What is the most significant river of the southwestern United States?

The most significant river of the southwestern United States is the Rio Grande. This river, which runs along the border between the United States and Mexico, plays a critical role in the ecology, economy, and culture of the region. The Rio Grande serves as a vital water source for both agricultural and urban areas, particularly in states like New Mexico and Texas. Its watershed supports diverse ecosystems and wildlife.

In contrast, while the Colorado River is crucial in the southwestern US, especially for its role in supplying water to a large population and for recreation along its course, it primarily serves areas like Arizona, California, and Nevada rather than being a defining feature of the entire southwestern region. The Missouri and Columbia rivers, although significant in their own rights, are not located in the southwestern United States and therefore do not hold the same relevance in this specific geographic context.