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Project Progress

Through a series of publications we report the following findings about the possibility of groundwater seepage-driven bedrock erosion of valley heads.

1. Seepage erosion seems difficult to achieve in resistant rock like basalt. First, mechanisms that might cause potential seepage weathering or erosion have not been identified in the literature, and field observations of ours at Hawaii and Box Canyon do not suppuires removal of sediment which is not trivial in coarse steep streams.
2. In Box Canyon (Idaho) we find morphologic features such as scours and plunge pools that point towards a large flood that passed over the canyon headwall in the past (Lamb et al., 2006). Sediment transport estimates confirm that this flood could have transported sediment, while the current seepage flow cannot. The necessary discharge is about 100 m3/s and the spring discharges a tenth of that. Dating efforts indicate that the erosion at the canyon lip occurred between 100ka and 50ka, and could have occurred in a single event (Aciego et al., 2007). These also indicate that the canyon has not experienced any significant erosion since 50 ka (despite 10 m3/s of seepage discharge).
3. In Hawaii, we find little evidence for seepage erosion or sediment transport by seepage. Instead it seems that waterfalls were initiated following a large collapse of Kohala volcano. We propose a new geomorphic erosion law to describe the process of waterfall retreat by vertical plunge pool drilling (Lamb et al., 2007).
4. The amphitheatre-headed valleys in Hawaii and Box Canyon have three commonalities. First they are both cut into vertically fractured, horizontally bedded basaltic rock. Second, waterfalls seemed to have been initiated externally (the slump on Hawaii and the Snake River Canyon in Idaho). Third, the canyons seemed to have been cut by retreating waterfall headscarps that allowed for preservation of amphitheater form.
5. These observations point to two processes that need further study. The first is to understand waterfall erosion in fractured rock, which is the focus of current experiments. Second is to understand the mobility of coarse sediment in steep canyons. A theoretical model has been developed for this and is currently in review (Lamb et al., in review).