PALOMINAS, Arizona (Border Report) — The majestic canopy of towering cottonwood trees can be seen for miles, like a green ribbon trimming the dry semi-desert grasslands of southern Arizona. The snaking treeline marks where the San Pedro River meanders through the Huachuca Mountain range that sits on either side of the U.S.-Mexico border.

The trees can grow to 100 feet tall and are a welcome source of shade for the millions of migrating birds that flock to the river each year. For some, it is the only water they will find for hundreds of miles. The San Pedro River is one of only two major rivers that flow north from Mexico into the United States and is one of the last undammed rivers in the Southwest.

But an unprecedented drought — the worst since 1924 — has struck Cochise County, Arizona. And in many places, the river that starts 10 miles south of the international boundary line in Sonora, Mexico, is just a riverbed of cool sandy silt to the feet.

That has made for ideal conditions for construction workers who are rapidly building a border wall here. And that has angered local environmentalists who worry that this massive structure will block the river flow when waters resume normal levels, especially when sudden torrential downfalls and flooding occur during the “monsoon” that runs from June to September throughout Arizona.

On Thursday, Border Report toured the area where the San Pedro River intersects both countries and where a 30-foot-tall metal bollard border wall is being built with special movable gate design features that are being added to accommodate torrential downfalls and regional flooding. This is part of the 74 miles of new border wall being built through the Huachuca Mountains as part of the Tucson Border Wall System Project, according to U.S. Customs and Border Protection.

Kate Scott, who led the tour, points out the various birds and mammal species that call home this “last free-flowing river.”

Scott, 62, runs the nonprofit group Madrean Archipelago Wildlife Center, a water conversation organization focused on the San Pedro River basin. She says the special gates that are being installed in the new border wall that can be opened to allow for greater water flow during the monsoon and storms are not hydraulic and must be turned manually. She’s met with CBP officials and said the Border Patrol agent in charge of this sector told her his agents will be in charge of opening the gates, and she worries about how quickly they will be able to respond to this remote region when unexpected storms hit.

“We don’t know how that will physically happen,” said Scott, a mechanical engineer, as she pointed to the rectangular structures being installed. “How they will actually open these gates.”

This area is about 6 miles south of Highway 92 in Palominas, a town with a population of just 212. It’s down a dirt road that has several signs cautioning motorists about bumps and deep holes in the unkept road. Scott has a good friend who lets her park on her property, which is about a half-mile walk from the river.

The beeping of construction trucks and hammering can be heard from quite a distance, and a giant crane marks where the crews are building a concrete bridge across the river, and currently installing these movable gates. The bollards here are 6 inches wide and have 5-inch spaces in between — that’s a gap 1 inch wider than other border wall designs, such as in South Texas. CBP has also altered plans for wall building over the Tijuana River in southern California.

But Scott says it’s still not enough for the 41 species of reptiles and amphibians to get through. She is worried about floodlights that will “disorient” owls and bats, and as she peered up at the wall Thursday afternoon, she said, “some birds don’t fly this high, they just don’t.”

She was visibly upset by the speed at which the wall is being built. Arizona is the state with the fastest construction of border wall in the Southwest. That’s because much of the land is federally-owned and environmental laws have been waived by the Trump administration to expedite construction.

During the second presidential debate on Thursday night, President Donald Trump touted that 400 miles of new border wall have been built under his administration. He wants to have 450 miles built by Election Day on Nov. 3. His Democratic opponent, former Vice President Joe Biden, said he would halt border wall activity if elected.

This area is part of the San Pedro Riparian National Conservation Area established in 1988. “The first of its kind. This was the place they wanted to make sure we protected,” Scott said. “The river is right here and the Huachuca Mountains are behind.”

She was last here two weeks ago and said that she was informed by CBP officials that the concrete bridge would not be built just yet and they were consulting with locals on the best design. But on Thursday the concrete bridge spanned halfway across the riverbank, and crews were busily adding to it.

A dirt road beside the wall has been cleared for a 150-foot-wide enforcement zone, which will include an all-weather roadway for Border Patrol vehicles, and the bridge for agents to traverse the river. On the other side of the dirt road, there are huge pieces of vehicle barriers, giant Xs like the tank restrains used during World War II on the beaches of Normandy, France. That is what used to mark this international boundary line.

On this spot two months ago, Scott organized a protest that drew 30 people, including some legislative candidates, “longtime river activists” and nonprofits, she said. In January, hundreds of people came to the river for a sit-in she helped organize when they first learned the border wall would be built here.

“There is a hardcore group of us who have refused to say, ‘OK, we’re not going to do anything more.’ We have held on and written letters and contacted our state and local leaders to try to get answers and so we felt like we had to come here and make a statement, almost like holding a congressional session with the river and to share with her,” Scott said.

On Thursday, Scott and her friend, Robin Motzer, a poet and writer, held a “prayer ceremony” on the dry riverbed as the noise of construction crews hummed in the background. They hung up “prayer flags” marked with handwritten words such as, “I am the sacred words of the earth … It is lovely indeed … Song of earth spirit.”

Motzer, 55, who lives in Tucson, put on a feathered mask and read “Bloated Border Men,” a fiery poem condemning the border wall. Scott read a prayer in a tribal tongue that she said was “gifted to me by Charlie One Horse of the Yaqui people.”

“We are here to bear witness to crimes against nature, crimes against humanity,” Scott said as she began the ceremony. “We are sending blessings of love and light in the four directions.”

“We, the people, are silenced by psychotic bloated men who draw lines in the sand with a tall metal band. Trespassers on Native American lands,” Motzer’s poem began. “We, the people, refuse bloated egos who proselytize their greed and their lies, imprisoned by fear and by hate with a border wall and a toxic electric gate.”

After an hour examining the area, the pair drove Border Report upriver about 3 miles — which took over 15 miles via roads and highways — to find a spot under a one-lane bridge where there is water still flowing in the San Pedro.

The arid, mountain air was cooler there and the towering cottonwoods, with their 75-foot-wide shade even greener. But as this drought — in which Cochise County has been declared a natural disaster persists — it’s uncertain how long the river will still flow at this spot.