Gatun Lake, impounded by Gatun Dam, has an area of 164 square miles when its surface is at the normal elevation of eighty-five feet above sea level, and is the largest artificially-formed lake in the world. The area of the water-shed tributary to the lake is 1,320 square miles. During the rainy season, from April to the latter part of December, the run-off from this basin exceeds considerably the consumption of water, and the surplus is discharged through the spillway of Gatun Dam. Toward the end of the rainy season the surface of the lake is raised to about eighty-seven feet above sea level, in order to afford a surplus or reserve supply to keep the channel full to operating depth during the dry season, in part of which the consumption and evaporation are in excess of the supply. It is calculated that when this level has been attained at the beginning of the dry season the reserve is sufficient to assure a surface elevation of at least seventy-nine feet at the end of the dry season in spite of the consumption at the hydroelectric station, and allowing forty-one passages of vessels through the locks each day with the use of the full length of the chambers, or fifty-eight lockages a day when the shorter sections of the chambers are used and cross filling is employed, which would usually be the case. This is a greater number of lockages than can be made in one day. [Illustration: STEAM SHOVEL LOADING ROCK These great machines, which are able to dig out and load several tons of material at each operation, made the rapid progress in digging the canal possible.] Gigantic Obstacles. The greatest difficulty encountered in the excavation of the canal was due to slides and breaks which caused large masses of material to slide or move into the excavated area, closing off the drainage, upsetting steam shovels and tearing up the tracks. The greatest slide was at Cucaracha, and gave trouble when the French first began cutting in 1884. Though at first confined to a length of 800 feet, the slide extended to include the entire basin south of Gold Hill, or a length of about 3,000 feet. Some idea of the magnitude of these slides can be obtained from the fact that during the fiscal year 1910 of 14,921,750 cubic yards that were removed, 2,649,000 yards, or eighteen per cent, were from slides or breaks that had previously existed or that had developed during the year. [Illustration: ONE OF THE GUARD GATES, GATUN LOCKS, PANAMA CANAL Each lock is provided with four gates. This shows the method of construction, the gate being only partially finished.] [Illustration: GATUN UPPER LOCKS, EAST CHAMBER The view is looking north from the forebay showing the upper guard gates and emergency dam.]