What river is named after a historic agreement involving Spain and the United States?

The correct answer to the question is the Rio Grande. This river was named in connection with various historic agreements, particularly in reference to the Treaty of Guadalupe Hidalgo in 1848, which ended the Mexican-American War and established boundaries between the United States and Mexico. The treaty designated the Rio Grande as a significant portion of the boundary, making it historically important for both countries.

While the Colorado River, the Red River, and the Mackenzie River are all significant bodies of water with their own historic and geographic importance, they do not have a direct connection to a specific historic agreement between Spain and the United States. The Colorado River primarily flows through the western United States and is known for its role in water supply and recreation. The Red River, which forms part of the Texas-Oklahoma border, and the Mackenzie River in Canada do not relate to U.S.-Spanish agreements.

In summary, the Rio Grande is directly tied to significant agreements between the United States and Mexico, particularly because of its role in defining territory after the Mexican-American War. This makes it the river associated with the historic agreement in question.