AZ vs. CA:04 California Exhibits, Vol. IV; Nos. 151-192

page 40

40 THE AIX-AjMERICAJS" CA1TAL. at the river's low stage. At high water the fall of the water surface at the structure-that is, the fall from water above the dam to water below the dam-it only about 5 feet. The shape of the crest of the dam is such that some of the large drift carried by the river is caught by the dam at certain stages of the water and hangs there until the water of a higher stage pushes it over. To some extent checking of drift in this way could be prevented by modifying the shape of the crest. As this is desirable and as there would be material advantage to both the Yuma project and the all-American canal project in holding the water above the Laguna Dam at low stages somewhat higher than heretofore, the board suggests that the crest of the dam be raised 2 feet and rounded, with a short approach from upstream on a gentle slope, so as to offer the least possible obstruction to drift. The advantage of thus raising the crest of Laguna Dam is represented by the saving of about 1,000.000 cubic yards of excavation for each foot that the grade line of the all-American canal is raised. The board realizes that due to the great length of the Laguna Dam and the broad expanse of the submerged area above the dam there will be a deposit of material to and above the full height of the structure over some areas, and a growth of willows and brush is to be expected, which may so encroach upon the structure that the dam can not function uniformly from end to end. There will always be certain points at which there will be freer approach for the water from above than at others, and from these points there will be some flow along the dam to the right and left. The water going over the crest of the dam will not, in other words, be at a uniform depth. Consequently the height to which the water will rise when the river is at a high stage will be somewhat above what would be calculated on the assure rtion that the overflow is uniform from end to end of the dam. Allowance for this fact has been made in planning the head-gate structures, bulkheads, and training walls, which are to be sufficiently high to avoid overtopping. The point at which there will be the greatest concentration of overflow over the dam should be maintained near its California end. The large diversion of water at that end of the structure will aid in drawing the stream in this direction. If the suggestion to modify the crest of the dam is approved, it is proposed to depress a section of the crest, possibly 800 feet thereof, from 1 to 2 feet below the general level, thereby definitely fixing the point of greatest water concentration, which will have more or less effect upon the course on which most of the drift runs and may assist in keeping the same out of the canal. Furthermore, during high stages of the river, when due to the flow over the dam, an increased head is not required, such a depression would reduce somewhat the maximum water elevation above the structure.