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Hillslopes comprise the majority of unglaciated upland landscapes; they are the primary source for the production of sediment from bedrock, and the routing system by which sediment is delivered to the channel network. Yet the nature of hillslope response to changes in tectonic, climatic or base-level boundary conditions is poorly understood in terms of the spatial and temporal distribution of hillslope morphology. Here I exploit a previously published framework for exploring hillslope morphology in high relief landscapes (Roering et al., 2007), to address several critical questions: Does high resolution topography allow understanding of the processes and rates by which sediment is redistributed on hillslopes? If so, can hillslope morphology be used to map the spatial distribution of erosion rates and facilitate interpretation of the timing and magnitude of tectonic forcing, particularly in transient landscapes which are adjusting their erosion rates? And to what extent does variation in lithology influence hillslope evolution and morphology, and the ability to interpret process rates from hillslope form? In this thesis I sought to explain hillslope adjustment to changing boundary conditions through combining the predictions of analytical and numerical models with detailed analysis of real, high resolution topographic datasets (derived from LiDAR), focusing on two landscapes where the influence of tectonic forcing on base-level history is relatively well constrained, the Middle Fork Feather River in the northern Sierra Nevada, and the Dragon’s Back Pressure Ridge, on the Carrizo Plain, both in California. The Sierra Nevada of California is a west-tilted fault block composed primarily of granitoids formed during Mesozoic arc volcanism. The block underwent acceleration in uplift 5 - 3.5 Ma which is hypothesised to be caused be the drop-off of a dense root from the lower crust and replacement by hot asthenosphere, causing crustal buoyancy. A relict landscape has thus been uplifted and dissected by the major drainage routes crossing the range, which have eroded rapidly to form deep canyons. The fluvial network is characterised by breaks in slope (knickpoints) which migrate into the landscape to transmit the signal of increased erosion, setting baselevel conditions for adjacent hillslopes. Theoretical predictions for the morphology of hillslopes governed by a nonlinear sediment transport law, if the hillslopes have attained steady state (i.e. they are eroding in concert with base-level fall in adjacent valleys) reveal that the curvature of hilltops will be linearly proportional to erosion rates or rate of base-level fall. I present innovative techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates. This work is carried out in granitoid lithologies where the influence of bedrock heterogeneity is assumed no to be a first order control on hillslope morphology. Existing and new cosmogenic radionuclide analyses in the Feather River basin, California, suggest that erosion rates vary by over an order of magnitude from the remnant upland landscape to the incised river canyon. Hilltop curvature increases with erosion rates, allowing calibration of the hillslope sediment transport coefficient, which controls the relationship between hillslope gradient and sediment flux. This in turn allows the estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature. Additionally, despite the landscape containing gradient-limited hillslopes, hilltop curvature continues to increase with rising erosion rates, reflecting higher erosion rates than can be predicted by hillslope gradient. The distribution of hillslope morphology conforms well to predictions of a nonlinear sediment transport model, with measured values of hillslope relief varying with the product of hilltop curvature and hillslope length (proxy for erosion rate) in a manner similar to that predicted by Roering et al. (2007). Hilltop curvature can thus be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions provided that the response timescale of hillslopes is short relative to channels. Having focused on a landscape with roughly uniform bedrock geology to isolate drivers of geomorphic change, I sought to evaluate whether these techniques could be extended across lithologic contacts and throughout the landscape. Underlying geology influences the efficacy of soil production and transport on hillslopes, and resistance to erosion by valley-forming processes. Here, quantitative analysis of LiDAR digital terrain models was performed to search for a topographic signature in two distinct lithologies in the Feather River catchment in northern California; granodiorite and deformed volcanics. The two sites, separated by <2 km and spanning similar elevations, are assumed to have similar climatic and denudation histories. Responding to increased erosion rates, transient hillslopes exhibit high gradient but low hilltop curvature in the metavolcanics relative to theoretical predictions for steady state hillslopes. However, hillslopes in the granodiorite have, for the most part, variation in hilltop curvature, hillslope length and hillslope relief similar to model predictions for steady state hillslopes. The curvature of hilltops adjacent to main stem channels implies that the coefficient of sediment transport is two times larger in the granodiorite (c. 8.8 m2 ka-1) than in the metavolcanics (c. 4.8 m2 ka-1). The data suggest that hillslopes get shorter as erosion rates increase due to the increased influence of debris flows in valley incision, suggesting that drainage density increases with erosion rate. The incision wave associated with more rapid erosion in the Feather River has propagated further into a basin developed on the metavolcanics and hence this substrate is less resistant to channel incision. I review an inventory of values for the transport coefficient for hillslope sediment transport but find that no clear patterns emerge with varying lithology. However in unconsolidated substrates, precipitation may play an important role in modulating sediment transport through variation in rain splash impact frequency and the frequency of wetting/drying, freeze/thaw, and expansion/contraction cycles. Finally I apply the same techniques to study hillslope morphology to a landscape where the tectonic history has a documented influence on landscape development. The Dragon’s Back pressure ridge, Carrizo Plain, CA, consists of a series of small catchments adjacent to the San Andreas fault, where previous detailed geologic mapping has allowed the spatial and temporal distribution of uplift to be constrained. This landscape offers a hitherto unique opportunity to study the temporal evolution of hillslope morphology via ergodic substitution. I show that the time evolution of a sensitive indicator of erosion rate, hilltop curvature, can be predicted using a nonlinear sediment flux law. Further to this, the temporal evolution of relief and hilltop curvature experiences hysteresis as the landscape grows and decays. Relative to steady-state predictions, hillslope morphologies exhibit higher than expected values for relief during active uplift or landscape growth, and lower than expected relief during landscape decay. Therefore landscapes growing due to fault activity can be distinguished from those with quiescent faults undergoing topographic decay.

