Floods of January and February 1993 in Arizona resulted in the most dramatic channel widening on the middle Gila River since 1905. An earlier flood in October 1983 had a larger instantaneous discharge but resulted in little channel change. The 1993 flood was of greater volume and duration, factors important in destabilizing flood-plain vegetation and eroding bank material. The 1983 flood was produced by a dissipating eastern Pacific tropical storm, whereas the 1993 flood was produced by a series of cold fronts from the northern Pacific Ocean supplied with subtropical moisture from a split jet stream. Meridional global circulation patterns enhance the frequency of winter storms that produce sustained flooding in Arizona and are more likely to result in channel widening and flood-plain instability on main trunk streams like the Gila River.