Sabino Canyon's new scars

The phenomenon is causing scientists with the U.S. Geological Survey and the National Weather Service to reconsider their long-held belief that homes in the Catalina Foothills that were built near historical evidence of "debris flows" are totally safe in our current climate.

Scientists also warn that any repairs to roads, trails and recreation facilities in Sabino Canyon could easily wash out again and that it won't take record rains to overtop Pima County's flood control channels now that all that sediment has been released into the Rillito and its tributaries.

Meteorologist Erik Pytlak said the National Weather Service is participating in the multiagency study because it needs to know whether to add rock slides to its list of warnings and alerts.

"We've always been under the assumption . . . that debris flows are not a concern here," said Pytlak of the National Weather Service in Tucson, "This is a problem similar to that in our offices in Los Angeles and San Diego," he said.

Pytlak, at a seminar held Wednesday at the Arizona Water Science Center, said the rain that caused mountain creeks and urban rivers to overflow their banks in the storm was a rare occurrence, "more than a 100-year event."

To geologists, it's an even rarer event. The Santa Catalina Mountains haven't eroded in this fashion for close to 10,000 years. A team of scientists is assembling to study the phenomenon.

A series of storms hit the Catalinas in the early morning of July 31, dropping up to 6 inches of rain in a seven-hour period, Pytlak said. That was unusual enough, but what made it historic was that the rain fell heaviest, not atop the mountains, but on the lower slopes of the front range and that it followed five days of significant rainfall that had already saturated the area.

The canyon walls came tumbling down. Robert H. Webb, a hydrologist with the U.S. Geological Survey in Tucson, said researchers have since mapped "240 slope failures in a place where only five are known to have occurred historically." The failures were found from Ventana Canyon on the west to Soldier Canyon on the east and included Rattlesnake, Sabino and Bear canyons.

The flow down Rattlesnake Canyon, a usually dry tributary of Sabino Creek, contained up to 90 percent debris, he said — rocks, sediment and boulders. Where it crossed the road at Sabino Canyon, it clogged the culvert, washed out the roadbed and buried the creek area in 15 to 20 feet of debris. It remains the first roadblock to clearing the popular canyon for visitors.

Webb said 19 major debris flows tumbled into Sabino Canyon that morning, followed by a flood that carried sediment downstream.

Some of the increased runoff can be traced to the 2003 Aspen Fire. But Webb said the major debris flows occurred in areas that weren't affected by the fire.

Sabino Canyon itself looks the same in some places. In others, it is scoured to bedrock or buried in sediment trapped behind dams of boulders and rocks. "Those sediment plugs will have to, at some point, come downstream," said Webb.

To the east, in Soldier Canyon, "whole hill slopes gave way," said Webb, cascading toward the valley below. Finely ground material mixed with water to speed rocks and boulders along, flowing like concrete from a cement mixer.

The Soldier Canyon debris flow made it through the huge box culverts under Catalina Highway but clogged the smaller culverts a short way downstream at Mount Lemmon Short Road. A boulder dam filled the channel, sending water into four houses there.

Residents were lucky, said Webb, that it was "run-out" from the debris dam and not the debris flow itself that hit them.

The county hasn't quite figured out what to do about Mount Lemmon Short Road, said Suzanne Shields, director of the Flood Control District, and is seeking help from Webb and the USGS scientists.

"The channel, from the road up 400 to 500 feet, is totally filled in. It redirected itself," Shields said. "It's going to take some design. It's not something we can just fix overnight. Some of those rocks are the size of Volkswagens."

Shields said the county is also measuring sediment buildup in the Rillito to see what needs to be removed or pushed around.

Shields said the county is being extra alert to future flooding in areas where debris flows occurred. "This is going to change how those watersheds react to rainfall," she said.

Shields said she isn't ready to change her view of the usually stable geology of the Catalinas in response to one rare event.

"We're looking at it and it's key to have both the USGS and the weather service looking at it. It was, though, an extreme event, a very unusual storm," she said.

Webb said scientists had predicted "this kind of thing could happen here, just not necessarily under present-day climate conditions. There is debris-flow evidence near where people are building houses in the Catalina Foothills. Everyone thought that was old, had no bearing on the present-day situation.

"Nobody dated them (debris flows) because we thought they had nothing to do with present-day. We're planning on dating them now," he said.

Wednesday's report included the work of a team of scientists, including: Webb and Peter Griffiths of the U.S. Geological Survey; Pytlak and hydrologist Mike Schaffner of the weather service in Tucson; and Philip A. Pearthree of the Arizona Geological Survey.

On StarNet: See photos from this epic monsoon season at azstarnet.com/ monsoon

● The U.S. Forest Service is holding a public meeting on the future of Sabino Canyon on Saturday from 10 a.m. to noon at the Best Western InnSuites Hotel, 475 N. Granada Ave.

Information about the changes wrought by flooding and the potential for continuing damage will be presented and participants will be asked to comment in writing. Comments may also be submitted online at www.fs.fed.us/r3/coronado

● Volunteers are needed to help "dig out" Sabino Canyon in honor of National Public Lands Day on Sept. 30.

Participants will help dig out picnic tables buried in sand and debris and remove invasive species of plants.

A slide show on the flooding will be presented that same morning by Friends of Sabino Canyon, which holds its annual meeting at the Visitor Center at 10 a.m.

Bring snacks and water and wear sturdy shoes, sunscreen and appropriate clothing. Bring a shovel and work gloves.