The floods of December 1964 and January 1965 in the Far Western States were extreme; in many areas, the greatest in the history of recorded streamflow and substantially greater than those of December 1955. An unusually large area--Oregon, most of Idaho, northern California, southern Washington, and small areas in western and northern Nevada--was involved. It exceeded the area flooded in 1955. Outstanding features included recordbreaking peak discharges, high sediment concentrations, large sediment loads, and extensive flood damage. The loss of 47 lives and direct property damage of more than $430 million was attributable to the floods. Yet, storage in reservoirs and operation of flood-control facilities were effective in preventing far greater damages in many areas, particularly in the Central Valley in California and the Willamette River basin in Oregon.
The floods were caused by three principal storms during the period December 19 to January 31. The December 19-23 storm was the greatest in overall intensity and areal extent. Crests occurred on many major streams December 23, 1964, 9 years to the day after the great flood of December 23, 1955. The January 2-7 storm produced extreme floods in some basins in California. The January 21-31 storm produced maximum stages in some streams in northeastern Oregon and southeastern Washington and a repetition of high flows in part of the Willamette River basin and in some basins in coastal Oregon. All the storms, and particularly the warm torrential rain December 21-23, reflected the combined effect of moist unstable airmasses, strong west-southwest winds, and mountain ranges oriented nearly at right angles to the flow of air. High air temperatures and strong winds associated with the storms caused melting of snow, and the meltwater augmented the rain that fell on frozen ground. The coastal areas of northern California and southern Oregon had measurable rain on as many as 50 days in December and January. A maximum precipitation of nearly 69 inches in the 2-month period was recorded in southern Oregon, and recorded runoff at several streamflow-measurement stations indicates that greater precipitation probably occurred at higher altitudes in these areas.
Flood runoff in streams, not affected by regulation, exceeded any previously recorded throughout much of the area. Some streams that had particularly notable floods are: Deep and Plush Creeks in the Great Basin ix Oregon, where the maximum flows were nearly twice those of the record floods of 1963 ; Thomes Creek, a west-side Sacramento River tributary in the Central Valley, where the maximum flow was 160 percent of the record peak of 1955; Eel, Klamath, and Smith Rivers in north-coastal California, where the catastrophic peak flows were about 1-1/3 times the floods of 1955 and the legendary winter floods of 1861-62 and inundated, damaged, or destroyed nearly all communities along the main rivers; Grande Ronde River in the lower Snake River basin, where the peak discharge at La Grande was 1.6 times the previous maximum flow during 57 years of record; John Day River in the lower Columbia River basin, where the peak discharge at the McDonald Ferry gaging station exceeded the historic peak of 1894; many Willamette River tributaries, where maximum flows exceeded previous record flows; and the Rogue River in coastal Oregon, where the maximum flow of about 500,000 cfs below the Illinois River near Agness was 86,000 cfs greater than the previous maximum in a 74-year record. The partly regulated flow of the Willamette River far exceeded that in 1955.
The suspended-sediment concentration and load of most streams greatly exceeded any that had been measured previously in the flood area. In Idaho, Washington, and Oregon, the ground thaw that preceded the period of high runoff resulted in conditions conducive to severe erosion of the uplands and subsequent deposition on flooded stream terraces. The greatest concentrations of suspended sedimen