHR conference paper.