at the bottom is 30 ft., but at an elevation of 30 ft. below sea-level its sides begin to batter at such a rate as to make the thickness of the wall 8 ft. at its top. On either side of this wall are heavy masses of earth embankment of selected material properly deposited in layers with surface slopes of one on three. As shown by the plans the lower portions of the core wall of this dam would be sunk to bed rock by the pneumatic process, the joints between the caissons being closed and sealed by concrete and by cylinders sunk in recesses or wells, also as shown by the plans.
The profile of this route shows that the summit-level would have an ordinary elevation of 85 ft. above the sea, but it may be drawn down for uses of the canal to a minimum elevation of 82 ft. above the same datum. On the other hand, under circumstances to be discussed later, it may rise during the floods of the Chagres to an elevation of 90 or possibly 91 or 92 ft. above the level of the sea. The top of the dam therefore would be from 8 to 10 ft. above the highest possible water surface in the lake, which is sufficient to guard against wash or overtopping of the dam by waves. The total width of the dam at its top would be 20 ft, and the entire inner slope would be paved with heavy rip-rap suitably placed and bedded.
This dam would create an artificial lake having a superficial area during high water of about 40 sq. miles. The water would be backed up to a point called Alhajuela, about twenty-five miles up the river from Bohio. For a distance of nearly fourteen miles, i. e., from Bohio to Obispo, the route of the canal would lie in this lake. Although the water would be from 80 to 90 ft. deep at the dam, for several miles below Obispo it would be necessary to make some excavation along the general course of the Chagres in order to secure the minimum depth of 35 ft. for the navigable channel.
The feature of Lake Bohio of the greatest importance to the safe and convenient operation of the canal is that by which the floods of the River Chagres are controlled or regulated. That river is but little less than 150 miles ]ong, and its drainage area, as nearly as can be estimated, contains about 875 sq. miles. Above Bohio its current moves some sand and a little silt in times of flood, but usually it is a clear water stream. In low water its discharge may fall to 350 cubic feet per second.
As is well known, the floods of the Chagres have at times been regarded as almost if not quite insurmountable obstacles to the construction of a canal on this line. The greatest flood of which there is any semblance of a reliable record is one which occurred in 1879. No direct measurements were made, but it is stated with apparent authority that the flood elevation at Bohio was 39.3 ft. above low water. If the total channel through which the flood flowed at that time had been as large as at present, actual gaugings or measurements of subsequent floods