abstract: Sedimentary basins are defined by extensional tectonics. Rugged mountain ranges stand in stark relief adjacent to muted structural basins filled with sediment. In simplest terms, this topography is the result of ranges uplifted along normal faults, and this uplift drives erosion within upland drainages, shedding sediment into subsiding basins. In southeastern Arizona's Basin and Range province extensional tectonics waned at approximately 3-5 Myr, and the region's structural basins began transitioning from internal to external drainage, forming the modern Gila River fluvial network. In the Atacama Desert of northern Chile, some basins of the Central Depression remain internally drained while others have integrated to the Pacific Ocean. In northern Chile, rates of landscape evolution are some of the slowest on Earth due to the region's hyperarid climate. While the magnitude of upland erosion driven by extensional tectonics is largely recorded in the stratigraphy of the structural basins, the landscape's response to post-tectonic forcings is unknown. I employ the full suite of modern geomorphic tools provided by terrestrial cosmogenic nuclides - surface exposure dating, conventional burial dating, isochron burial dating, quantifying millennial-scale upland erosion rates using detrital TCN, quantifying paleo-erosion rates using multiple TCN such as Ne-21/Be-10 and Al-26l/Be-10, and assessing sediment recycling and complex exposure using multiple TCN - to quantify the rates of landscape evolution in southeastern Arizona and northern Chile during the Late Cenozoic. In Arizona, I also use modern remnants of the pre-incision landscape and digital terrain analyses to reconstruct the landscape, allowing the quantification of incision and erosion rates that supplement detrital TCN-derived erosion rates. A new chronology for key basin high stand remnants (Frye Mesa) and a flight of Gila River terraces in Safford basin provides a record of incision rates from the Pliocene through the Quaternary, and I assess how significantly regional incision is driving erosion rates. Paired nuclide analyses in the Atacama Desert of northern Chile reveal complex exposure histories resulting from several rounds of transport and burial by fluvial systems. These results support a growing understanding that geomorphic processes in the Atacama Desert are more active than previously thought despite the region's hyperarid climate. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2014

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There is currently a project underway to restore many of the man-made salt ponds along the shores of South San Francisco Bay (SSFB) back to tidal marsh, potentially reestablishing these areas as sinks for SSFB sediments. While there have been recent studies examining the evolution of newly restored marsh areas in SSFB, there have been no recent projects focusing on the expected response of valuable tidal flat environments adjacent to the restored marshes. To help fill this void, this project seeks to characterize SSFB tidal flat morphodynamics, both spatially and temporally, through examination of historic morphologic variability and change along with variations in external forcings. Spatial and temporal trends in profiles of SSFB tidal flats are examined using bathymetric and LIDAR data collected between the 1890s and 2005. Eigenfunction analysis reveals a dominant mode of morphologic variability related to the degree of convexity or concavity in cross shore profile – classically indicative of tidally dominant, sediment rich, or wave dominant, sediment poor conditions, respectively. Two opposing areas of equilibrium shape – north/south of a constriction in estuary width located at the Dumbarton Bridge – are highlighted by the first mode of variability in the Eigenfunction analysis, accounting for 90% of the overall spatial variation in tidal flat shape. Additionally, the eigenfunction scores which quantify the spatial pattern of increasing/decreasing convexity in the inner/outer estuary are correlated to spatial variability in fetch length, sediment grain size, recent erosion/deposition, and tidal height. Results for spatial variation found herein are generally consistent with theoretical predictions of tidal flat morphologic response to waves, tides, and sediment supply. Trends for morphologic change between 1890 and 2005 in twelve geographically diverse regions within SSFB are compared to temporal trends in sediment discharge, mean sea level, diurnal tidal range, and Pacific Decadal Oscillation Index (as a proxy for storminess). Overall, convex vs. concave profiles were favored in the inner vs. outer estuary throughout the entire historical period. Furthermore, tidal flat morphology of the outer estuary displayed a steady increase in concavity with time. The trend of increasing concavity in the outer-estuary flats was consistent with temporal changes in hindcasted sediment discharge from the Central Valley. Although consistently convex, tidal flats located in the inner portions of SSFB exhibited greater complexity in their degree of convexity through time, and temporal changes could not easily be correlated to a given external physical forcing, suggesting a possible role for more localized variations in sediment supply. A set of criteria for establishing dependence between morphology and external factors was created, using results of a stepwise multiple regression. Using this criteria, trends sediment supply from the Central Valley were found to have a consistency with temporal trends in outerestuary tidal flat shape. Inner-estuary flat shape change was found to be consistent with local patterns in rainfall (as a proxy for local sediment discharge) in the innermost regions, and with recent deposition or erosion in all other regions.

