Although snowmelt from the San Juan Mountains of Colorado (Fig. 7) is a major source of water for the Rio Grande, runoff from northern Mexico also contributes to its flow. As in all arid to semiarid regions, the waters of the Rio Grande River are highly sought after and overallocated.

The Rio Grande, one of the most critical river systems in North America, primarily gets its water from snowmelt in the San Juan Mountains of southern Colorado. In 2026, this high-altitude snowpack remains the "lifeblood" of the river, providing the initial flow that carries it down through New Mexico. As the river travels south, it is also fed by various tributaries, most notably the Rio Conchos, which originates in the Sierra Madre Occidental in Mexico and joins the Rio Grande at Ojinaga/Presidio. This Mexican contribution is vital; in many years, the Rio Conchos provides nearly half of the total water volume for the lower portion of the river that forms the Texas-Mexico border. Other U.S. tributaries, such as the Pecos and Devils Rivers, also contribute to the flow. However, due to heavy agricultural diversion, urban demand, and persistent drought conditions in the Southwest, the river's volume decreases significantly as it approaches the Gulf of Mexico. In fact, in particularly dry years in 2026, the river occasionally "dries up" entirely before reaching the sea, highlighting the intense overallocation of its headwater sources.