Something to remember when flying in a dream:
Be careful, when flying over water, not to skim the surface too closely. Sharks can jump up out of the water to catch birds, and they could do the same to you.
Don’t assume that because you are following a river that there are no sharks in rivers. The Bull Shark “is a sea creature that can tolerate fresh water, allowing it to travel far up rivers and close to contact with humans. Its tendency to dwell in shallow coastal waters and rivers ranging from the Atlantic to the Indian to the Pacific Ocean, coupled with its unpredictable and aggressive behavior, lead many scientists to label it as the species responsible for the majority of shark attacks on humans.” I don’t know if it launches itself above the water surface to grab birds, but it might.
Using rivers to navigate is common in dreams since they’re good ways to go from town to town or town to city.