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In order to understand the processes controlling the temporal variability in settling velocity (Ws) and bed erodibility (ε), in the middle reaches of the York River estuary, VA, the relationships between the hydrodynamics and particle types were investigated with a near-­‐bed Acoustic Doppler Velocimeter (ADV) and the York River 3-­‐D Hydrodynamic Cohesive Bed Model. ADV observations of the flow characteristics that occurred over a strong temporal transition period indicated that Ws and ε were characterized by two distinct regimes with contrasting sediment and water column characteristics: (i) a physically-­‐dominated regime (Regime 1) which was a period dominated by flocculated muds (flocs), and (ii) a biologically-­‐influenced regime (Regime 2) which was a period dominated by biologically formed pellets mixed with flocs. During Regime 1, Ws averaged about 0.5 mm/s, and ε averaged about 3 kg/m2/Pa. In contrast, during Regime 2 average Ws increased to 1.5 mm/s, and average ε dropped to 1 kg/m2/Pa. The change between these two regimes and the transition in Ws and ε were linked with the arrival and departure of a seasonal density front. Comparison between ADV observations and the results from the York River 3-­‐D Hydrodynamic Cohesive Bed Model suggested that the current model version was not conducive to examining the temporal variability in settling velocity associated with the transition of the distinct sediment regimes. The existing model version estimated realistic values for current speed and concentration and resolved the daily variation associated with in current speed, bed stress, concentration, and settling velocity. However, model estimates of bed stress, current speed, settling velocity, and erodibility did not suggest the presence of two distinct sediment regimes. The model did a poor job of predicting peak bed stresses and settling velocities. Both were over estimated by a factor of 2 throughout most of the study period. Possible modifications to create a version that is able to simulate the bed stresses and sediment properties (i.e. erodibility and settling velocity) during each regime with more accuracy are: (1) define finer sediment classes in the model that are more representative of the water column and not just the seabed, (2) use a consolidation time scale of 5 days rather than 24 hours to allow more sediment to be suspended at lower bed stresses, (3) further reduce hydraulic roughness, and (4) turn on sediment induced stratification.

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A cohesive sediment bed model was implemented in the Community Sediment Transport Modeling System (CSTMS) to examine processes influencing sediment erodibility and suspended sediment concentrations. Estimates of eroded mass from the sediment bed model were calibrated and verified with erosion chamber measurements from the York River, Virginia, a tidally-dominated environment. A constant erosion rate parameter combined with depth-varying critical shear stress was sufficient to model erosion observations of depth-limited sediment cores. Sensitivity of total eroded mass to seasonal variations in erodibility and changes in consolidation time scale was evaluated during spring-neap variations in bottom stresses. Differences were greatest during spring tide and varied by as much as a factor of 2.5. Consolidation created an asymmetry between the spring-to-neap and neap-to-spring transitions with more sediment being eroded during the decreasing phase of maximum tidal stress. Consolidation time scales controlled the magnitude of this asymmetry with larger asymmetries occurring when slower consolidation time scales were assumed. Eroded mass estimates were potentially as sensitive to uncertainties in the consolidation time scale as they were to observed seasonal variability in critical stress. The cohesive sediment bed model was then implemented within a numerical model of the York River Estuary to examine feedbacks between sediment ux convergence and erodibility. Model results show the development of a highly erodible pool of sediment near the ETM location. Even when sediment convergence processes were diminished, suspended sediment concentrations remain high due to high sediment erodibility. Sediment concentrations and erodibility exhibited high spatial variability in both the along and across channel directions. As opposed to the results of the one-dimensional model, sediment concentrations and erodibility estimates were less sensitive to variations in the consolidation rate than to the initial bed conditions. Model calculations of sediment concentrations and erodibility showed similar patterns to observational data.