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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Geosciences

Year Degree Awarded

2016

Month Degree Awarded

May

First Advisor

Jonathan Woodruff

Subject Categories

Sedimentology

Abstract

The denudation of uplands and sediment deposition within lowlands comprises one of the most fundamental earth processes. Much like plate tectonics or isostasy, sediment transport runs through nearly every geologic sub-discipline. Its importance extends to other fields as well, such as soil and plant science, archaeology, and engineering. Research in the field of sedimentology remains current, with changes in global sediment transport invoked as a primary line of evidence for the arrival of the Anthropocene, and sedimentary archives frequently employed to evaluate current processes relative to the past. In this vein, my doctoral studies have centered on understanding some aspects of modern sediment transport in the Connecticut River watershed of the northeastern United States. Although each chapter varies with respect to time scale and specific location, a common thread of observing modern sediment transport and associated hydraulic or hydrologic conditions runs through the following chapters. Particular attention is given to how processes are changing or may change in the future given anthropogenic modifications of our climate and sea levels.

Hurricane Irene passed directly over the Connecticut River valley in late August, 2011. Intense precipitation and high antecedent soil moisture resulted in record flooding, mass wasting and fluvial erosion, allowing for observations of how these rare but significant extreme events affect a landscape still responding to Pleistocene glaciation and associated sediment emplacement. Clays and silts from upland glacial deposits, once suspended in the stream network, were routed directly to the mouth of the Connecticut River, resulting in record-breaking sediment loads fifteen-times greater than predicted from the pre-existing rating curve. Denudation was particularly extensive in mountainous areas. Sediment yield during the event from the Deerfield River, a steep tributary comprising 5% of the entire Connecticut River watershed, exceeded at minimum 10-40 years of routine sediment discharge and accounted for approximately 40% of the total event sediment discharge from the Connecticut River. Resultant sedimentation in Connecticut River floodplain cover was anomalously inorganic, fine grained, and enriched in elements commonly found in chemically immature glacial tills and glaciolacustrine material. These unique sedimentary characteristics document the crucial role played by extreme precipitation from tropical disturbances in denuding this landscape.

Lacustrine sediment archives spanning the last 100+ years indicate that flooding during Tropical Storm Irene caused the most severe erosion of any regional historic flood, surpassing that of events with greater precipitation and peak discharges. Compared to deposition from historic floods, Irene’s event layer was more massive and more enriched in unweathered upland sediments, indicating an anomalously high incidence of mass wasting and sediment entrainment. Precipitation records indicate that neither precipitation intensity nor total accumulation distinguished Irene from less erosive historic floods. However, cumulative precipitation prior to Irene exceeded the 95th percentile of all days in the record. When allowing for non-stationarity in 20th century background precipitation, we find a fourfold increase in the probability of Irene-like conditions, where impacts of extreme rainfall are enhanced by high antecedent precipitation. We conclude that irrespective of increases in extreme precipitation, the risk of highly erosive flooding in the region is increasing due to the influence of wetter baseline conditions associated with a changing climate.

Monthly sediment trap accumulation in an off-channel cove of the Connecticut River upper estuary showed that these settings comprise the main sites for sediment storage within high energy estuaries. At our study site, 20-25% of annual sediment trapping occurred during one month at seasonal low freshwater discharge and high estuarine salinity. Sediment deposited during these high accumulation times contained higher δ13C values, and lower 7Be activities, indicative of marine provenance. Therefore, this efficiently trapped sediment is not directly delivered from the watershed, but rather redistributed from within the estuary. Considerable seasonal variability in 7Be activity within the top 0.5 cm of cove sediments coupled with steady 7Be sourcing from the water column indicate that surface activity of 7Be can be used to reconstruct deposition rates at sub-seasonal time scales near the mouth of rivers. Water column observations as well as numerical modeling revealed that salt wedge dynamics play a critical role in resuspending estuarine sediment. The role of low discharge and high salinity in storing sediment suggests that sediment export from rivers is likely to decrease as sea level rises and warm season discharge decreases.

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