Travel times are approximately 18 hours from the upper basin to Folsom Reservoir. Flood peaks from the three adjoining forks arrive almost simultaneously. Some degree of snowmelt is often present, but rain on saturated soil provides the bulk of the runoff. Flow is oriented nearly perpendicular to the topographic ramp formed by the western slope of the Sierra Nevada. With a stationary pattern, orographic lifting provides a mechanism for prolonged heavy precipitation.
Nearby drainages including the Sacramento River are also often in flood at the same time.
The change in mid-century is not simply a peculiarity of this one gage or basin. Interestingly, the annual precipitation is increasing more slowly or not at all. The most recent two decades have brought an increase in both the number of wet years and the number of dry years. Thus, variability has increased. Floods on the eastern slopes of the Sierra, facing the Great Basin and caused by the same factors, also show an increase over the past several decades.