EFFECTS OF FLOODS AND CLIMATE ON CHANNEL PLANFORM CHANGE OF THE LITTLE COLORADO RIVER, ARIZONA

Planform changes of the Little Colorado River (LCR) channel were documented by repeat mapping from aerial photography flown in 1936, 1953, 1979–1980, 1992, 1997, and 2007. Dryland rivers, comparable to the LCR, have been noted for considerable changes in channel width in response to variations in flow without significant changes in bed elevation. The 90 kilometer sand-bed reach of the Little Colorado River, between Winslow and Leupp, Arizona, adjusted to flow perturbations by narrowing of the channel through floodplain development and by substantial lateral channel migration. Although the magnitude of change in channel width is variable along the study reach, there is an overall decrease in average width (175 to18 meters) from 1936 to 2007. In addition, the Little Colorado River channel has moved laterally as much as 1.8 kilometers in 71 years. Sinuosity, the ratio of channel to downvalley length, increased from 1.5 in 1936 to 2.0 in 2007 and 1.7 in 1936 to 2.6 in 2007, for two subsections of the reach, respectively. Pre-avulsion sinuosity of two additional subsections that have experienced major avulsions within the last few decades are 2.5 and 2.8. Much of this change appears to be driven by flow variation that in turn was related to precipitation variability, specifically the affects of the mid-20^th century drought, the late 20^th century wet episode, and the early 21^st century drought.