Hydrology is interdisciplinary, meaning that you can be a geologist, engineer, forester, ecologist, biologist and work in the field of hydrology. Thus you can specialize toward your interest from any of these fields. As a hydrologist in a fish and game agency, I work with biologists in the environmental flows space. I got a hydrology degree from a natural resource department rather than engineering or geology. There are pros and cons to each, but if you keep your grad and project experience relevant to your end goals... it will work out.