I am writing an article on wild water and have split "problems" into two categories: those that can be readily seen and those which cannot.
I have this definition for the word contaminant
a polluting or poisonous substance that makes something impure.
This seems to cover such as bacteria, viruses, and chemicals well. However, it does not work well for crocodiles, jellyfish, detritus, and sediment, say.
I'm also trying to crowbar rip tides, rocks, and entangling reeds in, but think they might be better served under the word "dangers", and they will be listed separately.