Saguaros don’t have bark. That’s the shortest explanation of why fire in the McDowell Mountains is a different problem from fire in a pine forest. A ponderosa forest is built to burn every few years, and its trees survive it. The Sonoran Desert isn’t. A McDowell Sonoran Conservancy biodiversity fellow, Kara Barron, wrote in 2018 that the desert “historically incurred fires only once every 100 to 1,000 years, so it is not considered fire-adapted.” Its plants grow in clumps with bare ground between them, and that bare ground used to stop most fires before they started.
Over the last thirty-odd years the McDowells have burned repeatedly anyway. This is the record as far as the public documents carry it. The blackened stretches you walk past on the trail are easier to read once you know it.
Why the desert burns now
The city, the county and the Conservancy tell the same story. Non-native grasses filled the bare spaces. Maricopa County’s 2019 master plan for McDowell Mountain Regional Park says historic cattle grazing and homesteaders brought more grass into the country. It says buffelgrass in particular was introduced to repair overgrazed land, and that it “increased the fire fuel load across the Sonoran desert, making damaging fires more likely by providing vertical and horizontal contiguous fuel load.” The Conservancy adds fountain grass and red brome to the list. These grasses “fill in these open spaces, outcompete native plants for space and resources, and increase the fuel load and likelihood of hotter, larger, and more frequent wildfires.” A wet winter grows a crop of grass. A dry June cures it. Then a spark finds it.
Then there’s people. The county plan notes that fire wasn’t historically common in this kind of desert, but “with intrusion of human influence and adjacent development, it is more of a risk today.” The Conservancy points to the “increase in surrounding urban development” around the Preserve’s narrowest corridor as part of the same problem.
1993: the TP Fire at Lost Dog Wash
The best-known McDowell fire story comes from the Conservancy’s history column, “Myths and Mysteries of the McDowells.” In 1993 a survey crew was working in Lost Dog Wash, assessing a site for a diversion dam. One surveyor stepped into the brush to relieve himself and, wanting to leave no trace, burned his toilet paper. “It was a breezy day. The burning paper cartwheeled along the desert floor and through its dry grasses. Two days and 2,000 charred acres later, the fire was extinguished. Officially, this burn is known as the Ancala Fire. Unofficially, local fire personnel refer to it as the TP Fire.”
The column says the 2,000-acre burn “flanks the Lost Dog Wash Trail,” which starts at the Lost Dog Wash Trailhead at 12601 N. 124th St. The Conservancy’s account is the only one we’ve found. The surveyor isn’t named, and the column doesn’t give a date beyond the year. Contemporary Arizona Republic coverage, or Rural/Metro’s incident records from the years it was Scottsdale’s fire department, would confirm the acreage and the date.
1995: the Rio Fire
The big one came on July 7, 1995. Maricopa County’s park plan says lightning from a summer storm, 30-mile-an-hour winds and “aggressively-growing non-native grasses” combined into the Rio Fire, which “burned nearly two-thirds of the Park’s total acreage (approximately 14,000 acres in all).” The plan elsewhere calls it the “Rio Verde Fire.” It says the damage “is still visible today” and that it will be “at least another six (6) decades before the native saguaro forest returns.” Because so much grass burned away, the county rated the park relatively low for fire hazard afterward.
The Rio Fire also crossed into what is now Scottsdale’s Preserve. The Conservancy has run “Fire in the Desert” education hikes from the Tom’s Thumb Trailhead that visit “the 1995 Rio Fire burn area” and then compare it with the Gooseneck Trail. Our own park page puts the fire’s total at roughly 23,000 acres over three days. The county plan’s “approximately 14,000 acres” could be read as either the total or the park’s share, and we haven’t found the incident report that would settle it.
The city’s 2015 Cultural Resource Master Plan adds a second year. It says portions of the Preserve’s eastern mountain slopes and pediment “were swept by wildfires in 1995 and 1996.” The 1996 fire isn’t named in anything we’ve read.
A quarter of the Preserve, by 2018
In 2018 Barron’s Conservancy article added up the damage: “Scottsdale’s McDowell Sonoran Preserve has had four fires over the last 30 years, burning approximately 11,000 acres, or one-fourth, of the land.” The article doesn’t list the four. The 1993 and 1995 fires are surely among them, and probably the 1996 fire. The arithmetic is a little loose. The Preserve today covers about 30,500 acres, against which 11,000 acres is closer to a third. The figure may count burns that overlapped or fell partly outside the boundary.
