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1 ' The following describes Colorado River flows in relation to the Preserve and then describes the Preserve wetland hydrologic system. (a) Colorado River Flows ' Yearly high flows occur in May and June and low flows occur in winter. Flow records for the Colorado River show a distinct pattern of cycles in flooding frequency ' and duration. These records also indicate a reduction of flood frequency and duration since 1959, when dams were constructed on the Upper Colorado River. The ' Preserve is currently flooded less frequently and for a shorter duration than prior to 1959. ' The river overbank flows occur at 40,000 cfs (Table 1). Previous to 1959, overbank flows occurred approximately once every two years. After 1959, overbank flows occur once approximately once every decade. It was ' determined by Cooper (1994) that only extremely high snow packs coupled with warm springs will allow flooding of the Preserve. Table 1. River Elevations at Certain Flows. -71 River River Flows at Cisco Gauge-30 Miles Upstream Elevations Recorded at North End of the Preserve 10,000 cfs 3,955 feet 20,000 cfs 3,958 1 feet 30,000 cfs 3,961 feet 40,000 cfs 3,963 feet ' Duration of flooding has also been reduced since 1959. Duration of overbank flows averaged 10 to 15 days prior to 1959. Between the years of 1959 and 1982, not a ' single year had more than 5 days with flows of 40 ,000 cfs. Additionally, records show clusters of two or more years exceeding 40,000 cfs occurring once or twice a decade prior to 1959. A cluster of high water years ' has occurred only once since 1959. This occurred 1983 through 1985. ' Analysis of total annual flow volume showed that the years from 1959 to 1993 were not abnormally low. Thus flows were similar to earlier years, but without ' seasonal high flows (Cooper 1994). I 16