Can Springs Cut Valleys Into Bedrock?

Valleys formed from groundwater sapping are thought to have a characteristic form including steep walls, flat floors, and amphitheatre-like heads. Observations of these features on Earth and Mars have led to the morphologic-based interpretation that groundwater sapping is an important valley forming process. This interpretation has significant implications for Mars in particular because it has been used to constrain Martian hydrology and the associated prospects for life. However, a mechanistic understanding of sapping erosion has only been demonstrated for granular mediums (i.e. sand). Many of the "sapping" valleys on Earth and (likely) Mars have been carved into bedrock, and the extension of previous work to bedrock erosion is unclear. To our knowledge, a process-based understanding of seepage erosion in bedrock does not exist, even though it is thought to be a first order geomorphic process on Earth and Mars. In order to address this knowledge gap, we are currently investigating Box Canyon, Idaho. Box Canyon, developed in the Snake River Plain, has many of the morphologic features often associated with sapping valleys. In addition, it was carved into basaltic bedrock and has a large spring emanating from its amphitheatre-like head, making it an ideal candidate for a sapping origin. There is currently no overland flow contribution to the canyon; however, based on mapping bedrock scours, a paleo-flood from an unknown upslope source did enter the canyon (and perhaps carved it). We present some first order hydraulic measurements, sediment transport calculations, and field observations to try and constrain the types of flows needed to carve Box Canyon. These flows could conceivably be derived from expansion of the current spring. Direct observation at the head of the canyon has not yet indicated how sapping could be responsible for the erosion of the headwall. We are using various dating techniques to constrain the timing and rate of headwall migration to constrain the possible mechanisms responsible for canyon development.