Rio Grande Streamflow Mystery: Solved?
In which we continue to ponder what's going on at Otowi Bridge
In one of last week’s dispatches, I asked: Why did the Rio Grande’s peak streamflows at the Otowi Bridge suddenly diminish beginning in about 1945?
Now, I’m not talking median flows or the total volume of water that passed under the bridge. I’m talking peak streamflows here, meaning the highest measured flow during a given water year (Oct. 1-Sept. 30). Before 1945, it was normal for the river to shoot up to 10,000 cubic feet per second or above during spring runoff and, occasionally, in September or October. After 1943, 10,000 cfs was a rarity.
An easy explanation is that a large upstream dam was constructed around that time, smoothing out the peaks and troughs of the river’s flow. Platoro Dam was constructed on the Rio Conejos, a Rio Grande tributary, in 1951.
But I’m convinced other factors must be in play. So I put the question to Twitter, where I received an overwhelming—and informative—response. Several folks mentioned the Middle Rio Grande Flood Control Project, which was established following floods in 1941-42 and resulted in the construction of Abiquiu Dam on the Chama River, one of the Rio Grande’s biggest tributaries:
John Fleck, a Western water expert, brought up the 1939 Rio Grande Compact, which divided the Rio’s waters between Colorado, New Mexico, and Texas.
Climate change was also offered up: Perhaps snowpack levels dropped and temperatures climbed around that time:
Or maybe it was beavers! (From Ben Goldfarb, of course, author of Eager: The Surprising, Secret Life of Beavers and Why They Matter)
Jerry Zink suggested that maybe the San Luis Valley had begun to dry up by that time due to overgrazing and decades of groundwater pumping, so that it began acting like a giant sponge, absorbing the runoff before it could make it to New Mexico. Jim O’Donnell said he suspected it had to do with San Luis Valley irrigators and the timing of their release dates.
Similar explanation, with a twist:
Ah, yes, the water-energy nexus!
Zink suggested looking at other streamflow gages to see if they showed a similar phenomenon. Hello! Why didn’t I think of that? First, let’s get oriented. Notice the Otowi Gage near the bottom of the map.
And here are peak streamflows for the Otowi Gage and the Embudo Gage. Embudo is one of the few gages on the Rio Grande that goes back to the late 1800s (though several years are missing). Note it is above the confluence with the Chama River.
Since a similar drop off is apparent at Embudo, it’s safe to say that the Chama River is not solely to blame. So let’s go further upstream to Del Norte, which is not only upstream from Platoro Reservoir, but also most of the San Luis Valley diversions (with the exception of the very large Rio Grande Canal, that is). Rio Grande Reservoir, built in 1910, is also upstream from Del Norte; it seems to have had zero effect on peak streamflows.
The pattern kind of repeats at the Del Norte gage, but the big peak flows seem to drop off in the late 1920s rather than in the 1940s. Notice the huge floods of 1911 and 1927. They just don’t make ‘em like that anymore.
Okay, now let’s look at the Rio Chama. This is the gage at Chamita, which is below Abiquiu and El Vado dams and just above the confluence with the Rio Grande. Unfortunately, records only go back to 1915.
Interestingly, neither of the dams had an immediate and noticeable effect on peak streamflows. Were they doing big releases for some reason? Or maybe the Rio Ojo Caliente—which runs into the Chama below Abiquiu Dam—raged during the 1970s?
So, where does this all get us? I don’t know. But it does seem like the key lies in the San Luis Valley. It appears that most of the Valley’s big canals were built between 1880 and 1900 or so. During World War I San Luis Valley farmers dug a lot of wells, perhaps to utilize groundwater that had seeped into the aquifers from irrigation. And then, in 1926, a drainage canal was built from the low point of the Valley (east of Saguache) to near Alamosa to alleviate the swamp-like conditions irrigation had caused. If the Valley’s a sponge, it was saturated by then.
Rural electrification roughly coincided with the signing of the Rio Grande Compact in the late 1930s. Both could have spurred more groundwater pumping. And the construction of Platoro Dam in 1951 also seems to have had some effect.
Another possibility is that climate change-related aridification and warming dried out the watershed enough that when big rains or snowmelts did come, the earth absorbed more of the water, diminishing downstream flows. In order to test that theory, I turned to my reliable control: The Animas River in Durango. There are no dams and few diversions upstream. Records go back to the late 1890s. And the Rio Grande’s headwaters are quite close to those of the Animas.
Well, there you have it folks. I’d love to hear your thoughts on this phenomenon now! I think we can rule out the Manhattan Project. But beavers? Well… what do you think?
In the meantime, I’ll leave you with this passage from David Starr Jordan’s 1889 report on rivers in Colorado and Utah and the fish that plied their waters. It doesn’t solve the mystery, but does show how big a factor ditches and diversions can be.