Bowman..... North Bloom field Co...... 500 100 425 $151,521 5,450 12,093 Saw Mill Flat. North Bloom The total amount expended on, all these dams and reservoirs, Fall Creek... North Bloom English field Co... Milton M. Co.. 395 Milton Dam.. Milton M. Co.. 10 Eureka Lake. Eureka Lake 131 5 6,690 331 $155,000 6,140 7,745 5,670 17.637 *Includes cost of the three dams, which form the reservoir. The height and length given are for the main structure. these upper reservoirs retain all the water flowing into them, reducing the catchment basin of the Bowman to about nineteen square miles. The mean annual rain and snowfall at the Bowman dam is about seventy-five inches, of, which seventy-five per cent. flows into the reservoir. Two dams are needed to impound the water. The main one, placed across the narrow gorge forming the outlet of the valley, has a maximum height of one hundred feet (96.25 feet above the datum base line) and an extreme length on top of four hundred and twenty-five feet, and is the largest on the coast. The smaller dam, placed across a gap near the mouth of the valley, has a maximum height of fifty-four feet and an extreme length on top of two hundred and ten feet. It is fitted with waste-ways, and over it is discharged all the surplus water from the reservoir. High-water mark is fixed at a point one and one-half feet below the summit of the main dam; at this height the reservoir contains 918,000,000 cubic feet of water with a surface area of over 500 acres. By placing temporary flush boards on the top of the waste dam the water is raised to the ninety-six feet line (above datum base), increasing the quantity of water stored to 930,000,000 cubic feet. The stream feeding the reservoir has a maximum flow during great freshets of 5,000 to 7,000 cubic feet of water per second. The existence of other reservoirs higher up the stream adds to the danger from great floods, and therefore the Bowman dams have been designed to withstand not only freshets in the cañons, but also any additional influx of water caused by the breaking of the upper dams. Main Dam.—Figure 5 A shows a profile across the cañon, being a longitudinal section through the dam. Figure 5 B gives a cross section at its extreme height. It rests on solid granite bed-rock, which is sufficiently free from seams to prevent any considerable leakage through crevices in the rock. 90 80 70 60 50 40 The dam was built in 1872 to the height of seventy-two feet, as shown by the sketch, being a timber crib formed of unhewn cedar and tamarack logs, notched and firmly bolted together, and solidly filled with loose stones of small size. A skin of pine planking, spiked to the waterface, forms a water-tight lining. During the years 1875 and 1876 the dam was increased to the height of ninetysix and one-fourth feet above datum line (one hundred feet extreme height) by filling in a stone embankment on the lower side of the old structure, faced with heavy walls Section across Cañon through main dam. Area to 95 ft. 20,230 Area to 72 ft. 12.025 of dry rubble stone of large size. The down-stream tace wall is fifteen to eighteen feet thick at the bottom, dimin ishing to six or eight feet at the top. Most of the face stones in this wall are of good size, weighing from threefourths to four and one-half tons, and there are many of equal weight in the backing. The lower portion of the wall is seventeen and one-half feet high, with a batter of fifteen per cent. It is built of heavy stone, with ranged horizontal beds and with the face stone tied to the backing by long iron ties. The upper portion of the wall is built with a slope of forty-five degrees, and the face stones are bedded on an angle of twenty-two and one-half degrees, thus dividing the angle between a horizontal bed and a bed at right angles to the face. No attempt at range work was made in this upper portion of the wall. Above the sixty-eight feet line ribs of flattened cedar, eight inches thick, are built into the up-stream face wall and are tied to it by iron rods three-fourths inch in diameter and five feet long. On these ribs a planked skin is firmly spiked. This planking is of heart sugar pine, three inches thick and eight inches wide, with planed edges fitted with an outgauge, similar to ship planking. The plank was put on nearly thoroughly seasoned, and swells sufficiently to make the face practically water-tight without battening or calking the joints. The openings at the joints made by the outgauge suck in small particles of vegetable matter, which take the place of calking to a great extent. At the bottom the plank is fitted to a firm bed-rock and calked with pine wedges. There are three thicknesses (nine inches) on the lower twenty-five feet, two thicknesses (six inches) on the next thirty-five feet, and one thickness on the upper thirty-six feet. From past experience it is believed that the planking will remain sufficiently sound for twenty years at least. A culvert extends through the dam, as shown by Fig. 5 B, through which the water is drawn from the reservoir. This culvert is built with heavy dry-rubble foundation and walls, and is covered with granite slabs sixteen to eighteen inches thick and six and one-third feet long. Three wrought-iron pipes of No. 12 iron, each eighteen inches in diameter, pass through the water-face of the dam. Their upper mouths are protected by a strainer, formed of two-inch plank, anchored to the bed-rock. A separate valve or gate is placed at the lower end of each pipe; the water passing through the gates, aggregating a flow of 280 cubic feet per second when the three are open, discharges into a covered timber sluice, seven and onehalf feet wide by one and three-fourths feet high, passing to the lower edge of the dam, and thence on to the solid rock of the creek bed. The gates are approached by a walk way above the sluice. The crest of the dam is formed by a coping of hewn heart-cedar timber, eight |