HOLTVILLE, Calif. — It’s late when I check into the Barbara Worth Country Club, 600 miles southeast of my home in the Bay Area. Spare rooms border dark fields, a dry golf course and a web of open irrigation troughs that help make Imperial County one of the biggest agricultural producers in California. Holtville calls itself the carrot capital of the world, and even now, after this season’s harvest, stray carrot tops bolt, blooming to seed.

Fifteen miles from my hotel is the border with Mexico, a boundary now marked by barbed wire that loops around the edges of the All-American Canal, an elaborate, 80-mile long aqueduct that diverts water from Colorado to irrigate farmland that would otherwise get around three inches of rain per year.

When I arrive in April 2019, Donald Trump has just visited Imperial County to stump for his wall. But I’m not here to talk politics, exactly. Instead, I’m on a pilgrimage to visit as many of the places Dorothea Lange photographed in California as I can.

Ms. Lange came to Imperial County during the late 1930s, capturing a different generation of migrants drawn here from Mexico, the Philippines, Oklahoma and the Dakotas, looking for work in the carrot fields. In 1937, she photographed ramshackle tents lining a canal; a group of Model T’s making haphazard camp in a gully. In other shots, cabbage pickers bend deep and hoist baskets high on their shoulders.

Ms. Lange, best known for her Depression-era photographs of migrant laborers, began photographing bread lines and labor strikes near her San Francisco studio in 1932. In the 1920s, she had made her living as a society portraitist, photographing San Francisco’s wealthiest families — the Levi Strauss and the Haas families among them.

As the Great Depression worsened, she began photographing people she saw on the streets: men curled up sleeping or in line for food. In 1935, she married the economist Paul Taylor; they left San Francisco together to photograph the living conditions of agricultural laborers up and down the state, from Davis and Marysville all the way to Imperial County. The Farm Security Administration supported their work.