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2.3 the total preCipitation depths in the study-area subbasins. Those areas east of the foothills receive more precipitation (a few tenths of an inch per event) than the areas adjacent to the foothills. The occurrence of precipitation varies; however, most of the rainfall occurs during May. Thunderstorms also occur irregularly throughout the summer months. The temperature extremes in Boulder are evidenced by mean maximums ranging from 46.30F in January to 88.30F in July, and by mean minimums ranging from 2l.60F in January to 6l.00F in July. Principal Flood Problems ~, The principal cause of flooding problems on the study streams is intense localized thunderstorms. Because the headwaters of these streams are below 9,000 feet, snowmelt does not influence the peak discharges. jo) Numerous floods have occurred in the Boulder area, with the most extensive flood occurring in 1894. This flood generally has been designated as the 100-year flood; however, there is little documentation of the flooding on streams other than Boulder Creek. Flooding was widespread during the 1894 flood, and nearly all bridges in the city were washed out. The most extensive flood since 1894 occurred on May 7 and 8, 1969. This flood was also the most expensive, with damages estimated to be $325,000. Bear Canyon, Skunk, and Twomile Creeks all overflowed their banks according to newspaper accounts. On May 8, 1969, the Boulder Daily Camera reported that "motorists were still being routed around the intersection of Table Mesa Drive and Broadway, where Bear Creek raged out of its banks, flooding the entire area Wednesday." (See Figures 2 and 3 for photographs of flooding on Bear Creek.) Twomile Creek has frequently overtopped its banks with silt-laden floodwater. The worst flood on Twomile Creek occurred in September 1933, while others occurred in 1909, 1941, 1942, 1949, and 1965. On August 19, 1965, the Boulder Daily Camera headline read: "Twomile Canyon Gully Washer Spews Mud Across Broadway and Into Yards." The article stated that "Water cascading down normally-dry Twomile Creek spewed mud across sections of Broadway and silted yards and basements of 8 or 10 homes near Broadway and Iris Avenue Wednesday afternoon." Fourmile Canyon Creek has flooded occasionally, with notable events occurring in 1916, 1941, and 1951. In 1916 and 1941, railroad bridges were washed out (Reference 4). Localized flooding along the lower reaches of Fourmile Canyon Creek occurs frequently, according to local residents. (See Figures 4 and 5 for photographs of flooding on Fourmile Canyon Creek.) Damages I';. )., 6