INFLUENCES OF ARROYO-SCALE GEOMORPHOLOGY on DOWN-VALLEY FLOOD FLOW AND SEDIMENT TRANSPORT, RIO PUERCO ARROYO, NEW MEXICO

The lower Rio Puerco Arroyo has been filling since the 1970s. However, some of the sediment contributing to aggradation across the arroyo bottom has come from sources within the arroyo, including from erosion of channel banks, floodplains, and valley-fill sediment in the arroyo wall. This work examines the sources of sediment deposited on the arroyo bottom during a large flood in August 2006. The peak flood flow was the largest since 1972, and it combined with a sequence of overbank flows in early August to cause extensive erosion of channel banks in a 12-km long reach sprayed with herbicide in 2003 to suppress the saltcedar population. The flood flow also undercut the 8 to 10-m high arroyo walls in some areas, causing collapse of large blocks of fine valley-fill sediment with subsequent transport of sediment downstream. Arroyo-scale geomorphic influences on the down-valley flood flow include the width and pre-flood topography of the arroyo bottom, orientation of the channel relative to the down-valley direction, distribution of woody vegetation, and height of the valley floor above the channel bed and floodplain. Data indicate that 15% of the total volume of sediment eroded in the sprayed reach came from sites of erosion of valley fill, compared to 85% of the total from erosion of channel banks. We determined the net change in arroyo volume for a 20-km long segment downstream from the sprayed reach by comparing volumes of sediment eroded and deposited within the arroyo. Volumes of sediment eroded within this reach were determined from pre- and post-flood high-precision GPS survey data and mapping from imagery. Volumes of sediment deposited on the floodplain were computed from pre-flood (2005) LIDAR topography and post-flood GPS survey data. Downstream from the sprayed reach, floodplain surfaces covered by dense woody vegetation aggraded by as much as a meter, and the estimated total volume of sediment deposited in this reach was about three times the computed volume eroded in this reach and supplied from the sprayed reach. Additional sources of sediment contributing to the observed deposits were gullies draining local hillslopes and sediment carried in suspension from the Rio San Jose and Middle Rio Puerco sub-basins. The net change in arroyo volume through the 20-km segment was a decrease of about 3% during this single flood event.