Well, somebody must be stealing it before it get to Hoover dam. Lake Mead is LOW. On the other hand, it appears that Lake Tahoe is at or slightly above it’s natural rim. Well, natural for the time being.
Here's one of the things that's stealing it:
Mature trees along riverbanks are said to consume about 200 gallons per day (per tree).There is a huge project in progress to try to control it. But one of the people at the local insectary (where they breed critters for bio-control) told me that environmentalists are trying to stop the tamarisk removal because they found a bird nesting in it. (This is also mentioned in the article cited.)