If you strip away, in your imagination, all of the roads, freeways, buildings, etc. and look at the Salt River Valley, you will see its overall shape is a bowl that slants from northeast to southwest. And everything drains that way. The Salt River is easy to see, but the other waterways aren't so easy to see anymore as they have been controlled. Well, most of them.
The massive flood plain on the west side of The Salt River Valley extends from the Agua Fria River (which is west of the 101 freeway) east across to the edge of Shaw Butte (about 19th Avenue) and then south to where it ends up at Tres Rios Wetlands. If you run your finger along the Arizona Canal on a map in this area you will see a gigantic diversion channel, called Thunderbird Paseo Park. And a lot of flood control is underground. The next time you see a lot of torn up roads on the west side, it's probably where they are putting in more flood control.
No, you won't see this stuff driving around in your car with your local real estate agent. But that's the reason for the lower prices. And when the flood control is finally finished along The Agua Fria River, the price of land there will go up. It's already happening.