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L’analyse géomorphologique d'une zone d’étude permet d’identifier et de comprendre le rôle des facteurs de contrôle tectonique et climatique sur l’évolution passée, récente et future de la surface topographique. Ce travail de recherche porte sur l’analyse géomorphologique des paysages du secteur de l’état de Sarawak, localisé au nord de l’île de Bornéo en contexte tropical. À travers l'analyse morphotectonique des deux plus grands bassins versants : le bassin versant du Rajang et du Baram, il a été possible de dresser une première évaluation du cadre morpho-tectonique de la région et des conséquences topographiques. Les bassins versants étudiés et situés au centre et au Nord de Sarawak drainent une superficie totale d'environ 75 000 km². L'île de Bornéo présente une couverture végétale dense spécifique aux régions tropicales, et demeurant souvent difficile d’accès. Cette île témoigne de terrains montagneux et accidentés, découpés par de vallées profondes aux flancs abruptes, entraînant des taux de dénudation notables depuis le Miocène. La première contribution est d’identifier les principales failles mineures et majeures ayant par réactivation participées au rajeunissement de la surface topographie à l’échelle régionale. À l’échelle des deux bassins versants, les facteurs géomorphologiques suivants comme l’intégrale hypsométrique, les facteurs d’asymétrie, les anomalies de pente identifiées le long des rivières ont été cartographiées à l’aide des techniques d'autocorrélation spatiale. Les principaux mouvements verticaux identifiés sont accommodés le long des accidents structuraux majeurs et des chevauchements spécifiques de la zone Nord de Bornéo. Parmi les autres résultats, il est également observé des surfaces planes reliques, à haute altitude, n’ayant pas encore réajustées leur surface d’équilibre depuis les 5 derniers Ma malgré de phases rapides de soulèvement connues. Enfin, à l’échelle de la zone d’étude, où les contrastes lithologiques sont absents, la présence de nombreuses ruptures de pente ou knickpoints sont observés le long des principaux profils longitudinaux des rivières. Les ruptures de pente fortes sont dans la majorité corrélables aux principaux accidents structuraux. Des observations de terrain viennent renforcer nos hypothèses par la présence de terrasses fluviatiles soulevées. Ce travail d’analyse d’indices géomorphologiques complétés par des observations de terrain permet alors de proposer un modèle synthétique des principaux facteurs de contrôle responsables du rajeunissement de la surface topographique de l’état de Sarawak jusqu’alors sous-estimé et méconnu. / Geomorphic assessment of a region is considered to be crucial in understanding the present day landscapeand forces that have acted and is currently acting on the ever evolving topography. This thesis explores the geomorphology of the tropical landscape of Sarawak, north Borneo through morphotectonic analysis of two of the largest drainage basins of the entire Borneo Island: the Rajang and Baram basin; making this work the first systematic tectono-geomorphic evaluation of the region. The island of Borneo is enveloped by thick rainforests, hostile rugged mountainous terrain with deep and steep valleys, and is characterized by high denudation rates since Miocene. The studied drainage basins flow across entire central and north Sarawak and drain a total combined area of ca. 75, 000 km². The first contribution to the field is by conducting a study on the presence of active tectonic forces that modify the topography through rejuvenation of major and minor faults. The analysis using basin-scale hypsometry, asymmetry factor, normalized channel steepness index and spatial autocorrelation techniques showed that the landscape has been rejuvenated and experiences tectonic deformation to present-day in the form of active folding of the fold-thrust orogenic belts of the Interior Highlands which form the backbone of Borneo. From the results, we highlighted the presence of relict surfaces of landscapes which were isolated at high elevations unable to balance a rapid uplift phase experienced after 5 Ma. We extended the study in order to identify the current stage of landscape development by conducting stream profile analysis which displayed an array of knick-zones and knick points devoid of lithological and climatic controls. Deep V-shaped valleys formed in the zones that demonstrated active folding of the highlands also revealed relief anomalies highlighted through topographic analysis. We showed that enhanced orographic precipitation following the rapid creation of relief has supported adjustment of the topography to a state of transience. In the next part of this work, we conducted swath profile analysis, minimum bulk erosion and channel steepness anomaly maps in order to identify the role of rapid incision in exacerbating erosion rates as a response to tectonic and climatic forcing. We show that there exists a coupling between incision rates, precipitation and channel steepness which shows a relation of direct proportionality. Extensive geomorphic and sedimentological field campaigns were carried out in order to substantiate our results and conclusions. The field work revealed the presence of uplifted fluvial terraces, waterfalls and cataracts corresponding to knick-points identified by us. Finally, we combine our results from the geomorphic analysis and stratigraphic field work in order to construct a conceptual model showing the geomorphic evolution of Sarawak, north Borneo.

