Transient Events and Landscape Response in Boulder Creek, Colorado Front Range

Climate affects the tempo and style of erosion and weathering processes, which together shape the Critical Zone. Here we examine how interplay between these processes over the Quaternary and longer timescales are reflected in the landscape of the Colorado Front Range. Boulder Creek watershed contains widely differing landscapes that arise from climatically controlled late Cenozoic events inscribed on what was probably a slowly evolving post-Laramide landscape. Although large tracts of the rolling, post-Laramide upland still exist, Pleistocene glaciation carved the headwaters of the range, and late Pliocene-Pleistocene fluvial downcutting sliced into the mountain front and adjoining plains (and continues to the present). Landscapes in the headwaters and mountain front continue to respond to these events. In the headwaters, Quaternary glacial erosion removed regolith, and its interglacial inventory of cosmogenic radionuclides, leaving behind a stepped, unweathered bedrock valley floor. Post-glacial modifications have been relatively minor in the ~15-13 ka since glacial retreat. Talus accumulations, with or without interstitial ice, dominate the mobile debris on the valley walls and floor. Soils are developed on deposits, such as under talus or on pockets of till, and contain important dust additions. In contrast, the upstream migration of a fluvial knickpoint into the crystalline rocks of the mountain front formed Boulder Canyon (and other Front Range canyons) and incited ongoing landscape adjustment over the last few million years. At the knickpoint, the canyon is narrow, defined by steep bedrock walls with up to 300 m of local relief, cut into a high, low relief landscape. Downstream of the knickpoint, two processes broaden and open up the canyon. The inclinations of channel-bounding hillslopes decline downstream of the knickpoint. In addition, canyon walls are dissected by the generation and extension of tributary channels that cut into the rolling upland; the frequency of 2nd or higher order tributary channels increases downstream of the knickpoint. In such tributary catchments we find both deeply weathered residual soils and slopes with thin to no cover of mobile regolith. The glacial headwaters are a landscape in which rapid Quaternary erosion outpaced weathering; future evolution of this bedrock-dominated landscape is determined by rates of weathering processes that must now produce mobile debris. The laying back of canyon walls is accomplished, however, by weathering and erosion operating together, rather than in sequence. Judging from the presence of regolith of varying thickness downstream of the knickpoint, after that initial impulse, erosion progresses at rates not substantially different from that of weathering front advance.