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Data Sources - 21 flood frequency analysis is the statistical nature of climatic variability. Flood frequency analysis is traditionally based on the assumption that flood magnitudes are independent and identically distributed in time. As previously discussed, changes in watershed vegetative cover due to humans or natural disturbances (fires, blowdowns, etc.) can change the probability distribution of floods, and invalidate the assumption that floods are identically distributed in time. Climatic variability can also pose a problem. Over decades it may be a useful approximation to assume that climate and flood magnitudes are independent and identically distributed in time. Over thousands of years, such an approximation may not be warranted. Several recent papers have provided evidence that flood magnitudes are not independent and identically distributed over the last 5,000 to 10,000 years. Based on a 7,000-year record of overbank floods for upper Mississippi River tributaries, Knox (1993, p. 430) concludes: "During a warmer, drier period between about 3,300 and 5,000 years ago, the largest, extremely rare floods were relatively small-the size of floods that now occur once every 50 years. After -3,300 years ago, when the climate became cooler and wetter, an abrupt shift in flood behavior occurred, with frequent floods of a size that now recur only once every 500 years or more. Still larger floods occurred between about A.D. 1250 and 1450, during the transition from the medieval warm interval to the cooler Little Ice Age. All of these changes were apparently associated \vith changes in mean annual temperature of only about 1-20C and changes in mean annual precipitation of ,;;10-20%." Knox's evidence suggests that during the past 7,000 years, floods on upper Mississippi River tributaries have not behaved as independent and identically distributed random variables. FIGURE 2.2 An ideal channel for studying slack-water deposits-the Escalante River in Utah. The person on the left is standing on a typical sequence of slack-water deposits, which were deposited where the flow velocity decreased in the canyon of the Escalante River. SOURCE: Robert H. Webb, U.S. Geological Survey.