Thesis (M.Env.Dev.) - University of KwaZulu-Natal, Pietermaritzburg, 2009. / Full text also available online. Scroll down for electronic link.

Mountainous regions are ubiquitously dissected by river networks. These networks are the main drivers by which climate and tectonic signals are transmitted to the rest of the landscape, and control the response timescale of the landscape to these external forcings. Furthermore, river systems set the downslope boundary conditions for hillslope sediment transport, which controls landscape denudation. Therefore, understanding the controls on the organisation and structure of river networks in upland landscapes is an important goal in Earth surface processes research. The recent introduction of high-resolution topographic data, such as airborne lidar data, has revolutionised our ability to extract information from the topography, providing new opportunities for linking geomorphic process with landscape form. This thesis is focused on developing techniques for analysing high-resolution topographic data to quantify and understand controls on the structure of fiuvial systems in upland landscapes. Firstly, I develop and test new algorithms for objective feature extraction from lidar-derived digital elevation models (DEMs). I present a new method for identifying the upstream extent of channel processes by identifying scaling breaks in river long profiles. I then compare this new method to three existing methods of channel extraction, using field-mapped channel heads from four field sites in the US. I find that the new method presented here, along with another method of identifying channels based on valley geometry, most accurately reproduces the measured channel heads in all four field sites. I then present a new method for identifying floodplains and fiuvial terraces from DEMs based on two thresholds: local gradient, and elevation compared to the nearest channel. These thresholds are calculated statistically from the DEM using quantile-quantile plots and do not need to be set manually for each landscape in question. I test this new method against field-mapped floodplain initiation points, published flood hazard maps, and digitised terrace surfaces from eight field sites in both the US and the UK. This method provides a new tool for rapidly and objectively identifying floodplain and terrace features on a landscape scale, with applications including flood risk mapping, landscape evolution modelling, and quantification of sediment storage and routing. Finally, I apply these new algorithms to examine the density of channel networks across a range of mountainous landscapes, and explore implications for fluvial incision models. I compare the relationship between drainage density (Dd) and erosion rate (E) using both analytical solutions and numerical modelling, and find that varying the channel slope exponent (n) in detachment-limited fluvial incision models controls the relationship between Dd and E. Following on from this, I quantify Dd for five field sites throughout the US. For two of these field sites I compare Dd to cosmogenic radionuclide (CRN)-derived erosion rates, and for each site I use mean hilltop curvature as a proxy for erosion rate where CRN-derived erosion rates are not available. I find that there is a significant positive relationship between Dd, E, and hilltop curvature across four out of the five field sites. In contrast to assumptions made in many studies of fluvial incision, this positive relationship suggests that the channel slope exponent n is greater than unity for each of these landscapes, with fundamental implications for both landscape evolution and sediment transport.

During the past decade, a series of record flood stages have occurred along the Tisza River, resulting in extensive damage and displacing the local floodplain population. Previous research on the Tisza River in Hungary showed increases in flood stages for fixed discharges (above bankfull). These results suggest that a loss of conveyance has taken place on the Tisza River, contributing to recent record flood levels. In order to assess the potential causes of flood conveyance losses, new hydrological and geospatial data were obtained from the Hungarian Federal Hydrological Authority (VITUKI) in order to develop hydrodynamic models for two reference conditions, 1979 and 1999, along two study reaches on the Middle Tisza River. In addition to these models of actual conditions at each time step (i.e., "retro-models"), four "scenario models" were developed in order to test the individual impacts of each of the hypothesized causal mechanisms: 1) channel geometry, 2) levees, 3) floodplain roughness (land cover), and (4) channel roughness. Comparison of the models for actual conditions ("retro-model") and the scenario models allows for quantitative assessments of the impacts of each of these parameters on flood conveyance. In addition to these four scenario models, an additional land cover scenario-model was created to further assess the impact of floodplain roughness on stage. Assessment of the two retro-models showed changes in flood stage along both study reaches over the 20-year time step. Along the upper study reach, changes in stage ranged between 0.64 and 1.64 m. On the lower study reach, changes in stage ranged from 0 to 1.75 m. These changes in stage were larger on average than changes in stage previously detected by Venzcel, 2008. Contributions from the channel geometry scenario model were significant on the upper study reach where channel geometry decreased flood stage by up to 0.58 m at Kisköre over the 20-year time step. This was attributed to the dam that was constructed immediately upstream from the Kisköre gauge on the upper study reach. Construction of a dam would cause incision downstream, and therefore, a decrease in flood stage over time due to channel geometry. On the lower study reach, change in stage due to channel geometry was negligible. Contributions from the levee scenario model were negligible on both the upper and lower study reach with a maximum change in stage over the 20-year time step at only 0.11 m. However, during the time step analyzed, no significant change in levee configuration occurred. Changes in stage due to changes in land cover were also negligible along both study reaches. An additional land cover scenario model was created to assess the impact of land cover change on modern flood levels. This scenario model set Manning's n values for floodplain roughness to 0.04, which approximates roughness for pasture and cropland. This scenario model showed that flood stages will decrease by 0.34-0.40 m, but only if the entire floodplain is converted from woodland to pasture/cropland. Channel roughness accounted for the majority of change in stage during the 20-year time step. Along the upper study reach, channel roughness accounted for an average of 1.51 m of change over the 20-year time step. On the lower study reach, channel roughness accounted for 1.03 m of change. Change in the stage/discharge relationship is the best explanation for changes in stage due to channel roughness. Most likely, the channel did not experience such a large change in roughness during the 20-year time step. Instead, the two flood years used in this analysis were different "types" of floods (with differing degrees of hysteresis), and the stage/discharge relationships could not be easily compared. Comparing these two floods may have exaggerated the total change in stage between the two retro-models. Backwater effects are a likely explanation for the change in hysteresis between the two flood years.

