When I was grad-school shopping, I learned about a field called limnology; it's the study of lakes. Fluid flow is very important here, so you'd learn a lot of fluid dynamics. If you want to study fluid dynamics, chemical engineering might be a good major. You could major in physics and take fluid dynamics as an elective (there are physics courses in fluid dynamics, but they generally aren't required courses).
If you like fluid dynamics, you might also consider space plasma physics. Plasma physicists treat space plasmas as fluids for the purposes of mathematical description. But there's a twist that makes it even more fun: the fluids are magnetic. Central to space physics are the magnetohydrodynamic equations, which take plasmas' fluid and magnetic properties into account. You might want to check it out.
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