Thesis advisor: Noah P. Snyder / Land cover and climate changes, attributed to natural and anthropogenic forcings, cause deviations in geomorphic processes that act to deliver sediment from watersheds to lakes. In New England, contradictory evidence exists as to the influence of deforestation associated with EuroAmerican settlement and major flood events on watershed erosion rates over the past ~250 years. Through combining sediment core analysis from Ossipee Lake, New Hampshire with geomorphic analysis of the Ossipee Lake watershed, this study quantifies Holocene through Anthropocene watershed erosion rates, and assesses variations in rates in relation to short-term historic events such as major storm events or deforestation, and long-term variations related to natural climate variability and post-glacial landscape evolution. An 8.63 m core was collected and spans the entire period from deglaciation to present. Bulk composition and age-depth modeling, utilizing both short-lived radioisotopes and radiocarbon dating, are used to quantify changes in deposition and inferred erosion rates over time. Additional insight on sedimentary processes is provided by measurements of magnetic susceptibility and bulk geochemistry. Lake-sediment data suggests clastic sediment mass accumulation rates vary between 0.0032 to 0.5870 g/cm2/yr, with deposits of increased terrestrially derived sediment focused between ~8500 to 7800, ~6500 to 2500, and 1600 cal yr BP to present. Geomorphic analysis is used to identify regions within the watershed that act to deliver sediment to Ossipee Lake. Potential sources of sediment supply include loose, unconsolidated proglacial deposits near Ossipee Lake that transition to primarily till in upland areas. Calculated bed shear stress along rivers highlights areas in the watershed capable of transporting sediment and areas that can serve as traps thus limiting sediment delivery to Ossipee Lake. / Thesis (MS) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

The Snake River flows across the dynamically uplifting hotspot plume of the Yellowstone region, cuts through the Snake River Range and ultimately enters the lowlying eastern Snake River Plain. Thermal and mantle-dynamic uplift around Yellowstone has been recorded by short-term geodesy and modeled by geophysicists, but measurements over Quaternary timescales and an understanding of how that uplift influences regional incision are absent. The Snake River is the only regional river that crosses the uplifting Yellowstone Plateau and flows into the subsiding eastern Snake River Plain (SRP), and provides an opportunity to investigate both ends of the phenomenon on the tailing margin of the Yellowstone region. This thesis consists of two related studies conducted in Alpine Canyon of the Snake River. The first is a study of fluvial terraces and steepness patterns along the Snake River considering the spatial distribution of bedrock or varying hardness and resistance to erosion and in the context of regional tectonics. This study uses surficial mapping, optically stimulated luminescence (OSL) dating, bedrock strength measurements, and steepness analyses of the mainstem Snake River and tributary drainages. Results include the first incision rate estimates for the southwestern part of the Yellowstone hotspot region and a discussion of the possible sources of baselevel fall along the Snake River. The second study documents the transitions between bedrock and alluvial channels in the study area and evaluates morphometric and transport capacity thresholds between these reaches. Alluvial bed-cover mapping with a side-scan sonar along with channel morphometric data, clast-counts, and sediment transport estimates allow us to explore what controls these two fundamental channel types. Results confirm that the Snake River has relatively fast incision rates for the interior western U.S. and that the Snake River is adjusting to an actively deforming landscape. Additionally, our dataset provides field documentation of the magnitude of bedrock-alluvial transitions and may be valuable for parameterizing landscape evolution models or assisting in the restoration of reaches that are in disequilibrium due to changes in land use or climate. This study will hopefully inspire future studies of tectonism and landscape evolution of the Yellowstone hotspot region.