Streams draining coniferous plantations contain higher loadings of hydrogen ion, aluminium, sulphate and nitrate, in comparison with streams in adjacent grasslands. Almost all of this ion-load is transported to streams via subsurface water-pathways. An incontrovertible, physical characterization of these pathways within a natural, layered hillslope, has yet to be presented. This research has sought to provide such a characterization for two hillslopes - one afforested with conifers, the other an improved grassland. Much of the uncertainty associated with the identification of soil-water-pathways stems from an inadequate characterization of the errors imposed by the use of each measurement technique. This research has, therefore, compared the predictions of a number of quasi-independent field and analytical techniques, to attempt to lessen the impact of measurement error upon the observed response of the true hydrological system. The impact of conifers upon the detailed water-pathways and lumped catchment response was monitored to educe any changes in the hydrological response which could account for the increased loading of acidic solutes within forest streams. The results of the analysis, indicated that the pathways of water through hillslopes could be predicted from the response of hydrological properties averaged over control volumes of soil-pores. The accuracy of these solutions was proven by the concordance of the response of all of the properties contained within the Darcy-Richards equation. The marked horizon development within the ferric podzol soil of the instrumented forest hillslope, in particular the presence of an indurated B horizon, deflects most percolation laterally within the 0/A and A/E horizons. This pathway was indicted by the results of techniques which included numerical and approximative calculations, discontinuities between the state-dependent hydraulic conductivity of each soil horizon, and the generation of steep, vertical potential gradients in layered porous media. The instrumented grassland hillslope was ploughed 11 years prior to instrumentation. This greatly increased the conductivity of the controlling B horizon, allowing almost all flow to percolate to depth. During winter-storms, the forest hillslope generated flows smaller than those within the grassland hillslope, concomitant with the 29 percent difference in the rainfall-runoff behaviour of the catchment areas. This increased loss of runoff within the afforested areas, may result from the high losses of wetted-canopy-evaporation (39 percent of gross-precipitation) from the Sitka spruce (Picea sitchensis. Bong. Carr.) trees. Individual conifer trees growing on the steep, ferric podzol hillslope appeared to enhance the lateral deflection of flow within the O/A and A/E horizons, probably as a result of their platy root systems, and the high rates of precipitation input to soil at the stem-base. The enhancement of both lateral near-surface flow and below-canopy ion concentrations could, therefore, generate the chemical signatures characteristic of streams draining coniferous forests.

