Doctor of Philosophy / Department of Geography / Melinda Daniels / Great Plains rivers are unique systems that vary from large, continental scale, to small intermittent streams with grain sizes that range from bedrock to cobbles to silt. These rivers have been subject to widespread hydrologic alteration both within the channel and the watershed, which has resulted in an alteration in their hydrologic and geomorphic regimes. Although there is an acknowledgement of this alteration, to date there has not been a synthesis of the hydrology of Great Plains rivers or of their longitudinal morphologies. Chapters in this dissertation provide, to my knowledge, the first comprehensive analyses of the hydrology and morphology of Great Plains rivers over a range of spatial and temporal scales. In the first study, I found that there was no uniform pattern of hydrologic alteration throughout the Great Plains, which is likely attributable to variable system-specific reservoir management objectives, land use changes, and climatic regimes over the large area the Great Plains encompass. Results of this study are the first to quantify the widespread hydrologic alteration of Great Plains rivers following impoundment. In the second study, I found an apparent decoupling between local moisture conditions and streamflow in intermittent prairie streams. Results of this study used statistical models to identify relationships between flow intermittence, mean annual flow, and flood flow characteristics with moisture to characterize flow in an intermittent prairie stream. In the final study, I found that the downstream trends in hydraulic geometry and substrate characteristics of the Ninnescah River were consistent with the expected trends proposed by hydraulic geometry and substrate theories. However, there were points that deviated from the expected trends, most notably where a substantially large tributary enters the Ninnescah River and as the Ninnescah River approaches the Arkansas River, and causal explanations for these deviations were explored. Results of this study are, to my knowledge, the first of its kind to assess the longitudinal hydraulic geometry and substrate characteristics of a large sand-bed river over a large spatial scale. To our knowledge, there have been no comparable studies exist that attempted to describe hydrologic and geomorphic characteristics of prairie streams.

<p> Field studies of Cordilleran metamorphic core complexes indicate that meteoric fluids permeated the upper crust down to the detachment shear zone and interacted with highly deformed and recrystallized (mylonitic) rocks. The presence of fluids in the brittle/ductile transition zone is recorded in the oxygen and hydrogen stable isotope compositions of the mylonites, and may play an important role in the thermomechanical evolution of the detachment shear zone. Geochemical data show that fluid flow in the brittle upper crust is primarily controlled by the large-scale fault-zone architecture. </p><p> We conduct finite element numerical modeling of groundwater flow in an idealized cross-section of a metamorphic core complex. The simulations investigate the effects of crust and fault permeability fields on groundwater flow. Results show that fluid migration to mid- to lower-crustal levels is fault-controlled and depends primarily on the permeability contrast between the fault zone and the crustal rocks. High fault/crust permeability ratios lead to channelized flow in the fault and shear zones, while lower ratios allow leakage of the fluids from the fault into the crust.</p>

<p> Regional climate models project that precipitation in the Great Plains of North America will become characterized by more intense rainfall events separated by longer dry periods. Changing seasonal precipitation patterns may differentially favor grassland productivity in ecosystems dominated by either cool or warm season grass species, and thus influence carbon uptake and loss in these systems. Furthermore, model estimates of ecosystem respiration based primarily on soil temperature could overestimate respiration by failing to account for the effects from saturated conditions during heavy precipitation events. This research contrasted water and carbon fluxes during two years with different intra-annual precipitation within a cool season mixed grassland and compared to a neighboring warm season grassland in Rocky Flats National Wildlife Refuge, Colorado, USA. Results from this study showed a significant positive relationship between the accumulated April/May precipitation and growing season carbon uptake in the cool season, smooth brome-dominated grassland. In addition, significant rainfall in the autumn of 2013 played a role in the early spring growth and carbon uptake in 2014. Comparisons between eddy covariance and soil flux-gradient observations and model estimates of soil respiration showed that during the extreme precipitation event in September 2013, processed-based models better characterized fluxes as compared to empirical models based on soil temperature. The study also found that the cool season grassland was a net sink of carbon during the spring and autumn whereas the neighboring warm season tallgrass prairie was a net sink during the summer. In addition, the study found that the grasslands had considerably different sensitivities to water limitations, with grasses in the tallgrass prairie having a higher water use efficiency (WUE). The comparison of the adjacent semiarid grasslands at Rocky Flats NWR improves our understanding of the response to changing precipitation between cool season and warm season dominated grasslands. This research underscores the importance of expanding grassland research to understand how the composition of grasses will influence carbon cycling, especially as precipitation patterns shift with changing climate. Moreover, this research will add to observations during extreme precipitation events, which can improve both empirical and process-based models of soil respiration.</p>

