Tree-ring reconstructions of centuries-long streamflows in four river basins of western United States were used to assess the severity, frequency and distribution of duration of droughts in the basins, and the probability of extreme drought in well-populated areas of California and Arizona. Our results show (i) greater long-drought threat than that inferred from historical streamflow records; (ii) weak statistical dependence of streamflows across the river basins; and (iii) significant probability of droughts in individual river basins. ¿ American Geophysical Union 1992