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Show and damage. Severe floods have occurred on an average of once every 10 to 12 years. Heavy cloud- bursts in 1949 over the mountains in Virginia and West Virginia were responsible for the loss of 11 lives, and damages estimated at over 9 million dol- lars. Discharge during these floods was calculated to exceed 1,300 cubic feet per second per square mile on an area of 25 square miles and 1,800 cubic feet per second on a 2.4-square mile area.2 This may be compared with a discharge of about 42 cubic feet per second per square mile for the maxi- mum flood of record in the main river near Wash- ington. Winter snowfall has little effect on the regimen of the main stream. Ice forms annually in the winter on most of the basin's streams, with spring thaws sometimes producing ice flows of large pro- portions. Floods are usually caused by three types of storms: (1) Extremely heavy convectional storms in summer cause local flash floods; (2) occasional tropical hurricanes produce heavy downpours of medium duration; and (3) general cyclonic storms, * U. S. Corps of Engineers. lasting for several days, have produced most of the disastrous floods. The shape and character of the basin are such that they favor rapid runoff, with high flood waters for short periods of time only. The Potomac and its tributaries thus are characterized by flash floods. Ground Water Ground water differs from place to place in qual- ity and quantity. In the central part of the basin there are many caverns providing natural storage basins whose springs may discharge as much as 4,000 gallons per minute. This water is clear, cool, and wholesome, but hard. In the Piedmont Plateau and Blue Ridge area the granites and other ancient rocks are relatively im- pervious; most of the springs are weak, but the qual- ity of their water is good. In the Coastal Plain water from some shallow wells is of poor quality,3 although artesian supplies of good quality have been found. Large municipalities and industries gen- erally have relied upon surface waters. * In some areas ground water is dark colored, mineral- ized, and of unpleasant flavor. 582 |