What causes the floods at Great Falls

Yearly departures of runoff were erratic in the northern part of the State; the general trend of the drought was interrupted by much greater than normal runoff early in water year 1946 and nearly normal runoff in water year 1948.

Peak flows in much of the area probably were the greatest since the 1861-62 floods.

Even at a given location, however, it is a matter of judgment whether a period of greater than normal runoff represents the end of a drought or just a minor interruption. In California, the total annual runoff generally is more important to the State's water supplies than the distribution of runoff within the year.

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Then during February 13-25, 1980, another flood occurred that had the greatest impact on the largest number of residents in Arizona history.

Botanic and geomorphic evidence indicates that floods exceeding the magnitude of the December 1964 floods may not have occurred since about 1600 (Helley and LaMarche,1973).

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On many streams, the floods ranked among the greatest since 1861 -62. On December 24, 1955, a levee failure on the Feather River flooded more than 3,000 homes in Yuba City and forced the evacuation of 12,000 people.

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During December 15-27, extremes of 40 inches of rain fell at several locations, and quantities greater than 20 inches were common in the coastal mountains and the Sierra Nevada. The floods of December 1955 produced peak discharges in much of the area that were in excess of any previously recorded.

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Departures from the average of the estimated annual flows indicate that the 1942-64 drought was the second most severe on record at each site: its recurrence interval probably is greater than 100 years.

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These larger peaks were not noticed by the public and caused no damage because no one had been at risk on the canyon streambed when the peaks occurred. Regional floods generally occur between September and March in drainage basins larger than 200 mi2.

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Thus, for example, no floods are included for 1983, even though the volume of runoff for that year is the greatest on record for most of the State. Streamflow records from more than 40 gaging stations were analyzed to determine the severity and areal extent of the floods and droughts.

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State Highway 70, which follows the North Fork Feather River, was closed for several months because of washouts, landslides, and damaged bridges. Runoff in the Sacramento River from the February 1986 floods was generally well controlled by timely reservoir releases and by operation of bypass weirs and overflow channels.