The river that forms part of the border between the USA and Mexico is the Rio Grande. Stretching approximately 3,051 kilometers, it begins in the San Juan Mountains of Colorado and flows into the Gulf of Mexico. The Rio Grande is significant for its environmental impact, biodiversity, and as a critical water source for agriculture in the arid southwestern United States and northern Mexico. It also serves as a geopolitical boundary, playing a role in the complex relationship between the two nations.