DETERMINING THE PROCESS OF DESERT STREAM RECOVERY FOLLOWING DEBRIS FLOW AND IMPLICATIONS OF CLIMATE CHANGE

Shinumo Creek, a tributary to the Colorado River within Grand Canyon, experienced a large debris flow after consecutive monsoon summer storms on the footprint of a recent 2484 ha wildfire. The debris flow aggraded the stream channel approximately 0.5 m, removed most riparian vegetation (impacts extending to 5 m above base flow), and changed the stream bed from predominately cobble to gravel. A sediment gauge on the Colorado River 95 km downstream of Shinumo Creek recorded a 4-fold increase in sand sediment load and 7-fold increase in silt/clay sediment load for a period of a few hours after the event.

The stream was monitored for several years prior to the debris flow for fish habitat and stream discharge. Following the fire the stream was monitored for three years at seven sites for repeat Wolman pebble counts, water quality stage, discharge, and riparian vegetation re-growth. Additional monitoring included hydrophones measuring bedload flux, empirical bedload measurements, and passive suspended sediment sampling. Results indicated substantial gravel saltation and bed movement during sustained spring snowmelt periods with less movement during the larger intensity, short duration monsoon summer storms. The fine material from the debris flow was mostly evacuated after three years with a channel that largely resembles the pre-event. Riparian vegetation, including native coyote willow (Salix exigua), arrow-weed (Pluchea sericea), and cottonwood (Populus sp.) have already re-established and providing habitat complexity and shade to the stream.

Regional climate change predictions, combined with long-term stream gauge trends for the Grand Canyon region, indicate that winter precipitation for Shinumo Creek is transitioning from a snow-dominant regime to rain-dominant regime. Long-term snowmelt maxima at the nearby Bright Angel Creek streamgauge (63 yrs) and at the Little Colorado River (70 yrs) has significantly occurred earlier in the spring season since observations began (0.2, 0.4 day/yr; p < 0.01, 0.005 respectively). If this pattern continues, desert streams that regularly experience debris flows from summer monsoon rains may not have sustained high flows from snowmelt for channel recovery, thereby increasing the time required for streams to recover for endemic desert fisheries.