Weathering, Water, And Slope Aspect
Anderson, Suzanne 1 ; Blum, Alex 2 ; Lee, Jeana 3 ; Cowie, Rory 4 ; Williams, Mark 5 ; Frederick, Zan 6
1 INSTAAR and Geography, CU-Boulder
2 CU-Boulder
3 CU-Boulder
4 CU-Boulder
5 INSTAAR, Geography, and ENVS; CU-Boulder
6 INSTAAR, CU-Boulder
Aspect controls solar radiation to hillslopes: north facing slopes are more shaded (in the northern hemisphere), while south facing slopes are not. Here we explore how this simple topographic control on energy balance plays out in the architecture of the Critical Zone of a subwatershed in the Boulder Creek Critical Zone Observatory. Gordon Gulch catchment is within the upper montane forest of the Colorado Front Range, with mixed lodgepole-ponderosa pine forest cover. Because the valley trends E-W, hillslopes are either N-facing or S-facing. The annual snowpack is deeper and longer lasting on the lodgepole pine dominated N-facing slopes. Snow is thin or patchy on the open ponderosa pine dominated S-facing slopes. These shading and snowpack differences can be seen in soil temperatures and soil moisture. In a series of soil pits, we found mobile regolith was nearly twice as deep on the moist, N-facing slopes, and saprolite was more weathered in these locations. Saprolite was found at shallower depths on south-facing slopes, and was less weathered, and more competent. We entertain two hypotheses. The depth of mobile layer and degree of weathering of saprolite reflect either differences in material transport rates or differences in chemical weathering rates. In the case of material transport rate control, creep and bioturbation remove highly weathered saprolite, keeping fresher rock closer to the surface on the S-facing slopes. In the case of chemical weathering control, soil moisture maintains greater rates of chemical alteration of saprolite, and physical disruption by creep and bioturbation is minimal on N-facing slopes. The differences in weathering profile development in association with slope aspect provide a natural experiment to unravel competing effects of weathering and erosion on landscape development.