CONTRASTING CHANNEL RESPONSE TO FLOODS ON THE MIDDLE GILA RIVER, ARIZONA

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.