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2.3 Principal Flood Problems Flooding in the study area can result from snowmelt runoff, general rains, .and cloudburst storms. The runoff from snowmelt occurs from late May to early July. During this period, the streams are vulnerable to out-of-channel flows, especially if warmer than usual temperatures and a heavy snowmelt are augmented by rainfall. Runoff from snowmelt is characterized by moderate peak flows, long duration, and large volumes of water. Most of the annual rainfall occurs from July through October. The rainfall intensity from cloudburst storms is usually high and runoff is characterized by high peak flows of short duration with relatively small volumes of water. The largest flood recorded on the streams studied in detail occurred July 1, 1957. Estimates on Cottonwood Creek indicate that this flood had a recurrence interval greater than 100 years (Reference 5). This flood, caused by a heavy, late spring snowfall, followed by abnormally warm weather, caused damage along most streams. Significant flooding also occurred in 1901, 1911, 1921, 1923, 1924, 1947, 1952, 1957, 1958, 1965, and 1973. Streamflow records on Chalk Creek show that the June 10, 1952, and June 11, 1973, flows of 1,050 cubic feet per second (cfa) and 1,260 cfs, respectively, approached the computed IOO-year discharge (Reference 4). Natural or manmade obstructions in the flood plain influence the depth and width of the area flooded. Trees and brush growing along the streambanks retard the out-af-channel floodflows, causing back- water and greater depths of flooding. Road crossings with inade- quate culvert or bridge openings often become channel constrictions and obstruct floodflows. Elevated roads in the flood plain act as barriers which raise the water surface at some locations. Trees, brush, and debris washed out during high runoff are carried down- stream to bridges and culverts. These obstructions act as dams, backing the water upstream, until the obstruction breaks loose, causing a sudden surge of water with additional destructive force. Backwater from clogged culverts and bridges results in a multitude of flow conditions which are unpredictable for any given flood event. The Chalk Cliffs lie along the northern side of Chalk Creek on the upstream half of the detailed-study reach. Between the base of these cliffs and the Chalk Creek flood plain lie a group of coalesced debris fans. The debris-flow hazard presented by these fans is distinct from that of the Chalk Creek flood plain. The Chalk Cliffs are dissected by many shor.t, steep canyons. Bet- ween major rainfall events, these canyons become choked with talus and colluvium. This material is saturated and mobilized by intense rainfall and flows down the canyon as a debris or mudflow. These flows have built active fans at the mouth of each canyon. 5