I’m still sorting out how much snow fell in the high country from this storm, and how it might influence runoff forecasts. The short answer is that it was a great storm (storms plural?), but we’re so far behind that it’ll take a few more great storms to catch up. I’ll have more on that later today.
But this storm also raised an interesting question about the “rain-on-snow” problem. The early part of the storm was so warm that rain was falling on top of snow in the high country. You can see a bit of a bump in flow on the Animas in Durango on Saturday during the warmest part of the storm. What does that do to the water supply? Is it a problem?
This all depends on what’s happening downstream. If there’s a dam downstream to catch it for spring and summer use, it may not be much of an issue. But if it flows right past your diversion gates, then the downstream folks might see benefit, but you won’t.
Carolyn Donnelly, a Bureau of Reclamation hydrologist, said she saw less of a jump in streamflow than she expected from the rain-on-snow portion of the storm. That’s in significant part because it was cold enough that, even though some fell as rain, the precipitation froze before running off significantly, according to Donnelly. But to the extent that you did have a pulse of water flowing in the high San Juan Mountains, it would have flowed right past the diversion gates for the San Juan-Chama Project, which brings water beneath the continental divide for use in the Rio Grande Valley. Why? Because at this time of year, the gates are frozen shut. There’s no way to get that water, according to Donnelly.