The top ten Northern California floods of the 20th century

The purpose of this paper is to summarize the big northern California floods of the 20th century. These were all winter season general floods covering a large area. They are caused by a strong flow of moist subtropical air from the southwesterly direction. When the moist winds hit the mountain ranges, air is lifted, with copious amounts of rain extending to high elevations. The primary cause of high river runoff is simply too much rain, but melting of a prestorm snowpack can add to the runoff, perhaps up to 15 percent on major Sierra Nevada west slope rivers.The history of large floods show that these occur at sporadic intervals. The one in 1907 became the impetus and guide for building the Sacramento River flood control project with its bypass overflow system. Flood control storage behind large multipurpose dams in the foothills added to flood protection. The system design was taxed by the enormous New Years flood of 1997 which set many new records for flood peaks, flood volumes, and in areal extent. There seems to be a trend for increasing flood size in the latter half of the 20th century.