<p> Several surface soil properties and topographical measures were studied at two hillslopes within the Crawford Lake Conservation Area. These measures were examined to establish any interrelationships to support the catena concept proposed by Milne (1935).</p> <p> The results of the study show that no similar patterns as found by Anderson and Furley (1975) and which include a decrease in organic matter and increase in pH, carbonate content, or, finer particles downslope could be found. Other factors which could be found within a three-dimensional soil landscape and may influence the soil processes along a catena should also be adopted. This may then describe all relationships that could affect soil development across a hillslope.</p> / Thesis / Bachelor of Science (BSc)

Although blanket peat covers many major headwater areas in Britain, runoff production within these upland catchments is poorly understood. This thesis examines runoff production mechanisms within the blanket peat catchments of the Moor House National Nature Reserve, North Pennines, UK. Catchments ranging from 11.4 km^ down to the hillslope and plot-scale are examined. Runoff from the monitored catchments was flashy. Lag times are short and rainwater is efficiently transported via quickflow- generating mechanisms such that flood peaks are high and low flows poorly maintained. Hillslope and plot-scale runoff measurements show that the flashy catchment response is the result of the dominance of overland flow. Typically 80 % of runoff is produced as overland flow. This occurs both on bare and vegetated surfaces. Most of the remaining runoff is generated from the upper 10 cm of the peat, except where well-connected macropore and pipe networks transfer flow through the lower layers. Below 10 cm depth the blanket peat matrix fails to contribute any significant runoff Thus most groundwater-based models of peat hydrological process are not readily applicable to these catchments. Suggestions that infiltration-excess overland flow may be largely responsible for the flashy regime of these upland catchments are not substantiated by the blanket peat infiltration data presented in this thesis. Monitoring of hillslope runoff mechanisms combined with rainfall simulation (at realistic intensities of 3-12 mm hr(^-1)) and tension- infiltrometer experiments shows that saturation-excess mechanisms dominate the response. Infiltration is relatively rapid in the upper peat layers until they become saturated and overland flow begins. High water tables result in rapid saturation of the peat mass such that even at low rainfall intensity runoff production is just as efficient as during high intensity events. While macropores have largely been ignored in blanket peat, results presented suggest that up to 30 % of runoff may be generated through them. Occasionally these macropore networks develop through the deeper peat such that runoff bypasses the matrix and runs off at depth from small outlets and larger pipe networks. Seasonal variations in runoff- generating processes can be exacerbated by drought which causes structural changes to the near-surface of the peat. This was found to result in enhanced infiltration and macropore flow which may encourage pipe network development. Flow has been monitored simultaneously from several natural pipes in a 0.4 km(^2) headwater catchment of the Tees. This catchment provides one of the few examples of pipeflow monitoring outside the shallow peaty-podzols of mid-Wales. Natural pipes are found throughout the soil profile and can be at depths of up to three metres. Ground penetrating radar was useful in identifying deep subsurface piping and suggestions are made for improvements to the application. The pipe networks were found to be complex and results demonstrate that outlet location and size may bear little relation to the form and depth of the pipe a short distance upslope. Pipes generally contribute less than 10 % to catchment runoff but on the rising and falling hydrograph limbs can contribute over 30 % to streamflow. Pipeflow lag times are short suggesting that both the shallow and deep pipes may be well connected to the surface. Thus while matrix runoff contributions at depth within the peat may be low, macropore flow mechanisms can be significant.

The geomorphic and geochemical characteristics of landscapes impose a physical template on the establishment and development of ecosystems. Conversely, vegetation is a key geomorphic agent, actively involved both soil production and sediment transport. The evolution of hillslopes and the ecosystems that populate them, are thus intimately coupled; their co-dependence potentially has a profound impact on the way in which landscapes respond to environmental change. This thesis explores how rates of erosion, integrated over millennia, impact on the structural characteristics of the mixed conifer forest that presently mantles this landscape, the development of the underlying soils and emergence of bedrock. The focus for this investigation is the Feather River Region in the northern Sierra Nevada in California, a landscape characterised by a striking geomorphic gradient accompanied by spatial variations in erosion rate spanning over an order of magnitude, from 20 mm ka-1 to over 250 mm ka-1. Using LiDAR data to quantify forest structure, I demonstrate that increasing rates of erosion drive a reduction in canopy height and aboveground biomass. Subsequently, I exploit a novel method to map rock exposure, based on a metric of topographic roughness, to show that as erosion rates increase and soil thickness consequently decreases, the degree of bedrock exposed on hillsides increases. Importantly, this soil-bedrock transition is gradual, with rapidly eroding hillslopes frequently possessing a mosaic of bedrock outcrop and intermittent soil mantle. Both the ecological and geomorphic trends are shown to be impacted by the underlying bedrock, which provides an additional source of heterogeneity in the evolution of the Feather River landscape. The negative correlation between AGB and erosion rate has potential implications for soil production. Using a simple hillslope model I show that if this decrease in AGB is associated with a drop in biotic soil production, then feedbacks between soil thickness and biotic soil production are capable of generating a complex response to geomorphic forcing, such that hillslopes possess multiple stable states: for intermediate rates of erosion, equilibrium hillslopes may be either soil mantled or bedrock. Hillslope evolution in these simulations is path dependent; once exposed at the surface, significant patches of bedrock exposure may persist over a wide range of incision rates.