Doctor of Philosophy / Department of Geography / Charles W. Martin / Disturbance regimes are important determinants of both terrestrial and aquatic ecosystem structure and function. Disturbances may linger in the landscape and lag temporally, influencing stream ecosystem form and function for decades, if not centuries. The recent enhanced Mountain Pine Beetle (MPB) infestation in pine forests of the Rocky Mountain region has resulted in extensive tree mortality, producing the potential for significant increases in carbon supply to stream channels. To better understand MPB impacts on in-stream large wood (LW), a census was conducted in 30 headwater streams within the Medicine Bow National Forest in south-central Wyoming, across the temporal spectrum from early- to late-stage MPB-infestation. A subset of those streams exhibiting mean conditions at each level of infestation was surveyed to determine any significant differences in channel morphology or aquatic ecosystem function. Results indicate that wood loads related to the MPB-infestation significantly increase with time since initial infestation. However, even in late-stage infestation streams, many of the fallen MPB-killed trees are bridging across the channels and have yet to break and ramp down sufficiently enough to enter between the channel margins. Wood loads will continue to increase as more trees fall and bridging pieces decompose, break and then enter the channel. Measurable increases in the amount of LW with time since initial beetle infestation have both positive and negative effects on channel form and function. Although forest MPB-infestation has peaked in the study area, streams are still early on a curve of rapidly increasing wood loads that are beginning to affect streams and have the potential to dramatically increase the carbon base of regional stream ecosystems.

No description available.

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: John C. Kraft, Dept. of Geology. Includes bibliographical references.

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 43-46). Also available on the World Wide Web.

Doctor of Philosophy / Department of History / James E. Sherow / During the period between 1848 and 1938, a combination of land-use changes and regional climatic alterations caused changes in the physical structure of the Canadian River. The Canadian River begins in the southern Rocky Mountains and flows south and then northeast across the High Plains of New Mexico and Texas. The Comanche Indians used the river as a transportation corridor, as a winter shelter for themselves and for their horse herds, as well as hunting the bison that visited the valley. The Comanches also valued the spiritual power, puha, found in the running water and on the mesas within the river’s lowlands. After the defeat of the Comanches in the Red River Wars and the destruction of the bison herds, New Mexican Hispanos moved their flocks of sheep into the valley and established settlements along the tributary streams. These settlers practiced “extensive” land use, drawing from a broad array of the valley’s resources and using them comparatively lightly in ways that drew from older Spanish laws and customs. The enclosure of parts of the valley by Anglo-Texan ranchers drove the Hispanos out of the Canadian watershed in Texas, although access to the open range in New Mexico allowed other Hispanos to retain their settlements. Corporations including the Capitol Lands Syndicate and Prairie Cattle Company introduced large numbers of cattle to the region at the same time that regional rainfall patterns shifted. This combination of heavy grazing and altered precipitation patterns led to erosion in the uplands that caused changes in the physical structure of the Canadian River. After 1903, the arrival of railroads into eastern New Mexico accelerated the development of dry-land farms in both states. Increasing calls for damming and controlling the Canadian led to the first interstate Canadian River Compact in 1928. The advent of a severe drought in the 1930s and the Great Depression led to federal resources becoming available and the first dam was built on the stream, ending the era of the free-flowing river and again starting physical changes to the Canadian.

