The future traveller through the Panama Canal will probably never guess the immensity of the labour that has gone to the building of the Gatun dam. Already, indeed, it looks so much like a part of the natural landscape that it might well escape special observation altogether. Yet nothing less than 21,145,931 cubic yards of material were laid down—enough to make a wall of earth three feet high and three feet thick reaching nearly halfway round the world. The spillway itself contains 225,485 cubic yards of concrete.
It will be noticed that in the dam proper there is no core of masonry or puddled chalk or clay whatever. It was at one time intended that there should be. I have alluded to the alarmist rumours that were raised again and again at Panama and created much uneasiness in the United States. These were especially concerned with the great dam, and that word must have frequently been on the lips of the engineers in more than one significance. Every possible test was applied to determine the exact character of the underlying materials, to ascertain whether there was any connection between the swamp areas to the north and south through the deposits in the gorges which the earthwork was to bridge, to prove the ability of the material below to support the structure, and to find out whether suitable material for the dam could be found in its neighbourhood. "As the result of all these investigations," wrote Colonel Goethals,[15] "it may be briefly stated that the underlying material is impervious to water; that it possesses ample strength to uphold the structure that will be placed upon it, and, the subsoil being impervious, that there is no connection between the swamps above and the sea below."
In order to make assurance doubly sure, Colonel Goethals planned the dam so as to include triple interlocking steel sheet-piling across the valley, driven down to bedrock, and decided to carry the dam to a height of 135 feet. Even so, the news of a collapse was wired home, and this so impressed President Roosevelt that he sent a commission of engineers to the isthmus accompanied by President-elect Taft. The investigations had a different result from what had been expected. Instead of being dissatisfied with the size and strength of the dam, the engineers declared that it was being built too high and that the steel piling was unnecessary. It must be admitted, therefore, that the efficiency of the Gatun dam has been subjected to the most rigorous tests, and that no further anxiety on the subject need be felt.
With the blocking of the Chagres outlet at Gatun, the waters of the lake have gradually accumulated until they cover an area of 164 square miles. Not only the Chagres itself but its tributaries, the Trinidad and others, are thus ponded back. The reservoir extends up a number of long and winding arms, and is thus very irregular in shape. The bed of the channel itself was cleared of brushwood and trees, but the rest of the valley was thickly overgrown. As the waters rose, therefore, and gradually submerged this primeval forest, a rather dismal spectacle was presented of decay and destruction. The lake has, indeed, completely altered the aspect of the country. Villages and even small towns, whose names had come down from the days of the old navigators, lie buried for ever beneath the waters of Lake Gatun. Even now the great expanse of water with its wooded islands looks like a natural feature of the landscape rather than yesterday's creation of engineering enterprise. The vessels in transit will, of course, keep to the dredged and buoyed channel, but the channel will itself be invisible, and the traveller, after tossing on the restless Caribbean Sea, will enjoy the full sensation of a cruise over a landlocked fjord or lake. Lake Gatun is indeed twice the size of Lago Maggiore and four-fifths the size of Lake Geneva. The journey from Gatun to Bas Obispo, where the waterway again assumes the appearance of a canal and enters the Culebra gorge, is 22 miles, but the same 85-foot level is maintained right to the locks at Pedro Miguel, where the waters of Lake Gatun are again retained by a dam connecting the walls of the lock with a hill to the west. The rest of the lake is held in by the natural configuration of the country, the only outlets being at the Gatun spillway and, of course, through the locks.
But we must not overlook the main purpose of the lake, which is to supply the water for the canal and the lockages. For this purpose everything, of course, depends on the rainfall at the isthmus, and the question arises whether this may be relied upon to replenish the canal with the needful water-supply. Colonel Goethals estimates that in an average dry season 58 "lockages," or transits of the canal, per day would be possible, which is a greater number than the twenty-four hours of the day would permit, allowing vessels to follow each other at intervals of one hour. Happily, a resource is still left if the supply of water should show signs of proving insufficient. At Alhajuela, on the Chagres River, some nine or ten miles above Obispo, there is an excellent site for a dam, forming a reservoir where some of the surplus water of the rainy season could be stored and supplied to the canal as required in the dry months. Details of the construction of such a dam were prepared in connection with a former canal-scheme, and would be available in case of need.
FOOTNOTE:
[15] The National Geographic Magazine, February 1911.