[jcrawfordor]

The streams and rivers are charged by rainfall. If stream and river water is property of the state, the rainwater must be as well, otherwise owners of well-placed property can greatly diminish the downstream flow of rivers through strategic earthworks. This isn't a theoretical problem, it has happened fairly regularly in high-rainfall areas. The legislation in Oregon is a result of past practices by ranchers who would dam the lower watersheds of their property and eliminate streams.

The concern here is much less in southwestern states like Arizona because rainfall is only an extremely small portion of aquifer charging. Most water entering this region is the result of snow and snowmelt in further north states, so you can't really change much through control of precipitation... consider, for example‚ that Arizona has had a very unusually wet monsoon season this summer, but this has had basically no effect on Lake Meade. The total rainfall in Arizona is a tiny portion of river and reservoir volume.

Broadly speaking, water in Arizona and New Mexico is the result of precipitation in Colorado and Wyoming. Water in Oregon is mostly the result of precipitation in Oregon (excepting the major Columbia river system which involves BC).