The Imperial Sand Dunes, sometimes called the Algodones Dunes, are the largest mass of sand dunes in California. This dune system extends for more than 40 miles along the eastern edge of the Imperial Valley agricultural region in a band averaging five miles in width. Rising to heights of over 300 feet above the surrounding desert floor, the dunes are a well-known landmark to local residents and the thousands of highway travelers who pass by them every year. While summer temperatures often rise above 110°F and annual rainfall averages less than two inches.