Her study is the most useful thing written about what fire does here. The burned areas “were left to recover naturally.” Botanists and stewards set up three plot lines inside old burn perimeters and three on unburned land nearby, and measured them in 2016 and 2017. The total plant cover in the burns was “approaching their pre-fire state.” The mix of plants was not. Little-leaf palo verde, creosote and cacti, the plants that define this desert, “showed a significant decrease in cover in the burned plots.” Globemallow and brittlebush moved in. Annual wildflowers such as scorpion weed and fiesta flower became scarcer, while western tansy mustard became more common. “Some experts think that deserts need 75 years or more to fully recover after a fire,” she wrote. Burn the same ground twice and “some species might drop out of the plant community completely.”
2023: the Diamond Fire
On Tuesday, June 27, 2023, at about 4:50 p.m., a fire started near 130th Street and Ranch Gate Road in north Scottsdale, just northwest of the McDowells. The Arizona Department of Forestry and Fire Management’s incident page describes a “wind-driven fire” in grass and brush that “moved quickly throughout Tuesday afternoon and evening.” The fire forced the evacuation of more than 100 homes and 1,145 residents. Large and very large air tankers dropped eight loads of retardant that first day. About 150 personnel were assigned, including a Forest Service hotshot crew. The evacuations were lifted at 7 p.m. the next day. The Diamond Fire finished at 1,960 acres. As of the incident page’s last update in July 2023, the cause was “Unknown, Under Investigation.” We haven’t found a published finding since.
Where it burned matters more than how much. The Conservancy says about 270 acres were inside the Preserve and the rest in the county park. The part inside the Preserve was the Gooseneck, “the narrowest part of the Preserve,” the strip that joins the northern and southern halves near Dynamite Boulevard and Rio Verde Drive. Wildlife biologists already worried about it because of the roads and new houses around it. In the Conservancy’s words, “With the recent burn, the Gooseneck is more vulnerable.” Our note on the Gooseneck and the Preserve’s field science explains why that corridor matters to deer, mountain lions and tortoises.
The response this time was active rather than wait-and-see. The Conservancy’s Parsons Field Institute, the City of Scottsdale, Maricopa County Parks and Northern Arizona University began monitoring the burn, reseeding, and testing whether nurse plants help young saguaros survive. The city’s 2024 Ecological Resource Plan notes that saguaro study plots have been measured in the Diamond Fire area since 2023. In May 2024 the Field Institute devoted its fourth symposium to fire and heat in the Preserve. That October it held a public lecture on community-driven restoration after the Diamond Fire.
How to read a burn from the trail
- Look at the saguaros, not the ground. Grass and shrubs come back in a few years. Saguaros don’t. A slope with plenty of brittlebush and few big saguaros, next to a slope with a normal saguaro forest, is usually a fire line. The county says the Rio Fire’s footprint is still visible in its park.
- Brittlebush and globemallow are clues. Barron’s plots found them filling in where palo verde and creosote had burned.
- Grass is the warning. Thick, cured grass between the shrubs is fuel. The city and the Conservancy pull buffelgrass and fountain grass every year for that reason.
- Walk the Gooseneck. The Fraesfield Trailhead and the Tom’s Thumb Trailhead both connect to the Gooseneck Trail. That puts you in the Rio Fire’s 1995 ground and near the Diamond Fire’s 2023 ground on one outing.
What you can do
No fires are allowed anywhere in the Preserve. The city code permits charcoal or camp stoves only in designated areas. The county park bans all fires, including charcoal grills, during dry periods; a typical ban runs from May 1 through September 30. Don’t smoke on the trail. Don’t park a hot car on dry grass at a pullout. If you burn your toilet paper, you are now part of a story the Conservancy has been telling for thirty years. Pack it out instead.
Sources: Maricopa County, McDowell Mountain Regional Park Master Plan Update 2019–2039; City of Scottsdale, Draft McDowell Sonoran Preserve Cultural Resource Master Plan (2015) and 2024 Ecological Resource Plan Update (working draft); McDowell Sonoran Conservancy, “Myths and Mysteries of the McDowells,” “Effects of Fire on the Preserve” (Kara Barron, 2018), and Ecological Restoration program pages; Arizona Department of Forestry and Fire Management, Diamond Fire incident page on InciWeb (July 2023).