The large lake that serves as the border between Arizona and California is Lake Mead. Formed by the construction of the Hoover Dam on the Colorado River, Lake Mead is one of the largest reservoirs in the United States. It plays a crucial role in providing water supply for urban areas and agriculture. Lake Mead is a popular recreational area, attracting visitors for activities such as boating, fishing, and hiking, while also showcasing the stunning desert landscapes of the surrounding region.