The Colorado River has its humble beginnings in Rocky Mountain National Park in Colorado. The river flows south through Colorado, Utah, and Arizona, as well as forming the border between Arizona and California and Nevada. The river empties into the Gulf of California in Mexico. The river has many significant sites, including the Grand Canyon and Hoover Dam. The Grand Canyon became part of Grand Canyon National Park in 1919. Construction started on the Hoover Dam in 1931 and ended in 1936, during the Great Depression. The first American to explore the Colorado River was John Wesley Powell. His expedition set off in 1869 and successfully travelled through the Grand Canyon. His expedition led to the publishing of scientific reports as well as a book entitled, The Exploration of the Colorado River and Its Canyons.