<p> Flow regime, the magnitude, duration and timing of streamflow, controls the development of floodplain landforms on which riparian vegetation communities assemble. Streamflow scours and deposits sediment, structures floodplain soil moisture dynamics, and transports propagules. Flow regime interacts with environmental gradients like climate, land-use, and biomass-removing disturbance to shape riparian plant distributions across landscapes. These gradients select for groups of riparian plant species with traits that allow them to establish, grow, and reproduce on floodplains &ndash; <i>riparian vegetation guilds.</i> Here I ask, <i>what governs the distributions of groups of similar riparian plant species across landscapes?</i> To answer this question, I identify relationships between riparian vegetation guilds and communities and environmental gradients across the American West. In Chapter One, I discuss guild-based classification in the context of community ecology and streams. In Chapter Two, I identified five woody riparian vegetation guilds across the interior Columbia and upper Missouri River Basins, USA, based on species&rsquo; traits and morphological attributes. I modeled guild occurrence across environmental gradients, including climate, disturbance, channel form attributes that reflect hydrology, and relationships between guilds. I found guilds&rsquo; distributions were related to hydrology, disturbance, and competitive or complementary interactions (niche partitioning) between co-occurring guilds. In Chapter Three, I examine floodplain riparian vegetation across the American West, identifying how hydrology, climate, and floodplain alteration shape riparian vegetation communities and their guilds. I identified eight distinct plant communities ranging from high elevation mixed conifer forests to gallery cottonwood forests to <i>Tamarisk</i>-dominated novel shrublands. I aggregated woody species into four guilds based on their traits and morphological attributes: an evergreen tree guild, a mesoriparian shrub guild, a mesoriparian tree guild, and a drought and hydrologic disturbance tolerant shrub guild. Communities and guilds&rsquo; distributions were governed by climate directly, and indirectly as mediated through streamflow. In Chapter Four, I discuss the utility of guild-based assessments of riparian vegetation, current limitations to these approaches, and potential future applications of the riparian vegetation guild concept to floodplain conservation and management. The classification of vegetation into functional trait-based guilds provides a flexible, framework from which to understand riparian biogeography, complementing other models frameworks for riparian vegetation.</p>

<p> This study investigated the effects of riparian vegetation on sediment transport rates and flow dynamics in the South Platte River just downstream of Fort Lupton, Colorado. FaSTMECH, a two-dimensional coupled fluvial and hydraulic model, was used to compute flow characteristics (velocity and depth) in addition to sediment mobility characteristics (shear stress and sediment flux) for four discharge levels ranging from 5% of bankfull flow to bankfull flow (Qbf). Estimates of a dimensionless drag coefficient (Cd) representative of the middle-aged bushy willows found on the river banks at the study site were used to create a spatially variable roughness in the model throughout the river reach. Model results show that during average annual flood events, vegetation on the river banks causes increased drag forces on the flow, leading to an increased proportion of flow being diverted into the main channel and resulting in higher velocities. The spatial distribution of shear stresses collapse under these conditions with an order of magnitude decreases over river banks and significant increases throughout the main channel. Sediment fluxes in the reach increase by nearly an order of magnitude with the presence of bank vegetation, however, the greatest differences occur during Qbf when the highest fraction of the sediment is mobile. Further analysis of vegetation effects was conducted by performing a sensitivity analysis by altering the representative non-dimensional vegetation drag coefficient by as much as +/- 400%. These alterations represent differences in vegetation density, height, orientation, leafy/leafless structure, age, rigidity, and vegetation type. Although there is a relationship between sediment fluxes and changes in Cd, there only exists a 14% increase in transport at Qbf between the two exterior limits of Cd.</p>

<p> The traditional paradigm of baseflow generation assumes a uniform water table contributes baseflow evenly across a watershed. This thesis considers an alternate paradigm in which baseflow originates from a mix of localized sources that drain at different rates. Four forested headwater catchments across the United States were examined for spatial variability in baseflow sources by analyzing fractional baseflow contribution from each subcatchment relative to the catchment outlet. This revealed that subcatchment flow contributions changed dynamically through time, supporting the idea of different drainage rates in different places. A parallel linear reservoir model, which is predicated on heterogeneity in flow sources and not groundwater hydraulics, was used to simulate results consistent with observations in some of the study catchments. These results support the idea that in some locations baseflow recession may be better explained by landscape spatial heterogeneity than by aquifer hydraulics. </p>

