The west slopes of the coastal ranges in the central and northern parts of the State also experience flooding as a result of heavy precipitation over a period of only a few days. These streams are usually longer than those of southern California and require a longer time to build up a flood potential.  The Eel and Klamath Rivers, as well as others in the northwestern part of California, are larger streams.  The Klamath drains a basin of more than 12,000 square miles.  In these streams a flood buildup may extend over a period of a week or longer.

The streams of the Sierra Nevada and Cascades overflow either as a result of rainfall or snowmelt, or from a combination of these.  With the construction of more dams and reservoirs on these streams the frequency of damaging floods decreases.  Most of the streams are still capable, however, of causing occasional major damage along their downstream reaches.

The extreme southern portion of the San Joaquin Valley has no direct drainage to the sea.  Excessive runoff from the southern Sierra Nevada into this area can result in the temporary enlargement of Buena Vista Lake and Tulare Lake.