Water supply can be addressed by asking the questions, "Where does your water come from? How does it get from its origin to your tap (or the tap of whatever or whomever the water is supplying)"?
So you're looking at the origin: In the case of Southern California, some of our water comes from the San Francisco Delta, some from the Colorado River, some from Owens Valley, and some from our local aquifer. You'd want to answer how much from each in your area (i.e. what's the balance of the mix?) and something about its quality. You could contact your local water supplier and then your regional water supplier, if you have one and ask them, and use some of what they say as quotes in your paper.
How water gets from its origin to you: In the case of Southern California, mainly through aqueducts—the first one being built from Owens Valley (and there are stories around that), the most recent being hundreds of miles long from San Francisco (and there are stories around that).
Depending upon your focus, you might emphasize a particular aspect of the water supply network. Are you more interested in history? You'll talk about it from a historical perspective. More interested in politics? Talk about it from the perspective of negotiations and power plays. Interested in construction? Emphasize the difficulties in construction and the machines and types of pipes used, the digging required, the labor. How about water quality? You'll look at what your water suppliers have to do to get the water up to drinking standards or, if you get water from wells, what you have to do.
If you're writing a long paper it is important to choose a focus, because "water supply" is a big topic, and the specific focus you give it will make the paper more interesting and also give you the length.