Big flood unlikely but possible

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There aren’t many old-timers around who can remember the huge 1929 flood when the Rio Grande raged through the valley, wiping out the little town of San Marcial south of San Antonio, or the last major Socorro County flood in 1944.

But events like last week bring the topic of river flooding into many Socorro conversations. What’s our chance of another major flood?

According to experts, not much.

“We had a classic heavy rain pattern for flooding on the Rio Puerco and eventually the Rio Grande,” said National Weather Service meteorologist Kerry Jones. “You had rain developing day after day. Rarely do we get this fire hose effect for six days.”

If last week’s event, with potentially even higher flows in the river, had occurred in the 1940s, he said the river certainly would have flooded. Cochiti Dam on the main stem of the Rio Grande and structures installed in tributaries lessened the amount of runoff that reached the river, even from arroyos draining areas burned during this year’s fires.

“You have to look at all the flood protection upstream,” said Middle Rio Grande Conservancy District hydrologist David Gensler. “Cochiti Dam prevents main stem flooding. Most of the burn arroyos end up in Cochiti. Cochiti released 350 cubic feet per second the whole time, which is normal for the last several weeks of September. That much water wouldn’t have reached Albuquerque. Once it started raining, they kept the release but wound up storing 20,000 acre-feet to protect the middle valley from flooding.”

An acre-foot of water is enough to cover one acre of land one foot deep — 325,851 gallons.

Most of the rising flow came from runoff from arroyos and tributaries, such as the Galisteo River.

“We saw a flow at San Felipe gage at 9,000 cfs,” Gensler said. “All that water came in from the arroyos below Cochiti. A lot of the biggest water came in from Galisteo that enters the river at Santo Domingo Pueblo. The concrete arroyos through Albuquerque also produced some big waters.”

Jones said side channels and ponds built to provide minnow habitat in the Albuquerque reach of the Rio Grande slowed much of the flow arriving from below Cochiti Dam.

But a major tributary contributing flow to the Socorro reach is the Puerco, 46 miles below Albuquerque. The Puerco was the star of last week’s rain event, funneling around 10,000 cfs into the Rio Grande at one point.

The small Crownpoint Dam near the headwaters of the Rio Puerco held back some of that runoff as well as other even smaller structures built on that and other tributaries feeding the Rio Grande, Jones said. Otherwise, the flow could have maxed out much higher.

“I don’t think we should be too complacent,” said Fred Phillips, New Mexico Tech hydrology professor and author of the book “Reining in the Rio Grande.” “To get comparable flows now you would have to have the same amount of moisture dumped on the headwaters of the Rio Puerco but for a much longer period of time. Saturate the system to fill it all the way up, give it a enough time to soak into the flood plain. It would take a much more extreme meteorological event, which becomes increasingly unlikely.”

According to Phillips, what protects Socorro now is better land management practices. The two main tributaries of the Rio Grande — the Puerco and Salado — are essentially undammed.

“In the case of a really massive flood, the Crownpoint Dam (on the Rio Puerco) is a brick wall in the way of a hurricane,” he said. “There are no large flood control structures on either river that are capable of holding back major flows.”

The Rio Puerco in the 1940s was a wide, deep, sandy channel that funneled massive runoff caused by overgrazing directly into the Rio Grande.

“At that time, the Rio Puerco was essentially a sluice box, an enormous slot carved into the landscape in many places 50 feet deep and a couple of hundred feet across,” he said. “The land upstream was heavily overgrazed back then, so it (rain) runs off really rapidly and gets into this gorilla of a channel network that just sluiced all this water down into the Rio Grande in very short order.”

Improved land management reduced average peak flows on the Puerco from 11,000 cfs in the 1940s to 1,200 cfs in the last decade. The all-time highest flow was 18,000 cfs in 1941. The 2006 peak flow was 6,200 cfs.

Now, runoff is slowed by grasses and salt cedars. More water soaks in, reducing runoff into the Puerco and eventually the Rio Grande.

“Now, you have a fairly narrow meandering channel just choked with vegetation on the sides that continues a long, long ways upstream,” he said. “When you do have a big flood peak coming down, it isn’t confined to a very large channel that can very efficiently convey it to the Rio Grande. Instead, it’s spread out over this floodplain filled with salt cedar. It’s a good lesson for watershed conservation.”

That said, a massive flood is still possible.

“This pattern is very similar to patterns that caused floods in the past,” he said.