Lake Mead began filling in 1935 while Hoover Dam was under construction. It is the largest man-made reservoir in the United States and is approximately 112 miles long with 247 square miles of surface and a depth of approximately 500 feet when at full pool. The white ring encircling the reservoir resulted from mineral deposits that accumulated when the rock walls were beneath the water. The visible ring reflects lowered water levels due to droughts, from full pool to the level seen in 2016.