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drainage areas. The reservoirs are often connected by canals. The dams typically have a normal operating pool several feet below the embankment top. Irrigation reservoirs maintain this pool during the late spring and early summer. The water level is usually 1Dwer during the fall and winter. Some reservoirs may be dry in winter, The normal operating pDo1 is the pool that could be expected during the flood season. The storage available to handle an incoming flood without overtopping, as shown in figure 1, is the volume between the normal pool and the maximum pool or top of the dam. . The outlet works, sometimes called the "service spillway", is usually a concrete conduit with a small intake structure and a gate with its controls 1Dcated on the top of the dam. It is used to drain the reservoir or release irrigation water, Most of the inspected dams have an emergency spillway; the spillway is usually an earth-cut channel through the natural ground to allow excessive flood inflow to bypass the embankment without overtopping, Some dams, however, were built without spillways, From the standpoint of dam safety, the hydrologic design of a dam aims at avoiding overtopping. Overtopping is especially dangerous for an earth dam because the downrush of waters over the crest may breach the dam embankment. The Corps of Engineers designs its dams to safely pass the esti- mated Probable Maximum Flood that could be generated in the basin upstream from the dam. The Probable Maximum Flood (PMF) is the flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the region. This is generally the accepted criterion for major dams throughout the world and is the standard for dam safety where over- topping would pose any threat to human life. The PMF is derived by using probable maximum precipitation information that is generally available from the National Weather Service. 40 .