Slide 11 of 35
Notes:
Wiezoreck and Snyder (1999) report that water seepage in the Glacier Point rockfall source area appears to respond to storms within a few days. This indicates that connections (fractures) exist which quickly convey some percentage of surface water to the rockfall sites. Furthermore, it indicates that groundwater sources feeding the rockfall sites are shallow and not deep. Water placed directly into the subsurface by other means should be expected to find its way to these fractures as well.
It has been suggested that water from snowmelt should be a trigger for slides at Glacier Point. The authors discount this as a major factor here on the basis that the majority of snowmelt would be expected to follow the surface drainage, which flows in a different direction. Also, a USGS research paper indicates that when snowmelt slides do occur, they tend to occur within a well-defined time frame based on average air temperatures and these slides fall outside of that time frame (http://geohazards.cr.usgs.gov/pubs/ofr/98-124/).
It should also be noted that the recharge area for shallow groundwater effecting the Glacier Point rockslides is quite small and does not include the large snow packs at the higher elevations in the Sierra’s.
It has been further suggested that storm water runoff may be a trigger for these slides. However, storms are of limited duration compared to other sources and are subject to the same limitations described above for snowmelt.
