Geology
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Item GEOMORPHIC AND HYDROLOGIC CHARACTERISTICS OF SMALL URBANIZED TRIBUTARIES TO A FALL ZONE STREAM(2024) Harris, John Allen; Prestegaard, Karen; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many rivers along the Atlantic Coast contain major knickpoints, which define the Fall Zone. These often-urbanized rivers straddle multiple physiographic regions with spatial variations in lithology, topography, and hydrology. This research evaluates the effects of mainstem channel incision and urbanization on channel and catchment morphology, bed substrate mobility, catchment water storage dynamics, and hydrologic response in tributaries of the Northwest Branch of the Anacostia River above and below the Fall Zone knickpoint. Topographic analyses show that differential incision below the mainstem knickpoint has initiated steep secondary channels incised into bedrock. Measurements at representative reaches show that bankfull shear stress exceeds critical shear stress in these newly initiated tributaries, resulting in erosive channels outside of threshold conditions. Increased urban runoff introduced at storm drain outfalls maintains these non-steady state conditions. Geophysical surveys reveal that regolith depth for water storage capacity is primarily below the flatter ridgetops of the tributary catchments, where development is concentrated. The secondary tributaries cannot access these upland storage zones, and thus have limited infiltration and recharge capacity. I installed streamgages in the tributaries and constructed catchment water balances to study storage dynamics and hydrologic response. Hydrologic consequences of urbanizing the steep secondary tributaries include flashy, elevated stormflows, greater total runoff, and reduced baseflows that are not maintained during drought periods. The combination of steep channels, thin regolith, and urban overprint limits infiltration to moderate storm responses and recharge storage. These effects were not seen in non-urbanized secondary tributaries, urbanized tributaries above the knickpoint, or the forested reference streams above the Fall Zone. These findings define the geomorphic adjustment of tributaries to differential mainstem incision and explore the hydrologic impacts of urbanizing small steep catchments with limited effective storage capacity. Supplementary files:S1: Table with the location, drainage area, stream gradient, bankfull hydraulic values, and grain size values at each Northwest Branch tributary and reference reach used in the study. S2: Spreadsheet with the water level logger gage height values collected at 5-minute intervals from April 2023-March 2024 and calculated discharge from the Northwest Branch tributary streamgages. S3: Spreadsheet with the monthly water balance values for the Northwest Branch tributary catchments and reference watersheds from April 2023-March 2024. S4: Table with the depth to bedrock values and corresponding slope angles measured from the seismic profiles and LiDAR-derived digital elevation models.Item TREE TRADE-OFFS IN STREAM RESTORATION: IMPACTS ON RIPARIAN GROUNDWATER QUALITY(2020) Wood, Kelsey Lynn; Kaushal, Sujay; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Restoring urban degraded stream channels in efforts to improve water quality often includes substantial alteration of the riparian zone which can require the removal of mature trees. This study assessed the impact of tree removal on riparian groundwater quality over time and space using a chronosequence of restored sites ages 5-20 years and well transects along groundwater flow paths. The response of multiple elements through various hydrologic conditions was evaluated by monitoring dissolved concentrations of inorganic carbon, organic carbon, total nitrogen, boron, calcium, copper, iron, potassium, magnesium, manganese, sodium, and sulfur over a 2-year period. Results revealed that concentrations of most bioreactive and organically derived elements were significantly elevated and increase along flowpaths at recently restored sites.Item Seismic Observations of Fluvial Energy Dissipation(2018) Goodling, Phillip James; Prestegaard, Karen; Lekic, Vedran; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Observing microseismic waves excited by turbulent flow is an emerging way to document river dynamics during extreme flood events. This thesis records fluvial-seismic observations in two contrasting systems at different scales. Two single-seismometer particle motion methods are introduced to characterize the seismic signal produced by rivers. In the large-scale system, the Oroville Dam spillway is observed when it is a simple rectangular channel and when it is damaged by erosion. The small-scale system is along the cobble-bed Northwest Branch of the Anacostia River. Particle motion analyses and the scaling between seismic power and discharge are suitable to characterize flow turbulence at the large-scale system. In the small-scale system, particle motion methods are found to be unsuitable and the scaling of seismic power is unable to resolve observed variability in flow dynamics within the study reach. This work suggests that methods of fluvial seismology are best suited to large-scale systems.Item Vegetation-Hydrodynamic Interactions and the Stability of Channel Inlets of Tidal Freshwater Wetlands, Chesapeake Bay System(2014) Statkiewicz, Anna; Prestegaard, Karen L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To maintain elevation, deposition of mineral and organic sediment in tidal freshwater wetlands (TFWs) must outweigh losses due to sea-level rise, erosion, decomposition, and compaction. Sediment loads into tidal marshes are controlled by inlet size and sediment supply, but