The American River flows westward from the Sierra crest near Lake Tahoe to its confluence with the Sacramento River in the city of Sacramento. The standard project flood with no protection would affect about $40 billion in property. The flood series shows a considerable increase of large floods over the past 50 years compared with prior years, an increase mirrored in precipitation amounts. Both phases of ENSO can affect the watershed, but in different ways. ENSO statistics themselves may vary in time, with possible modulations by factors elsewhere in the Pacific Basin or the remainder of the climate system. Other non-ENSO factors, poorly understood, are likely at work also. In addition, the seasonality and timing of precipitation, temperature and runoff show temporal trends. In the face of a possibly changing physical and statistical background, the adequacy of flood protection and preparedness is a subject of constant discussion. Changing values are similarly affecting the discussion over mitigation and response: what mixture of structural and behavioral approaches is the public most comfortable with? Two large floods in an 11-year span have further animated the debate. Analogous situations exist in other geographical settings