<p> This dissertation combines field and laboratory work to examine how delta hydrology and sedimentology affects trace element cycling in marsh porewaters. This work was prompted by our lack of understanding of the hydrogeochemistry in the lower Mississippi Delta and how biogeochemical reactions affect fluxes of groundwater constituents to the ocean. In particular, I measured the concentrations of a suite of redox-sensitive trace elements (Fe, Mn, V, As) to determine the dominant geochemical reactions operating in marsh aquifers. </p><p> Because much more is known about As, I first conducted a study comparing V geochemistry along flow paths in a reducing sand aquifer in Texas and an oxidizing bedrock aquifer in Nevada. In agreement with other studies, V concentrations are much higher under oxidizing alkaline conditions. Under the reducing, circumneutral conditions of the Carrizo Sand aquifer, V concentrations are low but relatively constant, owing to complexation with dissolved organic matter. Similar observations regarding V geochemistry are observed in marsh groundwaters. Specifically, in pore waters of organic-rich sediment experiencing sulfate-reducing conditions, V concentrations are high owing to V complexation with organic matter, whereas in coarser-grained sediments, V may be removed from solution by adsorption. Arsenic geochemistry in pore waters varies as a function of depth. In the shallow subsurface, As concentrations are high and stabilized in solution by formation of thioarsenates. At depth, As appears to be sequestered through coprecipitation with pyrite.</p>

<p> Groundwater plays an important role in runoff generation in the humid tropics, both as subsurface stormflow during rain events and sustaining baseflow during dry periods. Yet groundwater fluxes in tropical areas, particularly groundwater/surface-water (GW-SW) interactions, are not very well characterized at regional scales, thus preventing us from estimating how climate change and anthropogenic activities will affect future groundwater availability. In the case of Panama, abundance of water resources has caused its misuse and thus groundwater is exploited without previous knowledge of its distribution and availability. However, regions of the country such as the Central Pacific Region suffer from water scarcity during periods of extended drought, when streamflow reduces significantly and shallow wells get dry. Understanding groundwater dynamics, especially GW-SW interactions, is crucial for government authorities to make informed decisions in order to secure water availability for current and future generations. This thesis presents advances on the characterization of the La Villa groundwater basin, located in the Central Pacific Region of Panama. By building a groundwater conceptual and numerical model, and a surface water model, potential recharge areas and groundwater flow patterns were identified. Also, the model reveals that groundwater feeds the rivers, not only during dry periods, but throughout the year. Although this preliminary model is not yet capable of predicting the total amount of groundwater stored, and neither can be used to inform management decision, it can inform us of which features have the greatest influence on groundwater flow and it can tell us what types of data are necessary to improve the results of the simulation. The development of these models is the first step towards the development of an integrated hydrologic simulation that can be used to test different climate change and/or management scenarios.</p><p>

<p> The midcontinent of the U.S. is heavily karstified containing well developed subsurface drainage systems that are covered by beds of coarse-grained, poorly sorted fluvial sediments, resembling those found in upland surface streams. The movement of coarse sediment as bed load within karst streams has been considered negligible in the past as it was assumed that all karst is developed through dissolution rather than mechanical abrasion. The frequency and magnitude of sediment transporting events in karst streams has implications for models of fluviokarst landscape development and the stability of aquatic ecosystems. </p><p> Within Tumbling Creek Cave (TCC) in the Ozark Plateau of south-central Missouri, and Bear Cave Hollow (BCH), one of TCC&#129;es surface drainage streams, bed load entrainment and transport dynamics of coarse-grained (16-180 mm), mainly siliciclastic material, was evaluated using hydrological measurements and 670 painted tracer particles. Tracers are used in this research for the first time in a karst stream. Tracers are well suited for studying the stochastic and spatially variable nature of bed load transport because they reflect the movement of individual particles of known characteristics, and they are also inexpensive and simple to employ.</p><p> Median surficial sediment grain size in the study reaches ranged from 39 to 71 mm in TCC, and from 24 to 37 mm in BCH with bed and/or water slopes ranging from 0.006 to 0.077 in TCC and from 0.002 to 0.009 in BCH. TCC is classified as a pool-riffle channel morphology type and BCH is classified as a plane-bed channel. Preliminary data from surveys of the longitudinal (downstream) movement of tracers over a 10-month period indicate that minor amounts (0-13.2%) of coarse bed material in TCC are mobilized by relatively low flows (5-28% of bankfull) that recur somewhat frequently (less than 3.1 years). BCH transports a higher percentage of material (0-59.1%) during similar flows (2-29% of bankfull) and frequencies (less than 3.59 years). Bed load transport was observed to be in a state of partial transport for any one grain size class in TCC during the study, while the complete mobilization of tracer size classes was observed in BCH at the highest observed flow, indicating &ldquo;phase 2&rdquo; transport and the break-up of the armor layer. The differences are attributed to the wider observed range of grain sizes covering the bed in TCC compared to BCH.</p><p> The use of the Shields (1936) criteria tends to over predict the critical shear stress required for entrainment of the largest mobilized grain size of individual tracers, while the empirical equation of Bagnold (1980) performs much better. Thus, the Shields equation may be better suited as a gage for complete mobilization of a grain size class across a reach, while the Bagnold (1980) equation may be better suited for estimating entrainment of grains from patches of the bed.</p><p>

