Here is a comprehensive collection of abstracts from meetings at which my colleagues and I have presented the results of our research. The results presented below represent a "first-blush" of the results our work. Subsequent improvements to data analyses, models, and writing have led to peer reviewed journal articles, listed elsewhere on this site.

River-mouth tidal freshwater marshes (tfm's) are growing at a phenomenal rate in upper Chesapeake Bay tributaries. Because tfm sediments come from the associated upland watershed, they provide a unique perspective on the downstream impacts of watershed land use changes and the role of climate, watershed hydrology, and sediment transport in mediating those impacts. Paleoecologic and stratigraphic studies of tfm's show that these systems are highly responsive to land use changes on a decadal time scale. Meanwhile, the model of the Chesapeake Bay watershed used to assess Bay water quality goals assumes that sediments (and nutrients) from uplands are quickly transported to the main estuary. Thus, the question arises as to just how fast do tfm's (and beyond them the main Bay) respond to watershed events.

During the last two years a program of ecologic, hydrologic and geomorphic study aimed at understanding the on-going processes of tfm evolution has been underway at the head of the Bush River tributary. Measurements of bi-weekly deposition at different marsh locations shows that time-averaged sedimentation is occurring at rates ranging from 0 - 20 g/cm^2/yr. Furthermore, strong interdependencies exist between plant associations, sedimentation rates, organic contents, percents of time flooded, and elevations. Analysis of these interdependencies shows that tfm's are made up of distinct habitats with predictable geomorphic, hydrologic, and ecologic characteristics. In comparing these internal tfm variables with external forces such as precipitation and runoff, observed bi-weekly habitat fluctuations appear to be largely independent of watershed processes over the very short term. This suggests that rapid tfm growth is fed by tidal resuspension and transport of the vast subtidal muds further out in the subestuary that were deposited over the last few decades.

Tidal freshwater deltas in upper Chesapeake Bay receive significant amounts of contaminated sediment from rapid urbanization and agricultural runoff in upstream drainages. Once sediment is delivered to a delta, a variety of sub estuarine processes acting on different spatial and temporal scales can redistribute, sequester, or export the material. The question is which physical processes are responsible for these different outcomes?

From 1993-1997, data was collected on hydro meteorologic forces (e.g. precipitation, river discharge, wind vectors, and astronomical tides) impacting the Bush River tributary. Simultaneously, field monitoring of water level changes and intertidal sediment deposition was carried out in the 84 ha tidal freshwater marsh that makes up the central third of the delta. Spectral analysis was performed on the time series data to determine the significant frequencies in each signal. Coherency analysis was used to evaluate the correlation of time series pairs at all resolvable frequencies. The results show that the external controls are composed of forces that act on persistent frequencies (e.g. astronomical tides) and others that span a range of frequencies (e.g. wind vectors). Wind and astronomical tides play an important role in controlling water level, while river discharge and rainfall show almost no impact, even during major storms. Five hurricanes were found to primarily impact the delta via offshore pulses that propagated up the estuary to the Bush River. Measurements of bi weekly sediment accumulation throughout the system show significant spatial and temporal variability. Some of the variability is accounted for by hydro meteorologic forces, but most is due to seasonal growth of marsh vegetation and interannual variability in ice coverage. Consequently, wind and astronomical tides are primarily responsible for redistributing sediment between the subtidal front and intertidal marsh, while ice and vegetation determine the rate of sequestration.

Wetlands have long been characterized as sinks for metals and other potential pollutants, but factors affecting metal concentrations in freshwater tidal marshes are not well understood. Soil samples from sites corresponding to different habitats (high marsh, middle marsh, low marsh, floating leaf, pioneer mudflat) within a freshwater tidal tributary of the Bush River were obtained biweekly between March and November 1996. Samples were brought into solution by microwave digestion and analyzed for copper, zinc, and iron using mass spectrometry. Metal concentrations were used with sedimentation rates and organic fraction data to determine temporal and spatial trends among the different marsh habitats. Time-averaged data suggest that the high, middle, and low marsh zones have higher concentrations than the floating leaf and pioneer mudflat habitats. However, when metal concentrations are combined with sedimentation rates to calculate mass metal loading, the biweekly deposition for the pioneer mudflat far exceeds that of other zones. Loading decreases as site elevation increases. Furthermore, data indicate that tidal transport is primarily responsible for metal concentrations in the pioneer mudflat and floating leaf habitats, whereas this factor plays a smaller role for the low, middle, and high marsh regions. These and other conclusions suggest important implications for metal sources of freshwater tidal wetlands.

Throughout Chesapeake Bay fluvial sediments are rapidly building deltas at the heads of subestuarine tributaries. Past research has used stratigraphic and paleoecologic analyses of delta deposits to reconstruct the history of human-induced landscape change. Some studies use sedimentation history to reconstruct sediment supply, but natural delta processes that affect sedimentation have not been accounted for, so the impact of human activities is overestimated. Reconstructing sediment supply from sedimentation rates is an inverse boundary-value problem: knowing the effect (sedimentation rates) one deduces the cause (sediment supply). Mathematically, an equation governing delta progradation is needed along with output from the solution domain.

Otter Point Creek is a tidal freshwater delta at the head of Bush River in upper Chesapeake Bay. Twenty year average deposition rates at several locations on the delta were reconstructed using paleoecologic and stratigraphic methods. Because the delta is laterally constrained by the Bush River basin, delta progradation may be modeled using a 1D diffusion equation that solves for elevation. The model has two parameters: source function and diffusion constant. The diffusion constant was obtained from theory and model optimization schemes. The latter method yielded 6.1e4 square feet per year. The source function was found by trial and error solving of the diffusion equation, knowing the diffusion constant and progradation history. The source function had four distinct components and predicted total sedimentation within 7 percent. For the intervals 1840 to 1860 and 1920 to 1940 the model significantly underestimated deposition, while for 1940 to 1960 the model overestimated it. These discrepancies indicate periods when significant changes in land use were followed by a rapid geomorphic response. For example, the construction of a dam in 1942 decreased the rate of delta deposition at one location by a factor of eleven.

