This stripping of soil is indicative of the great care taken to secure a high quality of water for the city of Boston, but it is not done in the Croton watershed of the New York supply. It cannot be doubted that the quality of the Croton supply would have been sensibly enhanced by a similar treatment of its reservoir sites. Mr. F. B. Stearns, chief engineer of the Metropolitan Water-supply of Boston, states that in some cases the effects of filling reservoirs without removing the soil and vegetable matter have “continued for twenty years or more without apparent diminution.” On the other hand, water discolored by vegetable matter becomes bleached to some extent at least by standing in reservoirs whose sites have been stripped of soil. =178. Average Depth of Reservoirs should be as Great as Practicable.=—In the selection of reservoir locations those are preferable where the average depths will be greatest and where shallow margins are reduced to a minimum. It may sometimes be necessary to excavate marginal portions which would otherwise be shallow with a full reservoir. There should be as little water as possible of a less low-water depth than 10 or 12 feet, otherwise there may be a tendency to aquatic vegetable growth. The following table exhibits the areas, average depths, capacity, and other features of a number of prominent storage-reservoirs. COMPARATIVE TABLE OF AREAS, DEPTHS, AND CAPACITIES OF STORAGE RESERVOIRS WITH HEIGHTS AND LENGTHS OF DAMS. LEGEND: (A) = Area Square Miles. (B) = Average Depth, Feet. (C) = Length of Dam, Feet. (D) = Capacity, Million Gallons. ------------------------------+-----+---+--------------+-----+------- | | |Maximum Height| | | | | of Dam. | | Name and Location | | +-------+------+ | of Reservoir. | (A) |(B)| Above | Above| (C) | (D) | | |Ground.| Rock.| | ------------------------------+-----+---+-------+------+-----+------- Swift River, Mass |36.96| 53| 144 | ... |2,470|406,000 Nashua River, Mass | 6.56| 46| 129 | 158 |1,250| 63,068 Nira, near Poona, India | 7.25| 27| 100 | ... |3,000| 41,143 Tansa, Bombay, India | 5.50| 33| 127 | 131 |8,770| 37,500 Khadakvasla, Poona, India | 5.50| 32| 100 | 107 |5,080| 36,737 New Croton, N. Y. | ....| ..| 157 | 225 |1,270| 32,000 Elan and Claerwen, Birmingham,| | | | | | Eng., water-works | | | | | | (total for six reservoirs) | 2.34| 43|98-128 | ... |4,460| 20,838 All Boston water-works | | | | | | reservoirs combined | 5.82| 14|14-65 | ... |.....| 15,867 Vyrnwy, Liverpool, Eng. | 1.75| ..| 84 | 129 |1,350| 14,560