Rio Grande Valley map

Downstream the boundary is absolute: the Rio Grande meets the Gulf of Mexico at Boca Chica, the south-eastern corner of Texas and of the continental United States west of Florida. North the Valley fades into the King Ranch country of the Coastal Bend somewhere in the brush above Raymondville — Willacy County is the last county anyone counts. West, the four-county definition stops at Starr County and Roma; the practical break is Falcon Dam, completed in 1954, because everything below it is irrigated farmland and everything above it is ranchland.

The whole region drains to one river and has no reservoir of its own. Irrigation districts pump from the Rio Grande under a treaty that divides the flow with Mexico, and the water arrives from Falcon Reservoir upstream in Starr and Zapata counties and from Amistad far above that near Del Rio. On the Gulf side the Laguna Madre, a shallow hypersaline lagoon behind Padre Island, is saltier than the sea.

The Census counts the Valley as two metropolitan areas that run into each other: McAllen–Edinburg–Mission, which is all of Hidalgo County, and Brownsville–Harlingen, which is all of Cameron County. Between them they hold more than 1.29 million of the region's 1.38 million people, and the built-up strip along US-83 runs almost unbroken for 60 miles from Mission through McAllen, Pharr, Weslaco and Harlingen to San Benito.

Much of that growth happened outside city limits. Colonias — subdivisions platted on cheap farmland and sold without water lines, sewers or paved streets — spread across unincorporated Hidalgo, Cameron and Starr counties from the 1950s onward, and Hidalgo County ended up with more of them than any other county in Texas. Texas tightened the rules on new platting in the 1990s and has been retrofitting water and wastewater service ever since; a good many colonias have since been annexed by the cities that grew out to meet them.

Almost all commercial citrus grown in Texas comes from Hidalgo, Cameron and Willacy counties, and almost all of it is grapefruit. The Ruby Red appeared as a mutation on a single Valley tree in 1929; red grapefruit is now the state fruit, and the sweeter Rio Star varieties bred from it are what most groves plant. Onions, sugarcane, cotton, sorghum and winter vegetables share the same irrigated ground, and cold-weather harvests are the reason the Valley ships produce north in January.

The risk is a hard freeze. December 1983, December 1989 and February 2021 each pushed sub-freezing air all the way to the river and killed a large share of the trees; each time the acreage came back smaller. The other seasonal population is people — retirees from the Midwest and Canada, called Winter Texans locally, who fill RV parks from November through March.