. Development and electrical distribution of water power . spices it has been 194 DEVELOPMENT AND DISTRIBUTION OF WATER POWER deduced that although the minimum flow of the rivers in this particu-lar locality averages about J cubic foot per second per square mileof drainage area and for eight months in the year about f cubic foot may be dependedupon, the flood volumeagainst which precautionmust be taken in designis somewhat below 50cubic feet per second persquare mile, such flood-water flow being an ex-ceedingly high one, and,it is thought, the greateston record with the U. Survey.
. Development and electrical distribution of water power . spices it has been 194 DEVELOPMENT AND DISTRIBUTION OF WATER POWER deduced that although the minimum flow of the rivers in this particu-lar locality averages about J cubic foot per second per square mileof drainage area and for eight months in the year about f cubic foot may be dependedupon, the flood volumeagainst which precautionmust be taken in designis somewhat below 50cubic feet per second persquare mile, such flood-water flow being an ex-ceedingly high one, and,it is thought, the greateston record with the U. Survey. This developmentconsists essentially of alow spillway dam at thehead of Mountain Islandto deflect the water intothe western through thischannel nearly to thefoot of the island, it isthen forced through thehead-gates of the canal<* by another spillway extension of this damserves as an overflowweir between the canaland the river. From thispoint the stream is car-ried by a canal througha natural valley about1 \ miles to the power-. GREAT FALLS PLANT 195 house and retaining bulkhead built across the valley, while belowthe power-house the tail-race carries off the spent waters J mile toRocky Creek, in which channel it is again carried to the Head-works—-The deflecting dam at the head of Moun-tain Island is an ogee section overflow dam only 7 to 8 feet high,
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