Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997

SEDIMENT SOURCES, TRANSPORT, DEPOSITION, AND RETENTION TIMES WITHIN THE CHESAPEAKE BAY WATERSHED: AS PART OF THE USGS CHESAPEAKE BAY ECOSYSTEM INITIATIVE

Hupp, Cliff R.,
U.S. Geological Survey,
430 National Center,
Reston, VA 20192

Phillips, Scott W.,
U.S. Geological Survey,
208 Carroll Bldg.,
8600 LaSalle Rd.,
Towson, MD 21286

Chesapeake Bay, the Nations's largest estuary, has been affected adversely by both population growth and natural processes, such as large floods, during the past several centuries. High nutrient and associated sediment loads have decreased water clarity, in turn reducing populations of submerged aquatic vegetation, one of the most important components of the ecosystem because they provide critical habitat for shellfish and finfish and food for waterfowl. Resource managers are attempting to reduce the amount of nutrients and sediments that enter the Chesapeake Bay (Fig. 1).

The USGS, through the Ecosystem Program (Home Page: http://chesapeake.usgs.gov/), will collect and interpret earth science information to hutrient and sediment reductions. The project will be a 5-year effort withelp determine the effectiveness, and response of the ecosystem, to the n the goal of providing information on nutrient and sediment conditions and trends, watershed response times, and increased understanding of factors affecting nutrient and sediment dynamics, retention time, and response of selected living resources. Sediment and nutrient investigations began in FY 1996, as a collaborative effort involving most Divisions of the Geological Survey, the WRD National Research Program, the WRD Districts of Maryland-Delaware-District of Columbia, Pennsylvania, and Virginia, and several major Universities within the Bay Region. Scientists representing most of these agencies met in FY 1996 to help develop a workplan for this investigation. The present report is devoted to the approaches deemed necessary to accomplish the general objective through the identification and/or interpretation of: 1) significant sources of sediment (erosion and channel degradation); 2) the processes that entrain and transport sediment; 3) the processes, locations, amounts, and rates of significant sediment deposition; and 4) the retention times and remobilization of stored sediment. All four sub-objectives will be investigated at various spatial scales and at three time scales, namely from 1985 to the present, the last 50 years, and the past several centuries.

Sediment Sources

Three principle sources of sediment are: 1) soil erosion of upland areas from overland flow, including farmed areas, 2) head-cut or knickpoint migration in degrading low-order systems, and 3) remobilization of stored sediment through channel processes acting on flood plains or other storage sites, including channel migration, bank widening, and avulsion. At the scale and scope of the present investigation, sufficient information is available from previous or ongoing studies on soil erosion resulting from overland flow. However, incision of low-order streams and fluvial erosion of storage areas are poorly understood and may represent significant, if not major, sources of sediment in the Bay area. Similarly these two processes may strongly affect nitrogen and phosphorus dynamics and delivery to the Bay; erosion of storage areas exerts an equally strong influence on retention times.

Study approaches to the three principle sediment sources, outlined above, are: (1) Compilation and analysis of retrospective data set on overland erosion, drawn from the Smithsonian Environmental Research Center, the University of Maryland, and others. This effort will focus on small monitored watersheds located in physiographic regions or subregions identified as substantial source areas from gaging station sediment records. Whenever possible, these sites will be nested within increasingly larger watersheds where companion studies of channel and floodplain sediment aggradation and degradation have been or will be conducted as part of the present program. (2) Compilation and analysis of a retrospective data set on low-order channel incision occurring within the Bay watershed. Most of this information will come from studies conducted at Johns Hopkins University and the Universities of Maryland and Delaware. (3) Channel-stability monitoring along selected reaches of rivers to determine the long- and short-term trends in channel and flood-plain erosion or degradation. The alluvial areas of streams near the Bay and typically within the Coastal Plain (Fig. 1) are largely aggradational and represent significant sediment traps. However, rivers within the Piedmont and Great Valley (Fig. 1) function as both sediment sinks through flood-plain accretion and as sources through channel and bank erosion; at these sites most of our field efforts will involve the quantification of erosion of storage areas. Methodologies will include channel migration determination from aerial photography, analysis of current and historic cross sections, and dendrogeomorphic analysis of bank retreat. These techniques will permit quantitative estimates of current and recent sediment remobilization through degradation and channel migration and aid in estimation of retention time.

