Abstract

The apparent stasis of our current landscape belies the constant change it has undergone for millions of years before the present. The earth’s surface continues to transform as denudation balances slow uplift of rock material removed from slopes and by channels, which transmit climatic and tectonic signals through the landscape. In postorogenic terrains such as the Colorado Front Range, knickpoints—steep channels bounded by relatively shallower reaches—may reflect the influence of rock strength or slow, complex response to external forcing.

Most knickpoint research has focused on regions with high uplift rates, weak rock, and rapid landscape evolution. In contrast, the Front Range in Colorado—in the interior
of the North American continent—is a region with low rock uplift and low precipitation, relatively strong rock, and thus comparatively low rates of incision. Knickpoints are found in all channel orders in the Middle Boulder Creek catchment, likely the result of increased precipitation and channel mouth lowering initiated between 7 and 3 Ma.

This study focused on the Middle Boulder Creek catchment west of Boulder, CO. Middle Boulder Creek has a drainage area of roughly 350 km², an area that includes the other two study areas: Gordon Gulch (~4 km²) and Betasso Gulch (~0.45 km²). In the field, we surveyed channel profiles and recorded width, depth, and grain size measurements. We described the character of the hillslopes, surveying transects perpendicular to the channel and measuring rock strength using a Schmidt hammer. We supplemented these field data with LIDAR DEM observations with a pixel size of 1 m²..  We calculated relationships between spatial data, comparing downstream distance, channel slope, mean hillslope angle, rock strength, and stream power. In these channels, knickpoints are roughly twice as steep as the mean channel slope, producing local convexity in channel profiles that would be concave-up in steady state.In Middle Boulder Creek, the knickpoint slopes average 7.4%, higher than the 4.2% channel average. A second, minor knickpointis preserved upstream in a reach where average stream slope is only 4%.. Rock type is uniform along Middle Boulder Creek, but rock strength is highest in the middle of the lower knickpoint and at the second knickpoint. In those zones, Schmidt Hammer values range between 50 and 65, considerably higher than the values of 35-45 that characterize the rest of the basin.Gordon Gulch has two knickpoints, with mean slopes of 15.3%, compared to the 10.8% channel average. These knickpoints are located in Silver Plume Granite that is harder than the metasedimentary basement rock that underlies the rest of the basin. Schmidt hammer values average between 45-50 in the knickpoints, compared to values between 0 and 40 upstream. Betasso has three small knickpoints that do not dramatically disrupt the channel concavity, but instead appear as bedrock steps in the channel. These steps correspond to areas of hard rock in the channel and on the adjacent hillslopes, and have Schmidt hammer values of >45. Schmidt hammer values elsewhere in the catchment are between 5 and40, and are <5 above the third knickpoint, where a thick layer of colluvium and saprolite covers the bedrock.

In each catchment, knickpoints mark the boundary between steady-state and adjusting landscapes. Below and perpendicular to knickpoints, hillslopes are steep and rough relative to smoother and flatter hillslopes above. Steepening is a result of knickpoint-driven baselevel lowering. Where the hillslopes are steepened past the angle of repose, mass-wasting events hinder the adjustment process, overwhelming the ability of the channel to transport material downstream. Less dramatic baselevel lowering simply accelerates hillslope processes: rock strength, stream power, and continued channel disruption inhibit this adjustment and prevent a return to steady-state processes.
Bedrock strength plays a major role in knickpoint dynamics. In the Middle Boulder Creek and Betasso Gulch catchments, basins without local change in lithology contain knickpoints that mark the transition between resistant and non-resistant bedrock.  The strongest rock is at the knickpoint, and the lower margin of weaker rock lies immediately above. This localized hardness likely results from the removal of weaker rock by highly erosive flows within the knickpoint. Where bedrock in a catchment is nonuniform, knickpoints form in the zones of higher rock strength. In Gordon Gulch, incision has been less effective in resistant rock, and the resulting disparity in baselevel lowering has locaized the steepest reaches at the contact between less- and more-resistant rock.

Knickpoint migration in post-orogenic landscapes is difficult. Incision is not accelerated by rapid uplift, and must be driven by other forcing. The presence of resistant rock, low stream power, and the aggradation of sediment in the channel further hinder incision in the Front Range. Evidence for continued knickpoint migration is absent from this landscape and tentative evidence shows that the knickpoints may have stalled in each catchment. We must conclude that incision in the Middle Boulder Creek requires a synergy of many erosive processes in order to effectively incise the channels. This synergy is currently missing several key components, and incision is likely to remain stalled until the initiation of glaciation or other climatic forcing or the renewal of tectonic action in the Rocky Mountains.

Citation

Dethier, Evan N. (2011): Examining knickpoints in the Middle Boulder Creek Catchment, Colorado. A thesis Submitted in partial fulfillment of the requirements for the Degree of Bachelor of Arts With Honors in Geosciences WILLIAMS COLLEGE.