In Big Bend National Park, the Rio Grande is so low because of drought, locals are calling it the Rio Sand. The river that separates the United States and Mexico is shallow enough in some places that a person can walk across the river in ten steps, maybe less. American children skip stones across its surface — one, two . . . the third skip lands abruptly on the other side. The same stones are picked up by Mexican children who skip them back across to the other bank in Boquillas Canyon. The game continues back and forth until parents intervene. On one side of the Rio Grande, tourists stand. On the other side, men and boys are herding goats. Breach the border and you will be arrested, American or Mexican, it doesn’t matter. Border police could be anywhere. Black phoebes fly across the river, occasionally touching water like the stones skipped across international lines. In the twenty-first century, borders are fluid, not fixed, especially in our national parks.

Earlier in the day, I met a veteran from Desert Storm, the first Gulf War. His name was Bill Summers. Bill was a tall man with hair cut short; lean and muscular, rugged-looking in his camouflage fatigues — the kind of handsome that can’t be brought down by a few missing teeth. I had noticed him picking up trash along the Ross-Maxwell Scenic Drive; his backpack, with his sleeping bag and bedroll, was propped against the hillside by the side of the road.

We ran into each other at the Panther Junction Visitor Center on the interpretive trail. “Purple-tinged prickly pear — now there’s a mouthful,” he said.

“Yes, it is,” I said, “especially, if you try to say it fast.”

We began talking about cactus, how well adapted they are to drought conditions and arid country.

“I’ve been a volunteer in Hawai’i Volcanoes National Park,” he said. “The plants around the craters are also skilled at surviving harsh conditions.” A brown shirt with the Hawaiian park’s insignia was neatly tucked into his fatigues.

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“How long did you volunteer there?”

“Three years.”

“And you’re here now?”

“Hoping to be. Just turned in my application today, ma’am.”

“Does it look like they’ll hire you?”

“It’s lookin’ that way.”

“Why Big Bend?”

“The desert suits me, ma’am. Not a lot of people around here.”

We moved to the next plant — cholla.

“The Devil’s Stick,” I said.

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“That could do some serious harm to a man’s leg,” Bill replied.

“How long have you been volunteering in the national parks?”

“Since I returned home from Iraq in 1991. Served some time in the Grand Canyon; I’ve been all over. Our national parks are the most important thing we’ve got going in this country,” Bill said. “As the human population increases, the wild places not only become more valuable but more threatened. It’s another way for me to protect our homeland, ma’am.”

Bill Summers reminded me of my friend Doug Peacock, a vet from the Vietnam War. Doug and I met on a trail in Glacier National Park in 1982 and shared a similar conversation. Doug served two tours as a medic in the Army Special Forces, a Green Beret. A decade later, he would describe in his memoir, “Grizzly Years: In Search of the American Wilderness,” how a topographical map of Yellowstone National Park kept him half-sane in an insane war. Every night, he’d pull out the map and run his fingers over familiar country, transporting himself out of the jungle and into the mountains. He left Vietnam on the first day of the Tet Offensive, January 30, 1968.

Peacock returned home with wounds no one could see, and he disappeared into Yellowstone. Then he took a job as a volunteer on a fire lookout in Glacier National Park, where he not only watched for smoke, he watched for grizzly bears, and when he found them, he passed whole days in their presence. He didn’t fear them, he was in awe of them. As he came to know individual bears, his heart slowly began to open to the beauty of the world. The grizzlies returned him to a life he could believe in. As payback, Doug Peacock would become one of the grizzly bears’ fiercest advocates.

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“Where are you from?” Bill asked.

“Utah.”

“Now, there’s a place to live.”

“We live near Arches and Canyonlands.”

Bill nodded. “Gorgeous parks. Been there.”

“So, Bill, when you’re a volunteer in a park, what do you do exactly?”

“Anything that’s needed, ma’am, everything from backcountry rangering to trail maintenance to assisting people in trouble. You name it, I’ve done it, and believe me, with the Park Service hurting for funds, there’s a lot to be done.”

The conversation shifted to Big Bend.

“Have you been to the border of Boquillas near Rio Grande Village?” I asked.

“Not yet.”

I told him about the kids skipping stones across the border.

“I read today that Congress is trying to introduce legislation to build a wall along the entire U.S.-Mexican border,” he said.

“Can you imagine a wall in Big Bend?”

“Personally, I don’t think much of fences, ma’am, and that goes for walls, too.”

He bent down and rubbed his fingers across the small waxy leaves of the next plant. “Have you smelled creosote?” he asked. “Mighty fine scent.”

“Last night, after the rain, the air was fresh with it.”

“People don’t come to places like these to see a damn wall.” Bill shifted his weight and stuck his hands in his pockets. “I think there needs to be more emphasis on taking care of what’s here, not what’s over there.”

Our conversation grew more personal. He asked whether I had ever worked for the Park Service. I told him that I, too, had been a volunteer in the parks — Grand Teton National Park, in 1974. I was a year out of high school. I took early-morning bird walks down by Blacktail Ponds along the Snake River, but I kept seeing birds that had never before been reported in Grand Teton, so the park officials grew suspicious of me.

“I only lasted a season,” I said.

Summers laughed.

“In fact, one of the birds in question was an acorn woodpecker. I saw one today in the Chisos Basin and it was like seeing an old friend.”

Bill told me he grew up on a farm in central Florida. “Course any farm boy’s itchin’ to leave, so I joined the military, enlisted in the army and took advantage of what they could give me. Then Iraq blew up and I went over. Came home pretty messed up. Paddled around the swamps in South Carolina to clear my head. There was a lot going on with me, wild places can unwind a mind. You calm down a bit. I found my way to the national parks. It was a free place to live without being bothered. And then, I learned about volunteering. The Park Service gives you a place to live and enough money for food and incidentals. That was more than enough for me.”

“I have a friend who served in Vietnam and worked in Glacier National Park — grizzly bears saved his life . . . ”

“That wouldn’t be Doug Peacock, would it, ma’am?”

“You know him?” I asked.

“Doug Peacock’s my hero. George Washington Hayduke.” Peacock served as the model for the character of Hayduke in Edward Abbey’s novel “The Monkey Wrench Gang.” Bill turned around. “I love that book. Love Ed Abbey. They’ve been a real source of inspiration for me.”

Bill Summers’s eyes steadied for the first time. “You see, ma’am, I guess it’s a small world out here in the big open for us veterans. Tell Mr. Peacock hello for me next time you see him.” He paused. “And tell him, thank you.”

From “The Hour of Land: A Personal Topography of America’s National Parks,” by Terry Tempest Williams. Copyright © 2016 by Terry Tempest Williams. Published by Farrar, Straus and Giroux. All rights reserved.