Try Watershed Lab free in your browser, with nothing to install and no account. Shape a valley on a table, then make it rain. Watch the rain soak in or run off, gather in the low lines and form creeks, fill a pond behind a ridge you built, and see the divide where one watershed ends and the next begins. Cover the land with forest, farms, a town or a parking lot and rain the same storm again: the creek rises higher and runs browner. Then find the source: in a valley you have never seen, one place is leaking germs, nitrate, oil or mud into the creek. Fill sample vials, read the test kits, and follow the dirty water back to where it starts.