Arizona Bighorns Using Overpasses
Years of research, planning, and development showed early signs of success when video and still photographs captured two rams crossing the recently completed wildlife overpasses near the Hoover Dam.
It marks a small, but significantly meaningful achievement that Arizona Game and Fish Department and Arizona Department of Transportation (ADOT) officials hope will minimize the impacts of desert bighorn sheep fragmentation through the Black Mountains and improve traffic safety.
The overpasses are the result of years of collaboration between ADOT, Game and Fish, the Federal Highway Administration, the National Park Service, and the Arizona Desert Bighorn Sheep Society. They are part of a construction project which widened an existing 15-mile section of US 93 south of the Hoover Dam from a two-lane road to a four-lane divided highway. The project was completed before Thanksgiving.
When word arrived almost a decade ago of the planned widening of Highway 93 and the Hoover Dam Bypass effort, Game and Fish personnel pondered the fate of bighorn sheep throughout the range.
The Black Mountains bighorn sheep herd holds the largest contiguous desert bighorn population in the nation and has supported transplants to help restore historical populations in Arizona, Utah, Colorado, and Texas.
Fragmentation and habitat loss remain the leading cause of wildlife extinction. A four-lane highway in this area was going to increase the "barrier effect" for bighorn sheep, making it even more difficult for them to cross safely from one side of the highway to the other and raising concerns about the long-term sustainability of the herd.
"There was no question we had to move quickly," said Bob Posey, supervisor for the Game and Fish office in Kingman. "We had to explore the options and build on the knowledge and relationships we’d gained through our previous coordination with ADOT and Federal Highways on the highly successful Highway 260 project east of Payson."
With several underpasses already planned for wildlife, the problem wasn't just whether the crossings would be in the right locations; it was whether those underpasses would be used by bighorn sheep.
Federal Highways provided Game and Fish with the necessary funding to conduct research near the Hoover Dam and along Highway 68 (from Kingman to Bullhead City), where several underpasses were already in place.
The results along Highway 68 were surprising.
"What we discovered was that while many bighorn sheep approached the underpasses, few actually used them to cross," Posey said. "It was time to think like a bighorn. It became clear these animals wanted to remain high, and crossing under a road wasn't their preferred option.
"Bighorn sheep prefer to remain above potential dangers. Crossing under a roadway is avoided by bighorns because it is a great location for predatory animals."
The study on Highway 93 not only provided information on where the bighorns preferred to cross, it concluded they approached the highway from ridgelines.
When presented with the information, which included approximately 100,000 data points, plans to develop underpasses were scrapped in favor of three well-designed overpasses with fencing essentially 'funneling' sheep to the crossings.
"With current technology, our confidence and scientific predictions are more effective," Posey said. "The bighorn sheep themselves provided all the information. This is wildlife conservation-based science at its best.
"ADOT strives to have the least possible impact on the environment when it is building or improving a highway," said ADOT Public Information Officer Michele Beggs. "We are very pleased to see the first photos and video images of the Desert Bighorn Sheep using the wildlife overpasses, which were built to provide a safe crossing for the sheep and to protect motorists traveling this busy stretch of highway."
ADOT has dedicated nearly half a billion dollars to widening and improving US 93 from Wickenburg in Maricopa County to Hoover Dam over the last several years.
"Our long-term vision is to transform this highly traveled route into a four-lane divided highway through the entire 200-mile stretch," said Beggs."The US 93 series of projects is a priority for ADOT and has become a significant addition to our state highway system to more efficiently move people, goods and services."
The three bridges cost just under $2 million each, and Posey believes it’s money well spent.
"This was a fantastic collaboration between agencies," said Posey. "You get only one shot at this. If, 10 years from now we saw problems affecting this herd, there’d be nothing left to do but watch. This was an opportunity to make a bad situation better."
The research effort took place in three stages: the first to study movements and underpass use before construction, the second to study bighorn interaction as construction took place, and the third, which is currently taking place, will study the effectiveness of the overpasses.
"This is historic," Posey said. "As far as I know, these are the first overpasses built specifically for desert bighorn sheep in the nation. While animals such as deer and elk will use well-constructed underpasses, more visual animals, like bighorn sheep and pronghorn, are more hesitant.
"It's not enough to just put these overpasses in place. We need to know how well they work," Posey added.
While the first two sheep captured on the bridge is reason for optimism, the overall success of the project will be told over time.