Tidal freshwater marshes at the heads of Chesapeake Bay tributaries are growing at phenomenal rates. Field monitoring was carried out at one such marsh located in the Bush River tributary to determine the spatio temporal dynamics of sediment accumulation and the role of plants in those dynamics. By objectively clustering species abundances from 115 vegetation quadrats located throughout a 138.7 ha marsh, 9 plant associations from 5 distinct tidal freshwater marsh habitats were found. Independent sampling of physical conditions at 23 sites in a 3.8 ha subregion of the system from 7/95 to 3/97 revealed significant spatio temporal variability in sedimentation, with bi weekly net deposition ranging from -0.28 to 1.15 grams per sq cm. During winter there are no plants growing in the marsh and spatial variability in sediment dynamics is negligible. During summer, plant associations are fully grown and spatial variability in sediment dynamics is high. Statistical tests show that plant association, elevation, and distance to tidal inlet account for 90% of the spatial variability in summer sedimentation. We conclude that the annual growth cycle of tidal freshwater marsh plants is responsible for the annual cycle in sediment dynamics, while the spatial distribution of plant associations along with interannual variability in winter ice coverage controls the long term sequestration of sediment.

Landscape and river channel change caused by historic deforestation and intensive agriculture in coastal eastern U.S. has resulted in significant changes in the distribution, volume, and quality of riverine habitats. Where upland streams turn into lowland tidal freshwater rivers, historic sediment loadings caused a succession of habitats beginning with submerged aquatic, going to intertidal marsh, and ending in forested delta plains. In the face of this physico-chemical evolution, plant and animal populations occupied the landform as long as suitable habitats were present. While most species were forced out by habitat evolution, some, such as beavers and muskrats, might have engineered the landscape to protect their habitats from change. In this study of the geomorphology of landforms in tidal freshwater rivers, field monitoring of modern geomorphic processes revealed the extent and mechanisms by which modern mammalian populations combat landscape evolution.

Mapping and monitoring of vegetation, animal activity, substrate conditions, delta geomorphology and hydrology, and sedimentation rates were carried out April 1995 to November 1997 in the Otter Point Creek tidal freshwater delta at the head of Bush River on Chesapeake Bay. Bi-weekly sedimentation rates showed high spatial and temporal variability (-2.84 to 5.57 g per cm2), indicating that modern populations are faced with significant landform change. Out of 52 randomly located sampling sites, 6 were found to be in areas of historic or modern animal disturbance. Significant animal activites that were observed include plant uprooting, surface layer mixing, channelization, channel maintenance, dam building, and lodge building. These activities decreased elevation, increased flood depth and duration, locally redistributed sediment, and reversed the succession of plant community. One large area with indications of historic animal activity was found to be 2 stages behind the surrounding area in habitat evolution.

Throughout Chesapeake Bay sediment from 300 years of deforestation, intensive agriculture, and urbanization has built numerous deltas. Once sediment is delivered to a delta, several estuarine processes acting on different spatial and temporal scales redistribute, sequester, or export the material. The question is which processes are responsible for these different outcomes? From 1993-1997 data was collected on hydro meteorological and ecological processes impacting the delta at the head of the Bush River tributary. Simultaneously, field monitoring of water level changes and intertidal sedimentation was carried out in the delta. Spectral analyses of the time series shows statistically significant wind cycles of 3-4 and 7 days that drive water level changes over the same time periods. Modeling of wave conditions and sediment entrainment demonstrate that sediment is resuspended from subtidal areas daily, with the most resuspension occurring in spring and autumn. Sediment is transported upslope to intertidal marshes the majority of time, but sediment sequestration depends on a combination of summer biogeomorphology and the duration of winter freezing conditions. Field monitoring of sedimentation independently confirms model findings.

Tidal freshwater deltas in upper Chesapeake Bay receive large amounts of contaminated sediment from rapid urbanization and agricultural runoff in upstream drainages, and thus serve as critical buffers for the estuary. Based on historic maps (1836-1951), the Otter Point Creek delta at the head of the Bush River tributary has grown by a factor of 7.5. Physically-based computer modeling of present conditions has shown that once sediment is delivered to the delta front by distributary channels, winds and tides redistribute the material upslope into intertidal marsh habitats. The question is which processes are responsible for trapping the contaminated sediment in marsh habitats and thus driving ecological succession?

From September 1996 to November 1997, biweekly sedimentation rates were monitored at 29 sites spanning 5 distinct habitat types on the delta. At the same time, field surveys characterized topography, geomorphology, hydrology, and plant associations as well as sediment grain size distribution, organic content, and geochemistry. The mean deposition rate for the delta was 1.00 g/cm2/yr, with a range of Ð74.15 to 145.2, indicating net accretion with localized erosion. Three distinct temporal regimes were evident in the data, reflecting seasonal patterns in wind speed and vegetation life cycle. Multiple regression showed that 86 percent of the spatial variation in summer average sedimentation was explained by plant association and distance to the nearest distributary channel. In contrast, spatial variability in organic content was explained by elevation and distance to the subtidal front, while grain size parameters were explained by distance to the subtidal front and to the nearest distributary channel. Chemical results show that half of the Zn and Cu received by the delta stems from pollution. This research demonstrates that sediment sequestration is highly predictable and predominantly driven by geomorphic and vegetative factors.