Landscapes evolve through a number of processes in response to a wide range of forcing mechanisms. Many of the processes that drive landscape evolution occur at the interface between fluid and sediment. Sediment transport leads to changes in topography that, in turn, influence fluid flow. Feedback mechanisms between topography and fluid flow can lead to the formation of patterns, such as sand ripples, dune fields, parallel channel networks, and periodically spaced valleys. In many cases, the development and evolution of patterns within landscapes are heavily influenced by environmental conditions. Therefore, given relationships between landform features and the underlying processes, present-day landscapes have the potential to be used to infer a record of climatic conditions over the course of their development. An inability to make direct observations over geologically relevant timescales makes it difficult to study the processes that influence landscape evolution. Mathematical models provide a means of quantitatively linking natural patterns and landscape features with physical processes. Patterns in landscapes also provide a simple means of testing quantitative representations of geomorphic processes. In this work, we develop landscape evolution models to study the development of debris-flow-dominated hillslopes, rill networks, and cinder cones. Through a combination of theoretical modeling, analysis of experimental data, and remote sensing data, we attempt to better understand each of these three systems. While each system is interesting in isolation, these and similar studies add to our knowledge of the mathematical representations of processes that are used more generally within the study of landscape evolution.

The magnitude of global sediment flux from streams to the oceans over the last 5 Ma is poorly quantified, yet important for predicting future fluxes and deciphering the relative control of tectonic uplift, climate change, vegetation, and related feedback mechanisms on landscape evolution. Despite numerous proxy studies on global sediment delivery to the oceans, it remains uncertain whether bulk sedimentation increased, decreased, or remained approximately constant across one of the most significant global climate changes: the Plio-Pleistocene transition. New developments and strategies in the application of cosmic-ray-produced isotopes, in part developed by this thesis, provide records of pre-historic denudation of confined fluvial catchments in Texas and Yukon. Non-glaciated, tectonically passive regions were targeted in contrast to other studies on modern sedimentation rates in order to isolate the climate influence from glacial and tectonic controls. The results suggest that average catchment temperature, and surficial processes and other factors such as vegetation cover associated with temperature, are the primary controls on the variation in landscape denudation in regions lacking tectonics and direct glacial cover. Specifically, warmer temperatures yield higher denudation rates, both at the scale of glacial-interglacial climate change and over the Plio-Pleistocene transition. The implication is that stream sediment flux to the ocean from tropical and temperate regions was higher during the Pliocene than in the Quaternary. However, this may have been balanced by an increase in sediment flux from regions covered by warm-based glaciers during glacial periods, or by increased temporary continental storage during interglacial periods.

Landscapes respond to external perturbations over a variety of timescales, including million-year tectonic forcing, millennial to decadal climate fluctuations, and minutes-long high intensity storms or large magnitude earthquakes. In mountainous regions, understanding the role of landslides in driving the hillslope response to these perturbations is paramount for understanding landscape evolution over geologic timescales and hazards over human timescales. Here I analyze the landslide-driven hillslope response over millennial to decadal timescales using a variety of tools and techniques (e.g. tephrochronology, lidar and air photo analysis, field and subsurface investigations, and seismic refraction) in the Waipaoa Basin (New Zealand) and Oregon Coast Range (USA). For the Waipaoa study catchment, pervasive landslides have been sculpting >99% of the hillslopes in response to >50 m of fluvial incision following the shift to a warmer, wetter climate after the Last Glacial Maximum (LGM) (~18 ka). Then, starting in the late 1800s, European settlement resulted in deforestation and conversion of >90% of the landscape to pastureland—spurring a rapid increase in landslide-driven erosion. To quantify the landscape response, I first reconstruct LGM and younger paleosurfaces using tephrochronology and lidar-derived surface roughness to estimate the volume, timing, and distribution of hillslope destabilization. From these reconstructions, I calculate the post-LGM catchment-averaged erosion rate (1.6 mm/yr) and determine that the timing of the initial hillslope adjustment was rapid and occurred by ~10 ka. Second, I quantify the rate and volume of historic hillslope degradation using a 1956-2010 sequence of aerial photographs, lidar, and field reconnaissance to map the spatial extent of active landslides, create a ‘turf index’ based on the extent and style of pastoral ground disruption, correlate that with downslope velocity, and calculate the average annual sediment flux. From the sediment flux, I calculate an erosion rate over the past ~50 years (~20 mm/yr) that is 10x greater than post-LGM. Lastly, in Western Oregon, I confirm that seismic refraction can determine the size (e.g. depth) and failure style of landslides in western Oregon—data needed to incorporate these poorly studied landslides into future landscape evolution or hazard models. This dissertation includes both previously published and unpublished co-authored material.

