If you’re following flows on the Rio Grande through New Mexico this spring, no doubt you noticed the big drop this morning in releases from El Vado Reservoir on the Rio Chama. (Of course you noticed, right?)

Welcome to what we in the UNM Water Resources Program have come to call “the institutional hydrograph”. It is when the rule, not the climate, becomes the dominant variable influencing flow in a river. It happens all the time.

Up until this morning, inflows and outflows from El Vado Reservoir were roughly in balance. The La Puente gauge, seen in brown, is the inflow. The green line is releases from the dam. You’ve got a nice diurnal cycle at La Puente, which the dam smooths out, but basically whatever water flowed in was simply being passed through the dam.

Rio Chama flows

The river is operated this way because of Article VII of the Rio Grande Compact:

Neither Colorado nor New Mexico shall increase the amount of water in storage in reservoirs constructed after 1929 whenever there is less than 400,000 acre feet of usable water in project storage….

“Usable water in project storage” is, roughly speaking, the amount of water sitting in Elephant Butte Reservoir. “Reservoirs constructed after 1929” includes El Vado and Caballo Reservoirs. Basically this means that when Elephant Butte is empty, we (Colorado and New Mexico) can’t store water upstream, we have to send it all down to Elephant Butte. The accounting rules here can get a bit arcane, but over the weekend “usable water in project storage” topped 400,000 acre feet, so – boom! – we can start storing water in El Vado, cutting flows on the Rio Chama instantly.

There’s so much water piled up at the next reservoir downstream, Abiquiu, that the cut in Chama flows shouldn’t be seen here in Albuquerque, where the river has been edging out to the levees with some of the highest flows in years. And there’s so much snow still in the upper elevations in the headwaters that this will be going on for a while.

So if you’re in Albuquerque, head out to the river, early and often. It’s an amazing sight.

Rio Grande, overbanking into the bosque, Albuquerque, New Mexico, May 12, 2019