A Long-term (~50-yr) Historical Perspective on Flood-generating Winter Storms in the American River Basin

Because of the broad range of altitudes spanned by its catchment, the American River is notable for its occasional, very large winter floods. These floods typically are spawned by warm and exceptionally wet storms that deposit heavy rain or wet snows on broad expanses of the river’s watershed, generating rapid runoff and high streamflows. On flood and longer time scales, the American River is notable for the balancing act that characterizes its flows and floods. Overall, discharge of the American River derived in almost equal proportions from rainfall and snowmelt (Jeton et al., 1996), unlike the more snowfed rivers farther south in the range and the more rainfed rivers farther north. Because of the nearly equal contributions by rain and snow in the American River discharges, some storms bring substantial and immediate flood risks, whereas others are more inclined to build snowpacks with less attendant flood risk. Historically, the flood-generating winter storms in the American River basin have gained much of their impact from their intense precipitation, their particular orographic distributions of precipitation, and (often) their warm temperatures. Intense and warm storms are frequently associated with so-called “pineapple express” circulations that bring exceptionally warm and moist air from the subtropics and tropics into the central Sierra Nevada. Given the greater flood-generating capacity of heavy rains (compared to heavy snowfall), the extent to which precipitation rates in a given storm are heaviest at highest altitudes, where most precipitation falls as snow, or at lower altitudes, where the changes of rainfall are greater, also plays a key role in flood generation in the basin. Therefore, the long-term histories of two particular types (aspects) of winter storms of the American River basin are analyzed here: (a) the storm-to-storm, and winter-to-winter, differences in orographic enhancement of precipitation at high versus low altitudes, and (b) the recurring pineapple-express storms. Both of these aspects will be placed in longterm historical and geographic contexts spanning the 52-yr period from 1948-1999.