Thirty-five million years ago, the formation of the Rio Grande began, jumpstarting a region that would become home to millions of people. The past hundred years of land changing hands, water management and infrastructure development have created the Rio Grande we know today.

Geologically speaking, the Rio Grande is a relatively young river system, estimated to be between 1.6 million and 3 million years old in its current form. While the "Rio Grande Rift"—the tectonic depression the river flows through—began forming nearly 30 million years ago, the river itself did not always reach the sea. For most of its early existence, it was a series of disconnected "closed basins" that drained into ancient inland lakes. It wasn't until roughly 600,000 to 1,000,000 years ago that the river finally "captured" enough drainage to cut through the southern mountains and reach the Gulf of Mexico. This makes it much younger than other North American giants like the Colorado River. Today, the 1,900-mile river is a vital but endangered artery; in 2026, it is frequently cited by UN scientists as a symbol of "water bankruptcy" in the American Southwest, as the ancient geologic feature struggles to keep up with modern agricultural and urban demands in New Mexico, Texas, and Mexico.