California has 4,469 subwatersheds and 559 of them contain an active stream gage that measures flow. 37.4 % of the state's 60,569 licensed diversions sit in watersheds nobody currently measures — and two thirds of that gap is a network that was switched off, not one that was never built.
This portal reports availability and quality per watershed, and states on every number whether it was measured or modelled. Descend the hierarchy and watch the records drop out.