In a bend of the Canada del Oro below Biosphere 2, artist Cate McCarthy pointed out the water running like black coffee just feet from the historic home site she shares with her husband, Giovanni. The rocks along the creek are splashed with a thick gray-brown mud, an ominous watermark as high as 10 feet in places.
"I haven't seen it this high in a long time," she said. "We've been here four years and we used to watch the Canada from the rental (next door) for four years before that."
During the monsoon storm last Thursday night, McCarthy, 48, thought she heard car doors slamming and went outside to see who in the world would be visiting at that hour.
The noise turned out to be boulders crashing down the creek. "The Canada just raged," she said. "It scared the living bejesus out of me for half an hour."
By Friday afternoon, 300 sandbags had arrived, along with a friend's work crew to help protect the couple's well.
With each monsoon rain, black soot, ash and other debris from the Aspen fire in the Santa Catalinas courses down the mountain, the danger of flooding increased because of the mountain's fire-denuded slopes.
The black sludge reminded McCarthy, a former potter, of thick, viscous slip, a clay suspension used in ceramics. "This has a lot of ash," she said. "It would a pretty good Raku glaze."
The headwaters of the CDO start at Shovel Spring, just below Samaniego Ridge. The drainage heads north towards Oracle and then makes a U-turn, returning south into Tucson.
"If we have heavy rainfall on that watershed, we could see more flooding coming through there," said Heidi Schewel, public information officer for the Coronado National Forest. "It's more dangerous since the fire because there's less vegetation to hold back flooding."
McCarthy bought flood insurance last summer after the Bullock Fire on Mount Lemmon.
"Nobody knows what's going to happen this year, although it's been raining pretty consistently above us," she said. "During the day you can see the clouds developing up on the mountain."
Farthest from the creek is a historic ranch house, most likely built in the 1860s when the Samaniego family established a cattle station in the canyon. McCarthy only recently began restoring the house. "At least the historic house is safer than the studio or the guest house," she said.
Downstream, the forest service is more concerned about flooding in Sabino Canyon than in the CDO. "The CDO has a wider stream channel and a flood plain at lower elevations," Schewel said. Sabino 'is just a narrow, rocky canyon."
In Oro Valley, roiling black floodwaters in the lower Canada del Oro Thursday night swept two vehicles into the wash at the Overton Road crossing and trapped one in the wash at North La Cholla Boulevard. The occupants were unharmed.
"The third car ended up lodged in a sandbar several hundred yards west of the crossing at La Cholla," said District Chief George Good of Rural/Metro Fire Department. Rescuers found the driver sitting on the roof, cigarette in one hand and cell phone in the other.
"We believe runoff played a key part in that incident because of the color of the water," Chief Good said. "In years past, we'd get a certain amount of runoff based on the amount of rain - this year the water is coming through areas that have been eroded by the fire. We saw this in Sabino Creek and some other areas as well.
"We were told by the Forest Service that the runoff is going to increase between two and eight times normal," he said. "We don't know what that will mean for the CDO wash." But he suspects that the same storm that caused no problems last year could be a hazard this year. "Even though that fire is out," he said, "we expect that the after effects will be long lasting."
Annabelle Quihuis, spokesperson for Pima County's transportation and flood control district, said motorists need to avoid dip crossings like Overton and North La Cholla during flood conditions. "The drainage funnels over the top of the road during a storm - you're not supposed to cross when it's flooded," she said. "You're trusting that your car will get you through there and then all of a sudden you're stuck - that's the chance you take."
The black sludge may not be aesthetic, "but it's not harmful," Quihuis said. "The first couple of rains - flushes -will run dark. But it will clear up. We can't say when because it depends on the weather."
The ash, soot and other debris coming down the sludge in the CDO are just part of "nature's cycle," said Oro Valley town engineer Bill Jansen. "That stuff becomes a nutrient for vegetation."