Flood profiles and inundated areas along the Cowlitz River, Lewis County, Washington

Abstract

Extensive flooding occurred on a 37.2-mile reach of the Cowlitz River, Lewis County, Wash., on December 2-3, 1977. The flood profile, computed from high-water marks, was 25 feet above that for medium flow near the downstream end of the reach and decreased uniformly to 7 feet above medium flow at the upstream end. Overbank areas inundated averaged about a mile in width throughout the reach and caused damage to residential areas of the towns of Packwood and Randle. The peak discharge at the downstream end of the reach, was 89,300 cu ft/s and was the largest that occurred in a 30-year period of record. The peak discharge at the upstream end of the reach was 36,200 cu ft/s and was the second largest that occurred in a 49-year period of record; the largest peak discharge observed occurred on December 21, 1933, and was 36,600 cu ft/s. The recurrence interval for the December 2-3, 1977 flood, computed from a frequency analyses of recorded peak discharges, was greater than 100 years at the downstream end of the reach and about 45 years at the upstream end. The 100-year flood at the respective reach ends would have a peak discharge of about 82,500 cu ft/s and about 43,000 cu ft/s. (USGS)