Engineers have long appreciated the fact that it is seldom economically sound to design hydraulic structures either for the maximum previous floods or for the computed maximum probable floods, unless failure of such structures involves loss of life or serious property damage. Such floods may not occur more often, on an average, than once in a hundred or pos- sibly even several thousand years. In planning sound design for structures, such as culverts or bridge·s, which may involve flood frequencies of only 10 to 50 yr, the factors of economic~ and useful life of the structure should be con- sidered. In the past, records of streamflow have been so meager and of such short duration that litt~ reliable information could be obtained on the magnitude and frequency of floods in any particular region. As a result it has been the tendency of engineers to overdesign, often causing needless expenditure. The accumulation of reliable streamflow rec- ords over a relatively long period now permits satisfactory studies of flood probabilities in many regions. This report presents a method, based on actual flood data, for obtaining the magnitude and frequency of floods at any place in the watersheds of the Youghiogheny and Kiskiminetas River basins, an area comprising 3,651 sq mi in the upper Ohio River basin of western Pennsylvania and Maryland. The data contained in this report were com- piled and prepared for publication under a cooperative agreement on water resources investigations between the Pennsylvania Department of Forests and Waters, and the Water Re- sources Division, U. S. Geological Survey. J. W. Mangan directs the surface-water investigations in Pennsylvania for the U. S. Geological Survey. The data presented in this report were analyzed, and the text prepared, by the Pittsburgh suboffice, Max Noecker, engineer-in-charge, in collaboration with engineers of the Water Resources Division, U. S. Geological Survey, Washington, D. C.
