Record El Nino, wrong states: why this winter is noise for Colorado River siting

The distance to the bottom is easy to compute and easy to over-read. Powell's Thursday reading of 3,519.4 feet leaves 149.4 feet of elevation above the 3,370-foot level where water stops flowing downstream, and Mead's 1,039.6 feet leaves 144.6 feet above its 895-foot equivalent ^{[3][4][18][19]}. Those are heads at the dam, not volumes, and there are no storage figures here to convert them with, so the gap is a countdown in feet and nothing more precise than that.

The geography is what decides whether this winter counts for anything. The river is fed primarily by snowmelt out of Colorado, Utah, New Mexico and Wyoming, and that melt is what fills the reservoirs the Lower Basin draws on ^[8]. Zeke Hausfather of Berkeley Earth describes the reliable El Nino signal as a wet winter across the southern US, "from Southern California through Arizona and New Mexico to the Gulf Coast" ^[9]. Put the two lists side by side and New Mexico is the only name on both; Colorado, Utah and Wyoming, three of the four snowmelt states, fall outside the band ^[20].

Where the scientists part company is on whether magnitude changes that answer. Hausfather notes that the last very strong event, in the winter of 2015-16, produced a normal amount of Upper Basin snowfall, and that peer-reviewed correlations between El Nino and Upper Basin snowpack or streamflow are modest at best ^[10]. Daniel Swain of University of California Agriculture and Natural Resources argues that a normal El Nino and a "ridiculously strong" one are almost two distinct things, which leaves a chance that some Upper Basin states pick up a share of the precipitation ^[11]. As a planning input, that is an argument from the absence of an analogue rather than from a record of one.

Temperature is doing its own work either way. The past season combined record warm western temperatures, made much more likely by climate change, with some of the lowest snowpack on record, and warmer winters across the West are making snow droughts more common ^[15][16]. Precipitation that arrives warm is runoff, not storage.

The deficit itself was written on the demand side. The river's water has been over-allocated for decades to thirsty industries including agriculture, and the century-old compact assigning the rights runs out at the end of this year ^[14][13]. "People have talked about the Colorado River crisis for decades, we're now in it," says John Berggren, a policy manager at Western Resource Advocates ^[17]. A siting decision underwritten by one winter's snowpack is a bet on weather against an allocation that already promises too much to 40 million people in seven states ^[1][14].