Chute channel formation is a key process in the transition from a single-thread meandering to a braided channel pattern, but the physical mechanisms driving the process remain unclear. This research combines GIS and spatial statistical analyses, field survey, Delft3D hydrodynamic and morphodynamic modelling, and Pb-210 alpha-geochronology, to investigate controls on chute initiation and stability, and the role of chute channels in the planform dynamics of large, sand-bed meandering rivers. Sand-bed reaches of four large, tropical rivers form the focus of detailed investigations; the Strickland and Ok Tedi in Papua New Guinea, the Beni in Bolivia, and the lower Paraguay on the Paraguay/Argentina border. Binary logistic regression analysis identifies bend migration style as a key control on chute channel initiation, with most chute channels forming at bends that are subject to a rapid rate of extension (elongation in a direction perpendicular to the valley axis). Bend extension rates are shown to track variation in potential specific stream power, such that reaches and sub-reaches of the rivers studied fit within a planform continuum expressed though increasing bend extension rates and chute initiation frequency, and driven by increasing stream power relative to bedload calibre. Field observations of point bar geomorphology and vegetation dynamics illustrate the importance of rapid bend extension in forming wide sloughs between scroll bars that are aligned with the direction of over-bar flow, and in breaking the continuity of vegetation encroachment on point bars. Bathymetric surveys and Delft3D simulations for the Strickland River provide insight into flow and sediment division at bifurcate meander bends. Coupled with GIS analyses, these simulations show that stable chute channels have higher gradient advantages than chute channels subject to infill, but that upstream and downstream changes in bend orientation can also influence chute stability. The process of bend extension is typically associated with an increase in the chute gradient advantage, further elucidating the role of bend migration style in chute stability. At the reach scale, rivers with higher sediment loads (Qs/Q) are characterised by higher rates of chute infill. Strickland River floodplain sedimentation rates derived through Pb-210 alpha-geochronology are substantially higher adjacent to single-thread bends than adjacent to bifurcate bends, potentially due to an observed increase in channel capacity (and reduction in floodplain inundation frequency) associated with bend bifurcation. Further research is needed to determine whether this observation is significant in light of high uncertainty in the spatial variability of sedimentation rate estimates, but the data presented highlight a need for carefully considered stratified sampling approaches in floodplain coring campaigns, and illustrate the complexity of possible sediment dispersal mechanisms, and associated ecological responses. GIS analysis of the response of the Ok Tedi in Papua New Guinea to direct addition of mine tailings elucidates interplay between channel steepening due to the propagation of a tailings sediment slug, and mid-channel bar formation induced by the increased sediment load, with associated topographic forcing of bend and chute development. A temporal pattern of increased chute initiation frequency on the Ok Tedi mirrors the inter- and intra-reach spatial pattern of chute initiation frequency on the Paraguay, Strickland and Beni Rivers, where increased stream power is associated with increased bend extension and chute initiation rates. The process of chute formation is shown to be rate-dependent, and the threshold value of bend extension for chute initiation is shown to scale with reach-scale stream power, reminiscent of slope-ratio thresholds in river avulsion. However, Delft3D simulations suggest that chute formation can exert negative feedback on shear stress and bank erosion in the adjacent mainstem bifurcate, such that the process of chute formation is also rate-limiting. Chute formation is activated iteratively in space and time in response to changes in river energy, selectively targeting sites of greatest change, and thereby mediating the river response.

Morphological changes along several channels have raised concerns in the Province of Manitoba. This thesis presents a comprehensive study of fluvial morphological processes in open channels. Due to the recent concerns in the Province of Manitoba, the study mainly focused on the Red River in the city of Winnipeg, and two diversion channels in northern Manitoba. Morphodynamic conditions of these channels have become more complicated due to the cohesive nature of the channels bed and bank material and significant effects of subaerial processes. Several field measurement techniques, experimental setups, and numerical models were used to gain a better understanding of these complicated processes within the study reaches. Field measurements include soil sampling, water sampling, hydrometric surveys using an ADCP; the experimental setup includes several standard soil properties tests as well as an erosion measurement test; numerical modelling includes hydrodynamic and thermal modelling to quantify applied shear stress and seasonal freeze-thaw processes. Moreover, the effect of deposition processes on the final geomorphology of the study areas is discussed. / October 2016