PROCESSES, MECHANISMS AND VOLUMES OF DUST GENERATED FROM THE LITTLE COLORADO RIVER BASIN DURING REGIONAL SYNOPTIC WIND EVENTS, AND IMPLICATIONS FOR DUST ON SNOW IN THE SOUTHERN ROCKY MOUNTAINS, USA

The Little Colorado River (LCR) in northeastern Arizona is an important source of regional aeolian sediment, and transports a large sediment load of sand and silt. A significant decrease in river competence occurred over the 20^th century, and has had a major influence on processes of the LCR fluvial-aeolian system. Aggradation and deposition from the sediment-choked river, in combination with seasonal flooding events, and increasingly arid conditions from drought and higher temperatures promotes sediment supply to dunes, and the generation of dust events. Intensive water use, combined with decreasing annual snowpack, has resulted in declining discharge and perennial flow. Currently, only a short segment of the LCR channel, near its confluence with the Colorado River, flows year-round, leaving dry river sediment in and adjacent to most of the river channel during the windy spring. As discharge declined, the LCR channel narrowed by as much as 90% since the mid-20^th century. Winter and summer monsoon season discharge coincides with flood events, because the narrow river channel cannot accommodate peak flows. These flood events deposit silt and clay along a broad, low-gradient valley flood plain, forming playas adjacent to the river. Active dunes form on floodplain and terrace landforms as sediment blows out of the dry river channel. As dune sands saltate across flood plain deposits during high wind events, they produce large volumes of dust. During the dry, windy spring, transport of sand across adjacent overbank LCR flood deposits is now likely to be a significant source of dust contributing to “dust on snow” events in the southern Colorado Rockies. To estimate potential contributions to dust on snow from this source area, wind deflation was measured at three LCR overbank playas by using erosion-resistant animal track-ways that crossed the flood plain during the spring of 2010. Measurements provide an estimate of the volume of silt and clay-sized particles that were transported by wind during five major, well-documented dust events. Dinebito Wash playa, Tolani Lake playa, and Transwestern playa lost 0.168 km³, 0.843 km³ and 3.398 km³of sediment, respectively. These three playas contributed 4.41³ km of dust during March and April 2010 dust storms, or an average of 0.88 km³ of dust per wind event.