Riparian zones are increasingly being adopted as best management practices (BMPs) to control nonpoint source pollution. The effectiveness of these zones in mitigating pollution is a function of the distribution, nature, and rate of water and sediment movement through these zones. The intent of this research was to investigate the influence of site conditions on the hydrologic and sediment transport response of riparian zones/hillslopes. Research investigations were focused in two major areas: field investigations of riparian hillslopes and development of a riparian hillslope model. The objective of the field investigations was to characterize and quantify geomorphic features of riparian slopes that can be used to quantify flow concentration on hillslopes. The riparian hillslope model was used to investigate the dynamics of hydrologic and sediment transport processes. Field investigations revealed that riparian hillslopes were dissected into distinct convergent, divergent, or straight slope segments. In profile, these segments were either concave, straight, or convex. It was hypothesized that the size of such segments reflects the "representative hillslope scale". Probability distributions of catchment area showed that catchment area decreases with slope gradient. Distributions of catchment shape revealed that catchment shape elongates with increasing gradient. Distributions of drainage channel cross-sectional shape data showed a decreasing trend in width to depth ratio with increasing slope gradient. These results indicate that geomorphic features characterizing flow concentration vary with slope gradient and should not be neglected when simulating riparian hillslopes. Model simulations revealed that site conditions such as slope gradient, slope shape, flow concentration, and soil horizon thickness and characteristics play a significant role in shaping the hydrologic and sediment phenomena on these hillslopes. These results underscore the need for evaluation of riparian zones considering specific site conditions. Interflow was the dominant hillslope runoff mechanism. A large fraction of the interflow occurred via macropores. Macropore flow was orders of magnitude quicker than soil matrix flow. Overland flow was found to occur primarily due to saturation excess or return flow. Simulations showed that thinning of soil layers and/or concave-convergent slope shapes provide favorable conditions for generation of saturation excess or return flow. Sediment delivery down the slope increased with increasing flow concentration. / Ph. D.

Debris flows are a known hazard in southern California where growing numbers of people are moving into the urban-wildland interface, threatening lives and property. A common location to see a debris flow head scarp is the upper one-third to one-half of an unburned slope at or near the head of a first-order catchment, particularly in areas of relatively shallow soils overlying bedrock. Unburned, relatively steep slopes with gently rounded shoulders and thin soil over bedrock in southern California were investigated to determine if there is a position on these types of slopes where near-surface water levels and the associated pore pressures are relatively and consistently higher during and after rainfall events than the rest of the slope, resulting in an area of preferential shallow slope failure and debris flow initiation. It was hypothesized that this position, if it exists, would be on the upper one-third to one-half of the slope near a change from a shallower slope to a steeper slope (the slope shoulder). It was further hypothesized that elevated subsurface pore pressures at this location would contribute to it being an area of preferential shallow slope failure. The near-surface water levels at two field sites in southern California were monitored for three field seasons. In the laboratory, a meso-scale simulator was constructed and used to replicate field conditions using an adjustable artificial slope and simulated rainfall. The field research showed that areas of higher water levels can exist on the upper one-third to one-half of hillslopes meeting the designated criteria. The laboratory simulations showed elevated water levels in the same general area as the field data. Laboratory simulations also suggested that this is an area of preferential shallow slope failure. The angle of the slope influenced how long a slope took to fail and how much water was needed to do so, with gentler slopes requiring more time and approximately double the amount of water than steeper slopes.

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