SACRAMENTO, Calif. — Yosemite National Park officials wasted little time responding to the first reports of a fire burning in the Mariposa Grove on the afternoon of July 7.
The Washburn Fire started that afternoon along the Washburn Trail within the grove of iconic giant sequoia trees. By 2:45 p.m., the park’s Helicopter 551 was dropping water on the fire and ground crews had been dispatched to the area.
But two other aircraft assigned to another fire flew toward Yosemite in those early hours to offer an air drop of fire-retardant chemicals on the Washburn Fire. That drop, however, was declined by the park’s incident commanders.
“There was difficulty establishing communication with the unassigned aircraft as they did not initially have the proper (radio) frequencies for the incident,” a fire information spokesperson for Yosemite National Park told The Fresno Bee by email this week.
“This created momentary confusion for the firefighters on the ground and Yosemite Park Dispatch” because aircraft typically don’t arrive at an incident without being requested by incident commanders.
Once radio communication was established, ground crews learned that the pilots were offering to drop their load of retardant on the fire. Park officials cited two reasons why that offer was declined.
“The first was public safety. The fire had necessitated the evacuation of Mariposa Grove, which began at 2:24 p.m. and continued for approximately 90 minutes,” the email to The Bee said. “Somewhere around 450 people were being moved out of harm’s way from the Mariposa Grove to the Welcome Center two miles away” in park shuttles, private vehicles and on foot along routes close to the fire on the Mariposa Grove Road.
“On all fire incidents, retardant drops on people are avoided in all but the (most dire) circumstances,” the spokesperson wrote.
Another factor was how close the fire was to a fork of Big Creek. Under federal interagency policies, “firefighters and pilots are instructed to avoid applying retardant within 300 feet of riparian areas” (streams or rivers), the email explanation said.
“Because of the thick, dense canopy of the giant sequoia forest, it is unlikely the unassigned air resources could see either the creek or the people evacuating below them.”
Within several hours, however, other aircraft were requested and assigned to the Washburn Fire on the first day, including two medium-capacity helicopters, two large-capacity helicopters, two large tanker airplanes and three smaller tanker planes, and two air attack airplanes that fly surveillance over incidents and lead tankers on their drop points.
Wildfire retardant is mostly water, bur contains chemicals such as ammonium phosphate fertilizers and other ingredients including colorants, corrosion inhibitors, thickeners and bactericides, according to information from the U.S. Department of Agriculture.
The bright red hue that’s added to the chemicals helps pilots see where previous drops have been made on a fire. The chemicals are typically not dropped directly on flames, but outside the wildfire perimeter to limit the spread of the fire.
The U.S. Forest Service, in a 2011 environmental assessment on the use of aerial retardant chemicals reported that “in certain rare situations, when fire retardant comes in contact with water, the fire retardant chemicals can temporarily alter the water quality and may be toxic to aquatic organisms.”
For birds and land-dwelling critters such as deer or bears that are able to flee fire areas, there was minimal risk from the aerial chemicals, the assessment found. But for critters with more limited mobility, potential harmful factors “include the coating or covering of vegetation and food sources consumed by terrestrial species.”
“The level of ingestion of retardant on vegetation or insects depends on the amount of fire retardant used …, timing of ingestion after application, and the ability of an animal to avoid feeding on food sources bearing the chemicals.”
