Charging a 12V car battery from a stream
If there's enough continuous water flow to charge a battery to power a camera then there's enough to power the camera directly. No need for a battery - just a voltage regulator. 

The usual thing most people don't appreciate is that hydroelectric generators need head (height difference between inflow and outlet) to generate enough driving pressure to overcome the resistance of the generator.

Measure how much head is available and what is a reasonable pipe size you could install at that location. You will find plenty of information on the web to calculate what power you could extract with those parameters. When you have the information you can add them to your question and we can improve the answers.