‘Nothing Holding It Back’

PERALTA CANYON — It was barely drizzling last Sunday evening when Paul Herrera heard the water coming.

At 6:03 p.m., National Weather Service forecaster Brian Guyer, watching a thunderstorm on radar, had issued the latest in a series of warnings for Peralta and other canyons flowing out of the area burned by this summer’s Las Conchas fire:

“BOTH LOCALIZED AND DISTANT HEAVY RAINFALL WILL CREATE SUDDEN AND DANGEROUS FLOWS IN ARROYOS AND OVER LOW WATER CROSSINGS.”

Herrera, a 34-year-old ranger, has lived in Cochiti Pueblo his whole life and knows what to expect when a summer thunderhead settles in over Peralta Ridge and Bearhead Peak to the northwest. It doesn’t have to rain where you are. If the storm hits the mountain watershed above, you can expect the normally dry Peralta Canyon arroyo to begin flowing in about two hours. But nothing prepared him for the flood that hammered out of the mountains that day.

It sounded like a jet engine. He could feel the ground shake as a wall of water pushed rolling, tumbling boulders.

Down the road, Floyd Pecos, the 50-year-old field manager at Kasha Katuwe-Tent Rocks National Monument, shooed away the last tourists, then watched as water crested the road.

Summer flash floods here are normal. Pecos, Herrera and the other members of the crew of rangers on duty that day knew to watch for water when storms hit the mountains above. But the volume, they all said last week as they surveyed the damage left behind by the flood flows, was unprecedented.

Fire in the watershed, denuding the upstream landscape, had cleared the way for water to run off in a hurry. “There’s nothing holding it back,” Pecos said.

Firm numbers for the latest round of post-Las Conchas fire flooding are hard to come by, in part because some of the equipment officials hoped to use to measure the flows was destroyed by high water. But studies done after previous Jemez Mountains fires suggest that flows from heavily burned areas can be 100 times as great as the water running off an unburned watershed.

Cochiti Pueblo and Kasha Kathwe-Tent Rocks National Monument, located on higher ground out of the Peralta Canyon floodway, escaped serious damage, according to Pecos. But two canyons to the north, the famed Dixon Apple Orchard, located on the Cochiti Canyon flood plain, was devastated as two unprecedented waves of water roared out of the canyon, one last Sunday and a second the following day.

In Cochiti Canyon, a New Mexico Environment Department team estimated the Aug. 21 flow that caused the first round of damage at Dixon’s at 12,000 cubic feet per second of water — more than 10 times the entire current flow of the Rio Grande.

After that flood, Ralph Ford-Schmid and his Environment Department colleagues installed a new temporary flow meter in Cochiti Canyon upstream from the orchard. They put it six feet up in a tree on an embankment that was another 10 feet above the canyon bottom. “We thought it was safe that high off the ground,” said Ford-Schmid.

It was not. It was battered by Monday’s flood pulse, and Ford-Schmid said a calculation of the height of the water the second day suggested 16,000 to 19,000 cubic feet per second.

Kerry Jones of the National Weather Service, who was at the Dixon orchard surveying the first day’s flood damage when the second flood arrived, agreed with Ford-Schmid that the second day’s flood in Cochiti Canyon was larger.

U.S. Geological Survey scientist Craig Allen guessed that the flooded canyons had not seen flows that great in more than a thousand years.

As Jones walked the site on a visit later in the week, the landscape was so changed that he struggled to find spots he’d photographed two days earlier, before the flood completely rearranged the landscape.

Along one side of the orchard buildings, the second flood pulse had cut a channel more than 10 feet deep. Elsewhere, it had deposited a thick layer of gray sediment.

“This stuff is probably feet thick,” Jones said as he walked across the moonscape left behind by the flood.

Mountainside soil turned by flames into a waxy crust, allowing water to run off quickly rather than soaking in, is part of the reason flooding is so much worse following a fire, explained Grant Meyer, a University of New Mexico scientist who studies the erosion deposits left behind as water moves across burned western landscapes.

But a bigger reason, Meyer said, is a landscape denuded of the pine needles, grasses and small plants that would normally slow down and soak up rainfall. “The key thing is the ground cover,” Meyer said.

On a normal forested landscape, Meyer said, only 2 percent of the water that falls as rain flows off and downstream. Following a major fire, as much as 75 percent of the water can run off the landscape.

Once the water gains momentum, it can begin cutting little rivulets into the hillsides, “almost like the whole thing’s cat-scratched,” Meyer said.

As the running water picks up fine soil particles, it becomes more dense, which allows it to carve deeper and carry ever larger rocks and boulders, feeding on itself, creating a thick heavy slurry of flowing debris and gaining momentum as it goes. Jones’ photos of the second-day Cochiti flood show a wall of thick gray slurry plowing through the Dixon apple groves.

After the 2000 Cerro Grande fire burned nearby areas in the Jemez Mountains, peak flows in canyons downstream from the burn scars were measured at 100 times pre-fire runoff levels, according to analysis by Los Alamos National Laboratory and the Environment Department.

The extreme nature of the Las Conchas fire made things worse, Meyer said. Before human firefighting efforts, the forests burned frequently, but at low intensity. The kind of high-intensity blaze seen in Las Conchas and Cerro Grande, now common because of fuel buildup, was rare in past centuries. So floods like those seen in Peralta and Cochiti canyons over the last two weeks would have been extremely rare as well, Meyer said.

The lessons of the new floods are not lost on those spending their days right now in the flood zone.

“I hate to say this, but those storms building up there? They look similar,” Jones said Wednesday afternoon, pointing to fresh thunderstorms looming to the west. A bit later, a colleague back at the Weather Service office in Albuquerque who was watching the radar sent him a text message: “Get outta the canyon.”

Pecos, sitting on high ground above the Peralta Canyon arroyo, knew what to expect. “It’s going to flow for sure,” he said.

Loading ...