CHAPTER VIII.

RESERVOIRS AND DAMS.

STORAGE RESERVOIRS.

Sources of Water-Supply.-Running streams, melting snows and rains are the sources from which the mining districts derive their water-supply. The altitudes of the gravel deposits, two hundred to fifty-five hundred feet above the sea-level, necessitate the bringing of the water from still greater elevations nearer the sources of the streams. The supply from these streams is not always sufficient. Towards the end of winter and during the spring months, while the mountains are still covered with deep snow, rains and temperate weather cause sudden and rapid thawing, and enormous volumes of water are then discharged from the many water-sheds on the west flank of the Sierra Nevada into the Great Valley of California, and freshets are of quite common occurrence. Το make this supply of water available, storage reservoirs have been constructed by some of the large hydraulicmining companies.

The dry season in California is from May to November, but the streams do not run dry until the middle of June or July.

Requirements for Sites.-The principal storage reservoirs in the State are situated at elevations of five thousand to seven thousand feet above the sea-level. The location of a proper site for a storage reservoir is of paramount importance. In selecting a site especial attention must be paid to the following points:

(1) A proper elevation.

(2) The water-supply from all creeks and springs, and the catchment area.

(3) The amount of rain and snowfall.

(4) The formation and character of the ground, with especial reference to the amount of absorption and evaporation.

All of these points must be thoroughly investigated and determined. It is supposed that the catchment area has been ascertained, and that it is sufficiently large for its minimum discharge to supply all requirements.

Elevation. The elevation of a reservoir depends upon the location of the mines and the altitude and extent of the country which it is proposed to cover with the ditch. The reservoir should be located below the snow belt wherever possible, and so situated as to obtain the largest water-supply from the catchment area.

Streams. All the streams should be gauged carefully to determine the minimum and the average supply.

Rainfall.--In new and unexplored localities the water-supply due to rainfall can be determined only by actual measurement. It cannot be too earnestly impressed upon the engineer that for all such information he must depend on his own observations, which in some cases may require a prolonged stay of a season or more in the field. Under any circumstances rainfall data cannot be relied upon, unless based on many decades of observation.

The rainfall is always greater in mountain districts than in the lowlands. It is greatest on the slopes facing the direction from which the moist winds blow. Definite data of the rainfall of any catchment area can be obtained only by establishing rain gauges at different points, where the observations should be made daily during the season.

Snowfall.-The measurement of the snowfall must be taken on a level, and a given amount of snow reduced to water and calculated for rain.

Absorption and Evaporation. In reference to the ground, the most desirable formation is that of com

pact rocks, like granite, gneiss, or slates. Localities where the formation consists of porous rocks, sandstones or limestones, are not desirable on account of the great loss from absorption.

Steep and denuded slopes are always the best, as but little water will escape. The greatest slope will give the largest available quantity of water. The configuration of the ground influences and affects evaporation, and vegetation causes a large amount of absorption. The losses due to absorption and evaporation are reduced to a minimum where the site of a reservoir is in a compact formation with steep sides, and the surface area is consequently small. Evaporation varies with the season of the year and the weather (being most active in summer), while percolation, depending on the soil, varies from year to year. Percolation is greatest during melting of snows, and especially when a thaw follows small falls of snow. From reliable experiments made in France and England, the ratio of evaporation to rainfall was determined (1839 to 1852) in the former to have been 76.57 per cent., and in the latter, subsequently, 77.27 per cent.*

Finally, it must be added that the rule for estimating the total quantity available for storage varies in different districts. In some localities two-thirds of the total amount is estimated to be serviceable, and in others one-third. At the Bowman reservoir 75 per cent. of the total rainfall and snowfall, reduced to rain, is stored.

Reservoir Gauge.-In the construction of reservoirs the location selected must be sufficiently large to hold a supply necessary to meet a maximum demand. The exact area of the reservoir should be determined, and a table showing its contents for every foot of depth made, so that, from an inspection of the gauge and reference to the table, the amount of water available for service can always be known. A longitudinal section through the centre of the reservoir, with cross-sections and contour lines, five

*Harcourt," Rivers and Canals," p. 3.

feet above each other vertically, will enable the engineer to determine the height of the dam and to ascertain the contents of the reservoir with the water at any depth.

Reservoir Statistics. On the head-waters of one of the branches of the Yuba River in Nevada County, at an elevation of fifty-three hundred feet above sea-level, the North Bloomfield Company has established a complete system of reservoirs for the storage of water. Their Bowman reservoir and the small ones above it contain about 1,050,000,000 cubic feet of water. The catchment area is 28.94 square miles. The cost of the reservoirs and dams to date is $246,707.51, including the cost of distributing reservoirs.

The Rudyard or English reservoir of the Milton Company since its enlargement contains 650,000,000 cubic feet of water, having a high-water area of 395 acres, fed from a catchment basin of 12.1 square miles. The reservoir is formed by three dams. The back wall of the centre dam has a vertical height of one hundred and thirty-one feet. The walls are of dry rubble stone covering a solidly filled timber crib. The total cost of the reservoir to date is $155,000.

The storage reservoirs of the Eureka Lake and Yuba Canal Company consist of the French reservoir, 661,000,000 cubic feet capacity, area 337.32 acres; Weaver Lake reservoir, 100,000,000 cubic feet capacity; and Faucherie reservoir, 58,800,000 cubic feet capacity, high-water area 90 acres; having, therefore, an aggregate capacity of 819,800,000 cubic feet of water.* The catchment basins of most of these reservoirs are in a rugged, mountainous region, and in ordinary seasons 60 to 80 per cent. of the rain and snow fall flows into the reservoirs.

Distributing Reservoirs.-Independent of these reservoirs, all mines, at convenient distances from their works, have what are called distributing reservoirs, which receive the water from the main ditch for delivery to the

* See report of J. D. Hague, M.E., pp. 15, 16, and 17.