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The soils in and around the Town of Basalt are variably textured, water-deposited, sandy, and loamy, with varying amounts of cobble and gravel occurring in the valley bottoms and benches. The mountains vary from colluvial to the north, basalt lava to the southwest, and sandstone and shale to the east and west. The watercourses in the City of Aspen have channels, with heavily vegetated overbanks. covered with cobbles and boulders. steep, Channel well-defined bottoms are The average annual precipitation for Pitkin County is approximately 19.25 inches (Re ference 4). On the average, 11 . 8 inches of this precipitation is snowfall and 7.5 inches is rainfall. The average temperatures range from approximately 200F during the winter to 600F to 700F during the summer. 2.3 Principal Flood Problems Flooding from streams in Pitkin County usually occurs during May through August, with the principal cause of flood-flows being snowmel t runoff. Summer rains do not cause maj or floods in this area. Thunderstorm activity creates major floods only on small tributary streams because of the short duration of storms. Major floods on the Roaring Fork and Fryingpan Rivers result from rapid melting of mountain snowpack during the period from late May through early July. These snowmelt floods are characterized by moderate peak flows, large volumes, long durations, and marked diurnal fluctuations in flow. Rainfall on melting snow may accelerate the rate of the snowmelt, thus augmenting floodflows. The largest discharge recorded on the Roaring Fork River was in July 1957, with a peak of 19,000 cubic feet per second (cfs). A USACE letter report (Reference 5) describes the 1957 flood as follows: Floods on the Roaring Fork result from snowmelt and occur principally during the month of June. The flood of July 1, 1957 had an instantaneous peak discharge of 18,700 cfs at the Glenwood Springs gage. This flood was the maximum of 49 years of record, and its rnagni tude has an estimated frequency of occurrence of once in approximately 60 years. The river in the problem area is characterized by low banks, braided channels, and a considerable amount of gravel, cobbles, and snags deposited on gravel bars. The river carries a large bedload of gravel and cobbles, some of the latter being more than 6 inches in diameter. The capacity of the channel has been reduced by this sediment. In some instances, where channel changes took place during the flood, the original channels were so filled with sediment that nearly all of the present flows are discharging through the new channels. Generally, however, the new channels are old watercourses abandoned by the river in previous years. The littered condition of the channels, and the increased danger 6