interactions among vegetation, hydraulics, and geomorphology affect sediment retention. This study focused on these interactions in TFW inlets partially covered by aquatic vegetation (N.luteum, Z.aquatica, and H.verticullata). Measurements of hydraulic parameters and geomorphic change were correlated with observations of spatial and morphological characteristics for each vegetation type. The aquatic plants grew in significantly different water depths and well-defined platforms formed in areas occupied by emergent vegetation where effective shear stress is lowest. Net annual accretion data indicate an inverse relationship between maximum inlet depth and accretion rate. These results suggest that initial vegetation colonization modifies channel inlet morphology; both vegetation and morphology generate the shear stress distributions, which maintain channel form.Item Geomorphic, hydraulic, and biogeochemical controls on nitrate retention in tidal freshwater marshes(2012) Seldomridge, Emily; Prestegaard, Karen; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tidal freshwater wetlands are ideal sites for nitrate retention because of their position within the landscape (near the head of tide); they receive water, discharge, nutrients (N and P), and sediment loads directly from contributing watersheds. Nitrate retention (the difference between nitrate inputs and outputs in an ecosystem), however, is difficult to predict due to the complex interactions between flow processes and the multiple retention processes. The goal of the study was to evaluate both external and internal controls on nitrate retention, and to determine whether scaling procedures could be identified to estimate nitrate retention for an entire ecosystem. The external controls included temperature, dissolved oxygen concentrations, and incoming nitrate concentrations. Internal controls are the interactions among geomorphic, hydrologic, and biological systems within individual marshes that influence nitrate retention. This study was conducted in the upper Patuxent River Estuary where the ecosystem is composed of hundreds of individual marshes that are connected to the estuary through tidal inlets; marsh inlet geomorphology governs water and nitrate fluxes into the marshes. This study therefore took a mass balance approach to determine geomorphic, hydrologic, and biological influences on nitrate retention. Nitrate retention was measured over a 4-year period in three tidal freshwater wetlands, selected to represent a range of marsh sizes. An examination of the mass balance data suggest that nitrate retention is an outcome of complex interactions among inlet geomorphic characteristics, hydrologic flux, and biogeochemical processes. In cases where nitrate concentrations and temperatures are greater than critical (limiting) values, an emergent behavior in which nitrate retention is a simple function of water volume is observed. The wetland ecosystem is composed of numerous, small wetlands that process a small percentage of total nitrate; approximately 50% of retention is processed by the large marshes that comprise only 4% of the total population, but over 80% of the marsh area; therefore, any processes that affect tidal water volumes in large marshes is likely to affect net nitrate retention. The growth of vegetation in these large channels reduced ecosystem nitrate retention.Item Geomorphic and Hydrologic Controls on Tidal Prism and Inlet Cross Sectional Area for Chesapeake Bay Lagoons(2011) Jenner, Brittany A.; Prestegaard, Karen L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Previous studies have defined a power function between tidal prism and inlet cross sectional area for many lagoon systems. The goals of this study are to first, determine underlying processes that generate the area-prism relationship and then, examine whether the area-prism relationship extends to the small lagoons of Chesapeake Bay. Geomorphic data were measured, compiled and compared for Chesapeake Bay lagoons, Chesapeake Bay regional tidal marshes, and New South Wales, Australia lagoons and creeks. These data generated two inter-regional emergent relationships: 1) An area-prism relationship that included Chesapeake Bay data and 2) A relationship between lagoon surface area and drainage basin area. Examination of Chesapeake Bay data suggests that lagoon water surface area, tidal prism, and inlet geometry are primarily determined by streamflow. Results also indicate that Chesapeake Bay lagoon inlet geometry is modified over time by wave processes, which generates two alternate states for inlet characteristics.Item EFFECTS OF BAR FORMATION ON CHANNEL STABILITY AND SEDIMENT LOADS IN AN URBAN WATERSHED(2009) Blanchet, Zachary; Prestegaard, Karen L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study investigates channel adjustment due to urbanization in the Little Paint Branch creek of the Anacostia River watershed. In the past 15 years, large gravel bars have formed in the channels, more than doubling the active channel width of some reaches. Field data was collected to analyze downstream hydraulic geometry and the effects of gravel bars on shear stress, turbidity, and morphological change. The watershed was gauged at three locations to document the contributions of discharge and sediment to the downstream Anacostia Estuary. The results indicate that Little Paint Branch Creek generates proportionally more runoff per basin area than the watershed does as a whole, even though the impervious surface area is lower in the upstream tributaries, like Little Paint Branch Creek. Bar formation induces channel widening, which decreases flow depth and thus shear stress for bankfull and higher stages. This shoaling limits bed transport and will eventually limit bank erosion.