abstract: Climate and its influence on hydrology and weathering is a key driver of surface processes on Earth. Despite its clear importance to hazard generation, fluvial sediment transport and erosion, the drawdown of atmospheric CO₂ via the rock cycle, and feedbacks between climate and tectonics, quantifying climatic controls on long-term erosion rates has proven to be one of the grand problems in geomorphology. In fact, recent attempts addressing this problem using cosmogenic radionuclide (CRN) derived erosion rates suggest very weak climatic controls on millennial-scale erosion rates contrary to expectations. In this work, two challenges are addressed that may be impeding progress on this problem. The first challenge is choosing appropriate climate metrics that are closely tied to erosional processes. For example, in fluvial landscapes, most runoff events do little to no geomorphic work due to erosion thresholds, and event-scale variability dictates how frequently these thresholds are exceeded. By analyzing dense hydroclimatic datasets in the contiguous U.S. and Puerto Rico, we show that event-scale runoff variability is only loosely related to event-scale rainfall variability. Instead, aridity and fractional evapotranspiration (ET) losses are much better predictors of runoff variability. Importantly, simple hillslope-scale soil water balance models capture major aspects of the observed relation between runoff variability and fractional ET losses. Together, these results point to the role of vegetation water use as a potential key to relating mean hydrologic partitioning with runoff variability. The second challenge is that long-term erosion rates are expected to balance rock uplift rates as landscapes approach topographic steady state, regardless of hydroclimatic setting. This is illustrated with new data along the Main Gulf Escarpment, Baja, Mexico. Under this conceptual framework, climate is not expected to set the erosion rate, but rather the erosional efficiency of the system, or the steady-state relief required for erosion to keep up with tectonically driven uplift rates. To assess differences in erosional efficiency across landscapes experiencing different climatic regimes, we contrast new CRN data from tectonically active landscapes in Baja, Mexico and southern California (arid) with northern Honduras (very humid) alongside other published global data from similar hydroclimatic settings. This analysis shows how climate does, in fact, set functional relationships between topographic metrics like channel steepness and long-term erosion rates. However, we also show that relatively small differences in rock erodibility and incision thresholds can easily overprint hydroclimatic controls on erosional efficiency motivating the need for more field based constraints on these important variables. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2014

This thesis reconstructs the landscape evolution of the Billy Big Spring site (24GL304, Glacier County, north-central Montana) from the last glacial maximum to present through the analysis of sediment and soil samples collected from a transect of auger tests that bisected the site and surrounding landforms. Interpretations were drawn from stratigraphy, pedologic data, sedimentologic analysis and radiocarbon dating. The site landscape came into being in the late-Pleistocene, after Wisconsin-age glaciers retreated. Glacial retreat left a meltdown depression on the land that filled with water to form a pond, which persisted through the early-Holocene. The onset of the mid-Holocene (Altithermal) occurred before ~8,415 cal. yrs. BP, when increasingly arid conditions caused the water level to drop. The first radiocarbon dated human occupation of this site occurred during the Altithermal, ~7,030 cal. yrs. BP, after the eruption of Mount Mazama (~7,633 cal. yrs. BP). Arid conditions continued until ~7,000 cal. yrs. BP, when pond water re-expanded across the basin, marking the transition to the cooler late-Holocene. Sometime before 2,100 cal. yrs. BP, dry conditions returned, and the extent of the pond water decreased again. Since this time, overland alluvial processes have deposited sediments in the basin. Many hypotheses on how the Altithermal impacted the people of the Northwestern Plains have been proposed since the 1950s, but little agreement has been reached. This is due to the fact that there was great variation in how the Altithermal expressed itself throughout the Northwestern Plains. The human reactions to this phenomena cannot be explained simplistically for the region as a whole. This study shows that the Billy Big Spring site experienced drying during the Altithermal, but despite this, people continued to occupy this site. This evidence adds to the argument that the Altithermal climate of the Northwestern Plains did not have severe enough impacts to impose much hardship on its occupants.

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