ARIVACA, Ariz. — Monsoon rains tinge the desert with deceptive hints of green at this time of the year, but migrants crossing illegally from Mexico continue to risk death from thirst and exposure in the blazing heat. A fortunate few who become lost might stumble upon lifesaving gallon jugs of drinking water, scattered by a band of volunteers along makeshift footpaths that have been carved through the mountains and washes.

From a primitive base camp here, volunteers trained by a group called No More Deaths patrol the desert, offering water, food, clothing and medical care to lost, injured and exhausted migrants, no questions asked. The group’s mission is as simple, though not uncontroversial: to end migrant deaths along Arizona’s borderlands.

On average, 144 gallons of water a week are carried out of the camp, where bathrooms are buckets equipped with toilet seats, and bedding is a sleeping bag stretched out on the dirt.

Byrd Baylor, 89, a best-selling children’s book author living on 35 acres here, had been helping migrants “get to where they need to go” for years, she said, by distributing food and water to those who crossed her land. A decade ago, she gave permission to No More Deaths to set up a base near her home — because, she explained, “we were all doing the same thing, more or less.”