KNICKPOINT-CONTROLLED LANDSCAPE EVOLUTION IN THE MIDDLE BOULDER CREEK WATERSHED, CO

In post-orogenic terrains such as the Colorado Front Range, knickpoints—steep channel reaches bounded by relatively shallower reaches—are bellwethers of landscape evolution. This study, conducted in the Boulder Creek Critical Zone Observatory, reports the response of slopes to knickpoint migration along tributaries of Boulder Creek in the Colorado Front Range. In this relatively dry, tectonically stable region, physical weathering and transport processes prevail in the critical zone, mobilizing sediment on slopes and in channels. Knickpoint migration, a wave of increased incision moving upstream, accelerates these processes, transmitting climatic and tectonic signals through the landscape. Knickpoints also may reflect rock strength. All channel orders in the Boulder Creek catchment support knickpoints likely initiated between 7 and 3 Ma. by changing climate.

Field measurements and analysis of LiDAR DEMs show that study catchments—Betasso Gulch (~0.45 km²), Gordon Gulch (~4 km²), and Middle Boulder Creek (350 km²)—support channels that differ in size by roughly an order of magnitude. Knickpoints in each basin mark the transition between relict landscape above and transforming terrain below. In each instance, observations and Schmidt-hammer measurements show that rock within and downstream of knickpoints is relatively fresh and strong in compression. In contrast, pervasively fractured rock and saprolite in the channel and on the hillslopes upstream of the knickpoints indicates long-term weathering.

Channel and hillslope evolution in the post-orogenic landscape of the Colorado Front Range must be driven by non-tectonic forcing. Resistant rock, low stream power, and channel aggradation hinder incision. Knickpoint migration will remain stalled until forcing factors, particularly climate, change in the Front Range.