The vast reservoir of Gatun Lake holds the water supply that operates the locks systems of thre Panama Canal. It was created by the construction of Gatun Dam across the Chagres River near its mouth at the Caribbean Sea. The dam was the largest ever built at the time, and the 164-square mile lake was also the largest man-made lake on earth. The surface of the lake forms a major portion of the navigable canal, being approximately 20 miles of the total raised length. It also offers the opportunity for east- and westbound ship convoys to pass, allowing two-way traffic through the canal. Its mechanical function is to supply the 53,400,000 gallons of water required to operate the locks for each ship’s passage. The source of the freshwater is the Darien rain forest watershed of the isthmus draining into the Chagres River. The lake holds approximately one year’s average rainfall amount.