Which river is backed up by Hoover Dam and flows through the Grand Canyon?

The Colorado River is the correct answer because it is the river that is backed up by Hoover Dam, which is located on the border of Nevada and Arizona. This dam was completed in the 1930s and creates Lake Mead, which is one of the largest reservoirs in the United States. The Colorado River continues to flow downstream through the Grand Canyon, a natural wonder known for its stunning geological formations and significant depth.

The other rivers mentioned do not fit this description; for instance, the Mississippi River flows through a different region of the United States and is not associated with Hoover Dam or the Grand Canyon. Similarly, the Rio Grande serves as a border between the United States and Mexico, and the Amazon River is located in South America, far from any connection to Hoover Dam or the Grand Canyon. Thus, the Colorado River is uniquely tied to both the dam and the canyon, making it the correct choice.