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170 At Gatun, the valley of the Chagres is only about a mile and a quarter wide, and by closing the gap between the hills on either side with an artificial hill—for that is what the Gatun Dam really is—we accomplish two things: first, by backing up the river behind the dam, we form a deep lake that will float our ships up against the side of the hills at Bas Obispo, and make so much less digging in the Gaillard Cut; and, second, a flood that would cause a rise of twenty-five feet in the river would not cause one of a quarter of an inch in the big lake, that will have an area of nearly two hundred square miles.

In building the dam that is to hold back all this water, two trestles were driven across the valley, and from them were dumped many train-loads of hard rock from the Gaillard Cut, to form what the engineers call the "toes" of the dam. To fill the space between them, dredges pump in muddy water that filters away between the cracks of the toes, leaving the sediment it carried to settle and form a solid core of hard-packed clay, over a quarter of a mile thick. When the dam is finished, the side toward the lake will be thoroughly riprapped with stone to prevent washing by the waves, and so gentle will be the slope that you could ride over it on a bicycle without rising on the pedals.

To keep the water from running over the top of the dam, the engineers have cut a new channel for the Chagres through a natural hill of rock that stands in the center of the valley, and this, lined with concrete and fitted with regulating works, is what they call the "spillway." When the dam is finished, the spillway will be