Sediment Transport

We will use historic and current gaging station sediment records to determine sediment (and nutrient) loads in relation to stream size and discharge. This information will be used in conjunction with sediment trapping, remobilization, and estuarine storage data to develop a regional sediment budget.

Sediment Deposition/Storage

Reaches will be selected along rivers that are representative of the basin and typically significant sites of deposition, i.e. substantial historic or current flood-plain development. Long-term rates of sedimentation will be estimated from stratigraphic analysis, flood-plain cores (using markers including palynology and isotopes), and dendrogeomorphic analysis. Short-term and current rates of deposition will be estimated from placement of feldspar clay pads. These data will be combined with sediment transport and source information to develop a sediment budget at various spatial and temporal scales.

Sediment Retention Time

The study of retention dynamics of sediment and nutrients requires information on their sources, transport, and deposition. Further, the approach will require the consideration and quantification of many geomorphic processes. Retention time is strongly influenced by sediment size, deposition rates, channel aggradation and degradation, bank erosion, channel migration, and meander dynamics. Estimation of retention time will be approached through stratigraphic analyses, repeat cross sectional analyses, age determination using tracers such as cosmic and atmospheric isotopes, artifacts, coal, and pollen, determination of percent stored fraction, and perhaps from models of fluvial change or meandering. Retention may be affected by long-term trends like deposition from post-settlement agriculture, potential channel modification, and present trends in land use. Biogeochemical processes will be investigated to estimate the fate of associated nutrients after deposition.

Site Selection

Sites will be selected to utilize existing gaging stations, previous studies (including USGS Habitat Level II, and bridge scour), and the results of the Potomac and lower Susquehanna NAWQA Study Units. Sites for intensive field efforts will be selected to represent significant source areas, storage areas, transport zones, or areas of significant change within each physiographic region. Several streams or basins are targeted due to their importance in sediment and nutrient delivery to the Bay because of their significant existing data bases, and/or are of substantial interest and allow for integration among Initiative researchers. Sites will be nested as much as possible to allow for ease of scaling up and economy of effort. Within each major basin, characterized sites will be extrapolated to similar reaches and combined through GIS for an entire watershed perspective. Additionally, sites will be selected to represent the major sediment and nutrient generating landuse and results will be analyzed and combined through GIS.

Sediment Budget

We shall initially use │Jacobson╣s Simple View of Sediment Routing▓ (JSVSR) to conceptualize and model the routing of sediment through the system (Fig. 2). Our efforts will focus on remobilization and retention times using a box or compartment approach to study the rate of sediment accumulation and retention. Using the data from sites that are typical of basin to fill the boxes in the JSVSR and transport information from gaging stations to estimate fluxes we shall estimate yields from areas of different landuse (principally agricultural, urbanizing and suburbanizing), estimate deposition amounts focusing on the annual and low-flow conditions, define conditions necessary for erosion, deposition, and associated retention times, and then scale up and down.

Additional field and lab work will include mapping of the surficial deposits and determining the geomorphic form of channels, flood plains, and valleys in both plan and cross-sectional views, using aerial photography, existing maps, cross sections, and stratigraphic information. Critical reaches will be resurveyed in detail. Sediment budgets will be integrated with estuarine deposition information from offshore river and bay sediment cores.

Hupp, Cliff R., U.S. Geological Survey, Water Resources Division, National Research Program, Reston, VA: Currently serves as Project Chief of the Vegetation and Hydrogeomorphology Project in the Branch of Research, Eastern Region. Experience and research during 19 years with USGS includes plant ecological and tree-ring analyses of mass wasting, floods, forested wetlands, and the riparian zone. Currently the Project is concentrating on the sediment and nutrient trapping function of forested wetlands and multi-disciplinary investigations of mountain watershed hydrology and ecology.

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