At San Felipe Pueblo the afternoon of Friday, Sept. 13, the Rio Grande rose rapidly, fed by heavy storms hitting already saturated soil from what will go down as one of New Mexico’s wettest storms. Preliminary estimates from the U.S. Geological Survey’s Rio Grande valley stream gauges suggest it might have been the largest flood pulse since August 1967.
The flood pulse was impressive. But it is instructive to look at how much larger it would have been if not for a pair of flood control dams built in the second half of the 20th century to protect us from our habit of building our cities in flood plains. On Sept. 13, they were two dams in the right places at the right time.
At the U.S. Army Corps of Engineers’ nerve center off Jefferson in northeast Albuquerque, Ryan Gronewold was preparing as much as a week before the Sept. 13 event. Forecasters had warned him an unusual storm was on the way. “We had quite a bit of warning,” he said.
Jemez Canyon Dam, built in the 1950s, is an earthen structure on Santa Ana Pueblo, astride the Jemez River just upstream from where the river dumps into the Rio Grande. Gronewold dispatched corps staff to close the big dam’s gates, preparing to catch water if any flood flows came down the Jemez.
Upstream, Cochiti Dam, which plugs the Rio Grande itself just north of Cochiti Pueblo, also was ready.
Authorized by Congress in 1960 and completed in the mid-’70s, Cochiti is one of the largest and arguably the most controversial dam in New Mexico. Its completion drowned farmlands, summer homes and culturally significant sites at Cochiti Pueblo, and seepage beneath the dam waterlogged remaining pueblo farmlands downstream. In addition, the way it altered the Rio Grande’s flows has caused lasting environmental problems downstream, depriving the riverside ecosystem of natural floods.
But on Sept. 13, it did the job we gave it when we built it, which was to protect the Middle Rio Grande Valley from flooding.
As the rains came that Friday afternoon, Gronewold had done all he could to prepare. All that was left was to watch the weather radar and the river gauges and wait.
The San Felipe gauge began rising shortly after noon. The water came from spot storms hitting watersheds downstream from Cochiti, with no flood control dams to hold them back. By 1:30 p.m., according to preliminary estimates, water was flowing past San Felipe at 9,450 cubic feet per second. “Cubic feet per second” is the arcane measure that water mangers use to gauge river flows in the United States, and it might not make much intuitive sense to nonwater people. So here’s a benchmark: At San Felipe, that was the most water measured at that point since a summer thunderstorm on Aug. 10, 1967.
By the time that flood pulse hit Albuquerque at midnight, natural “attenuation” had cut the flood peak by half as the water spread out and slowed down, with 4,310 cubic feet per second of flow at the Central Avenue bridge.
That’s the part you saw if you were one of the legion of Albuquerque residents who went out that Friday and Saturday to watch the show.
Here’s what you didn’t see, because it didn’t happen. While 9,450 cubic feet per second of water was flowing past San Felipe, other heavy storms upstream were dumping another 10,000 cubic feet per second into the Rio Grande that Gronewold and his colleagues were catching behind Cochiti Dam. Another 1,000 cubic feet per second was headed down the Jemez River, with all those flood pulses timed such that each would have added to the other to create a flood through the Middle Rio Grande Valley. A post-storm numerical analysis by the corps concluded that, without the two dams, the peak flow at Central would have been three times as large.
The big levees flanking the river through Albuquerque itself would likely have held the water at bay, Gronewold said. But there are known weak spots.
“Just upstream of Bernalillo there’s a weak point,” Gronewold said. “That could have broken and potentially flooded Bernalillo.”
Around Bosque School and the Montaño river crossing on Albuquerque’s West Side is another vulnerable spot the flood control managers call “the Montaño gap.”
To the south, old levees could very easily have let water through in the Isleta-Bosque Farms areas that could have flooded communities all along the river in Valencia County.
In his monumental “Environmental History of the Middle Rio Grande Basin,” historian Dan Scurlock identified 92 significant floods since 1591. For better or worse, we’ve built our cities in a flood plain. But the dams we built worked, and Sept. 13, 2013, will not go down as No. 93.