DOES A 100-YEAR STORM ALWAYS CAUSE A 100-YEAR FLOOD?
No. Several factors can independently influence the cause-and-effect relation between rainfall and streamflow. When rainfall data are collected at a point within a stream basin, it is highly unlikely that this same amount of rainfall occurred uniformly throughout the entire basin. During intensely localized storms, rainfall amounts throughout the basin can differ greatly from the rainfall amount measured at the location of the raingage. Some parts of the basin may even remain dry, supplying no additional runoff to the streamflow and lessening the impact of the storm. Consequently, only part of the basin may experience a 100-year rainfall event; for example, in July 1997 only 30 percent of the McAlpine Creek Basin experienced rainfall amounts greater than or equal to a 100-year event (table 1). Existing conditions prior to the storm can influence the amount of stormwater runoff into the stream system. Dry soil allows greater infiltration of rainfall and reduces the amount of runoff ente
