LINKING GULLY EROSION TO PRECIPITATION INTENSITY AND MAGNITUDE: A TWO YEAR RECORD AT LEES FERRY, ARIZONA

Gully erosion along the Colorado River corridor in Grand Canyon has increased in recent decades, and may be related to either the impacts of Glen Canyon Dam or climate change. We are working to link measured erosion and precipitation intensity and magnitude in order to help land managers protect cultural sites from destruction. Erosion of two gullies has been monitored at a site along the Colorado River just downstream of Lees Ferry over the monsoon seasons of 2002 and 2003. Collection of field data included precipitation measurements from an on-site instrument and a weather station at Lees Ferry, survey of two cross-sections in each gully, and gully thalwegs.

Most precipitation events in the two years resulted in no erosion, whereas a single storm during the 2002 monsoon season (4.57 cm in less than an hour) caused gullies to incise from 10-25 cm and knickpoints to retreat up to 15 cm. The 2003 monsoon season was marked by a series of four milder storms during which little incision was observed, but 10-15 cm of widening occurred at cross-sections in each gully, and knickpoints continued to retreat from 2-10 cm. Erosion along the profiles is primarily focused at knickpoints, and increases with contributing catchment area.

Relations observed between erosion and precipitation intensity support an erosional threshold between 3-5 cm/hour. Although more data are needed, knickpoint retreat and gully widening observed thus far are consistent with alternate cycles of widening and incision as a result of knickpoint migration. An alternative hypothesis is that the processes of widening and incision are dependent upon precipitation rates.