Annual maximum instantaneous discharge data for 527 gaging stations with at least 10 years of peak-discharge record on unregulated streams were used to compute the magnitude and frequency of floods on Washington streams. Flood magnitudes having exceedance probabilities of 0.5, 0.1, 0.04, 0.02, and 0.01 were related to physical and climatic characteristics for 504 of the 527 gaging stations using generalized least-squares regression analyses. The regression relations can be used to estimate flood discharges having these exceedance probabilities for ungaged stream basins.