American River Flood Events - History & Analysis

Background

We plan to share research on the history and analysis of American River flood events. With improved understanding of historical flood events, we are in a better position to make informed flood risk management decisions to protect life and property in the American River watershed.

The American River is a key player in the history of California. The Gold Rush began with the discovery of gold on the South Fork of the American River in January 1848. The establishment of the City of Sacramento at the confluence of the American and Sacramento Rivers began a history of people responding to floods repeatedly inundating the new city. Their response was based upon a lack of knowledge about the nature of river flooding in California; it's different from other parts of the world.

The hydrometeorology of California, Oregon, and Washington is controlled by atmospheric rivers moving water vapor across the Pacific Ocean and the water vapor being forced over tall mountain ranges condensing the vapor into rain and snow. In the American River watershed, flood events occur fast compared to other river systems because the distance from the Sierra Nevada mountains to the valley floor is about 100 miles. Rain in the mountains can bring flood flows to Sacramento in less than 24 hours compared to other river systems where flood waters rise slowly over days and weeks.

American River Paleofloods

This document is a review and critique of the US Bureau of Reclamation's Flood Hazard Analysis — Folsom Dam, Central Valley Project, California [PDF*; 12 MB] (January 2002) (referred to hereafter as the FHA-FD or USBR 2002), a study which developed paleohydrological bounds or paleoflood estimates at four sites in the American River basin — three in the upper American River watershed and one along the lower American River (LAR).

The FHA-FD was intended to develop an estimate of peak discharge frequency of the American River at Folsom Dam for dam safety purposes. The floods of primary interest had annual probabilities in the range of 1-in-1,000 to 1-in-10,000. "This peak discharge frequency information is combined with historical hydrographs to develop probabilistic hydrographs based on paleoflood information." (USBR 2002 [iii])

This present review concentrates on problems seen with the FHA-FD's paleoflood assessment for the lower American River (LAR) site. It raises significant scientific questions about the validity of the conclusions reached in that study. Two broad issue areas are addressed:

1. First is the FHA-FD's use and application of "stream power" concepts in developing estimates of paleoflood occurrence, magnitude, and age-date.

2. Second is the FHD-FD's interpretation of LAR geomorphology and its development and use of paleoflood evidence