The effect of bedrock jointing on the formation of straths in the Cache la Poudre River drainage, Colorado Front Range

Abstract

The North Fork Poudre and Poudre Rivers have incised deep, narrow canyons into Precambrian‐age granite and gneiss along the Colorado Front Range. Longitudinally discontinuous active strath surfaces and strath terraces are present along each river. Although the crystalline bedrock of the study area is resistant to weathering and erosion, differences in joint spacing and orientation create significant differences in rock mass strength along each river. The vertical spacing of active straths along the North Fork Poudre correlates strongly with the spacing of prominent subhorizontal joint surfaces. Strath terraces along the Poudre River occur in areas of wider valley bottom upstream from a profile convexity. Especially wide segments with the greatest downstream extent reflect the locations of shear zones that create more densely spaced joints and lower rock resistance. I interpret the absence of strath terraces in wider canyon segments downstream from the convexity to indicate that the Poudre River is capable of laterally eroding the entire valley bottom as the river incises, leaving behind only fill terraces. Upstream from the convexity, in contrast, lateral erosion has created canyon segments with wider valley bottoms, but the entire valley bottom has not been lowered at the same rate, leaving discontinuous strath terraces. The Poudre River is regionally unique in having relatively well‐developed strath terraces, but strath terraces in other major drainages of the Front Range are also only present upstream from convexities in the longitudinal profile.