Evolution of fluvial systems on the Colorado plateau
Keywords: Geological and Environmental Sciences.

As major conduits for both water and sediment, rivers are responsible for shaping much of Earth's surface. The river morphology we observe today is result of complex interaction between geology, tectonics, climate, and erosion over time, an evolution that is recorded in sedimentary deposits left behind as rivers changed course. This dissertation focuses on the arid, bedrock landscape of the Colorado Plateau, assessing three major factors required for interpreting desert drainage networks and the fluvial material left behind: history, process, and rate. The Colorado River is the largest river draining the Colorado Plateau and its history is central to understanding the impact of fluvial erosion in shaping the American West. In Chapter 1, we reassess a constraint on the timing of Grand Canyon incision using modern geochronology. Conflicting interpretations of thermochronologic and provenance data in western Grand Canyon led us to reassess deposits on the Shivwits Plateau, north of Grand Canyon, which were reported to contain clasts derived from south of Grand Canyon. These deposits are confirmed to be deposited ~5.4 Ma, a critical period in Colorado River history, however no clasts of unequivocally southern origin were observed. Instead, this work demonstrates the gravels likely represent a small catchment eroding geologic units north of Grassy Mountain, removing a constraint on the timing of Grand Canyon incision. Chapter 2 is focused on the processes that control drainage initiation and expansion in bedrock landscapes. I use a combination of topographic analysis and geomorphic observation of drainages on the dipping flank of Raplee Ridge monocline near Mexican Hat, Utah to show that threshold shear stresses sensitive to lithology and structural geometry likely control the location of drainage initiation and the extent of headward drainage expansion. Drainages initiate where shear stresses exceed that necessary for vertical plucking of blocks; subsequent disaggregation of the limestone surface occurs via block sliding. Once sufficiently incised, knickpoints form in differently erodible lithologies, leading to headward drainage expansion. Buttressing of the drainage heads by large limestone blocks suggests the limit of channelization is set by the inability to transport blocks. Finally, the pace of regional denudation is typically set by the rate of incision on major rivers, thus Chapter 3 is focused on establishing the short-term incision rate of the San Juan River. Previous research on fluvial deposits in the San Juan River region suggest an incision rate of ~110 m/m.y. over the past ~1.4 Ma, however our analysis of fluvial material using a combination of cosmogenic radionuclide exposure age dating and post-Infrared Infrared Stimulated Luminescence (p-IR IRSL) dating suggests incision on the San Juan River over the last ~28--39 ka is 759--1151 m/m.y., nearly an order of magnitude higher. We demonstrate that the fluvial deposits were abandoned due to a bedrock meander cutoff, which partially explains our accelerated incision rate. The complex interactions between planform river evolution, lithology, alluvial cover, and climatic factors likely have a strong effect on short--term incision rates and calls into question the applicability of short--term incision rates in assessing whether modern systems are in equilibrium with geologically recent changes to regional fluvial and tectonic systems