<p>The Peruvian/Bolivian Altiplano is an important hydrologic system for paleoclimate reconstruction because it is unique in its ability to record climate variability associated with the near-continental scale South American summer monsoon (SASM), which is responsible for much of the precipitation over the Amazon basin and the southern subtropics. Over long timescales moisture on the Altiplano fluctuates in intensity due to changes in precessional insolation forcing as well as teleconnections to decadal-to-millennial scale abrupt temperature shifts in the Northern hemisphere Atlantic. These long-term changes in moisture transport to the Altiplano have been observed in multiple paleoclimate records, including drill core records and paleo-lake level records, as apparent advances and retreats of large lakes in the terminal basin occupied by the Salar de Uyuni and the Salar de Coipasa. </p><p>Presented here are the results from three studies that utilize different methods to create a refined reconstruction of paleohydrology, as well as paleoclimate, on the Altiplano. A major goal of this research is a more detailed understanding of millennial scale climate variability as it relates to insolation changes and abrupt warming and cooling in the north Atlantic. The first study discusses the creation of a paleohydrologic profile to reconstruct north-south hydrological history using previously reported lake core sediment records the northern and southern basins of the Altiplano, and a new 14 m core from the Salar de Coipasa representing the last ~45 ka. The second study uses a terrestrial hydrology model to simulate lake level changes through time given changes in precipitation and temperature. The third study uses strontium isotopic measurements of carbonates and halites in a 220-m core from the Salar de Uyuni to determine how source waters to the southern basin have changed through time. </p><p>The paleohydrologic profile in the first study is constructed using records from three major basins within the Altiplano: Lake Titicaca in the north, and Salar de Coipasa and Salar de Uyuni in the south. The new continuous sediment core from Salar de Coipasa indicates a lake that has fluctuated between deep and shallow phases for the last 45 ka. Lacking sufficient calcium carbonate, we instead take advantage of the general correlation between d18O and d13C in closed basin lakes to approximate water balance using d13C from organic carbon. This reconstruction is validated with diatom paleoecological records. The isotope measurements and diatom records indicate that from 45-36 ka Coipasa was moderately deep, consistent with paleoshoreline evidence of paleolake Minchin (46-36 ka). From 36-26 ka a shallow lake <10 m deep occupied the Coipasa basin. During the LGM (26-21 ka) the lake varied from moderate to shallow and during the Holocene (< 10 ka) the lake evolved from a shallow lake to a salt flat. </p><p>The hydrologic model in the second study was run through many scenarios including increases in precipitation, decreases in temperature, and combinations of the two. During the LGM southern Altiplano lakes fluctuated between 3,660 - 3,700 masl. Model results suggest that during this period basin wide precipitation increased up to 250 mm/yr compared to modern values dependent on a temperature decrease of 5 °C relative to modern values. To create a lake at elevation 3,760 masl consistent with the highest paleolake phase (Tauca, ~16 ka) the model requires an increase of 350 mm/yr compared to modern values dependent on a 5 °C decrease in temperature (relative to modern values). An increase in temperature alone of 2 °C above modern values causes Lake Titicaca water level to decrease ~30 m, creating a closed basin lake. Results indicate that Lake Titicaca outflow is necessary to sustain large lakes in the southern basin, providing ~40-60% of total input via the Rio Desaguadero. </p><p>Analysis of a 220 m core from the Salar de Uyuni suggests periods of alternating wet and dry phases (indicated by alternating mud and salt units respectively) at the salar. Evident in the record is a transition at ~60 ka from sediments consistent with dry conditions ("playa lakes") to sediments consistent with deep lakes ("great lakes"). It has been shown that rivers and lakes in the Bolivian and Peruvian Altiplano display a range of Sr isotopic ratios that can be connected to the lithologies of specific drainage basins. Measurements of Sr ratios of the alternating halites and carbonate sediments are used to determine when paleolakes in the Salar were supplied by flow from the northern and central basins of the Altiplano, and when they were more a product of increased precipitation in the Uyuni basin. The results from Sr isotope analysis suggest that prior to ~60 ka the primary source of Sr to the Uyuni was local runoff and direct precipitation. Following the state change from the "play lakes" phase to the "great lakes" phase Sr isotope measurements suggest a significant influence from more radiogenic waters originating in the central and northern Altiplano basins. The reason for this state change is attributed to a combination of a general increase in precipitation following the onset of the MIS-4 (~70 ka) glacial period and downcutting of the Laka Jahuira hydrologic divide, which connects Lago Poopó in the central basin to the Salar de Coipasa. </p><p>This approach of reconstructing hydrology using the combination of multiple paleolake records, hydrological modeling, and isotopic tracers allows for a better understanding of how precipitation and temperature changes affect the advance and retreat of large lakes on the Altiplano, and ultimately a more accurate understanding of how decadal-to-millennial forcings influence the climate of the subtropical Andes.</p> / Dissertation