Total storage in Lake Mead at the end of September (the end of the western “water year”) was lower than it’s been since they first filled it in the 1940s. But this is a bit more complicated than that stark statement might appear.

Storage in Lakes Mead and Powell

If you click through to look at a larger version of the graph (numbers on the Y axis are thousands of acre feet of water), you’ll see that total storage in lakes Powell and Mead, the two huge rivers on the main stem of the Colorado River is pretty much the same this year as it was last year. Mead has been dropping while upstream, Lake Powell has been filling. In other words, what we’re seeing here on the short time scale of a single year is the result of a management decision, not a change in the overall supply of water available. It’s worth noting that the last time we saw something similar – Powell rising while Mead fell – was in 1964, after the completion of Glen Canyon Dam, when water was being held back to fill Powell for the first time.

I made the graph over the weekend from a couple of U.S. Bureau of Reclamation datasets because I’ve been having a hard time visualizing how the whole system works. Because of the legal and political distinctions between the Upper Basin and Lower Basin water management systems, much of the discussion involves separate conversations about one of the lakes or the other, not both taken together. And FYI, this is just a quickie analysis, I haven’t done any quality control on the data, so this is not publication quality information.

Oh wait. I guess I just published it, didn’t I?