River basin - I have come across "nadi kshetra" quite frequently.
Watershed is "jal vibhaajak", though I've also seen the English term in Hindi documents.
Water buffalo - Not sure if there is a specific term for water buffalo - isn't a "bhains" a water buffalo? Not sure.
Black buck - Most often referred to by its Eng. name, though the Hindi "krisnasaar" does exist.
Hope that a couple of these work for you!