USING ROCK STRENGTH ANALYSIS TO DETERMINE THE SPATIAL DISTRIBUTION OF EROSION AND EVOLUTION OF KNICKPOINTS IN A STREAM CHANNEL WITHIN THE SACRAMENTO MOUNTAINS, NM

In the Sacramento Mountains, rainfall and snowmelt from high elevations supply water to ephemeral streams. The mountains are bound on the western flank by a steep escarpment formed by normal faults associated with extension. We are investigating the spatial distribution of erosion in an arid climate stream channel to better understand where erosion is focused and what controls knickpoint evolution. Knickpoints (sharp changes in slope) within longitudinal profiles are formed by resistant bedrock units that are difficult to erode or by slip and offset during faulting. If knickpoints are created by uplift or faulting, they may occur anywhere along the stream channel regardless of the strength of bedrock material. However, if the knickpoint results from the bedrock’s resistance to erosion, there will be a strong correlation between the position of the knickpoint and rock strength. Based on this, we hypothesize that the rock strength of bedrock with knickpoints will be greater than the rock strength of bedrock units without knickpoints. To test this, we used the Schmidt Hammer and Selby rock strength classification to determine the rock mass strength (RMS) of rock units parallel to the channel in Dog Canyon. We then compared RMS results to changes in slope along a longitudinal channel profile created in GIS identifying geologic units and knickpoint features. We also compared RMS values to the average slopes for each bedrock unit. Initial results suggest that the knickpoints occur in the three strongest rock units tested, which were all carbonates. Dolostone had the highest RMS values on average. RMS appears to generally decrease with increasing elevation. Also, RMS and average slopes of bedrock units show a weak positive relationship. Knickpoints were located near formation contacts and between weaker rock units. These results suggest that knickpoints may be controlled by rock strength and their position along the stream profile. Therefore, the height and extent of these knickpoints may be influenced by the thickness of the bedrock units. Future work on this issue will involve analyzing the spatial distribution of erosion with detrital apatite minerals from samples collected in Dog Canyon.