Site Description

The ETM zone of Chesapeake Bay is almost always found between latitudes 39° 10' N and 39° 28' N, a range of approximately 40 km (Figure 10.1). This region of the Bay has a mean volume of approximately 2.63 km3 and a mean depth of approximately 4.1 m,37 not counting the small tributaries. It is incised by a narrow shipping channel maintained by dredging at a depth of approximately 12 m, which connects the Chesapeake and Delaware Canal with the Port of Baltimore and serves as the primary pathway for up-Bay salt intrusion. The average astronomical tidal range increases northward from 0.36 to 0.5 m, with typical maximum tidal current speeds of 0.5 m sec-1 in the channel and 0.3 m sec-1 over the broad shoals.38 Wind-forced water level fluctuations can be much larger than the astronomical tide (up to 1 m in range), with associated current fluctuations of up to 0.2 m sec-1.6,39,40 Net non-tidal gravitational circulation varies strongly in response to fluctuations in river flow, changing from a riverine, barotropic downstream flow above the limit of salt to

FIGURE10.1 Map of Upper Chesapeake Bay with station locations. Gray shading indicates depth intervals of 3, 7, 10, and 20 m. Xs indicate axial stations repeated during each axial hydrographic survey. Os indicate locations of 12-h anchor stations.

FIGURE10.1 Map of Upper Chesapeake Bay with station locations. Gray shading indicates depth intervals of 3, 7, 10, and 20 m. Xs indicate axial stations repeated during each axial hydrographic survey. Os indicate locations of 12-h anchor stations.

a well-developed estuarine circulation below the limit of salt. Almost all of the fresh water flow enters from the Susquehanna River. At the average Susquehanna River flow of 1100 m3 sec-1, the freshwater replacement time of the ETM region of the Bay is approximately 1 month.

More than 80% of the sediment entering the upper Bay comes down the Susquehanna, with almost all of the rest resulting from shoreline erosion.41'42 The bottom sediments gradually fine southward from the broad, sandy delta known as Susquehanna Flats, through short transition zones of silty sands and sandy silts, through the clayey silts that dominate the ETM region, to the silty clays that dominate the broad lower reach of the upper Bay. The deep channels of the upper Bay sediment rapidly6,43 and require constant dredging to maintain navigable depths. Suspended sediments comprise silts, clays, and aggregates thereof, with a low organic matter fraction.13,16,41 Total suspended sediment (TSS) concentrations in the entire upper Bay are elevated relative to the rest of the estuary, with typical background concentrations of very slowly settling particles between 5 and 25 mg l-1.6,13,19,44 The ETM itself typically has TSS concentrations 20 to 100 mg l-1 higher than background, with the largest concentrations resulting from tidal resuspension in near-bottom waters.6,13

Freshwater flow and sediment load data from the Susquehanna River were obtained from the United States Geological Survey (USGS). The sediment load data were calculated from flow using a calibrated USGS estimation procedure.45 Monthly averaged data for 2002 are presented in Figure 10.2 for comparison to the 23-year mean monthly averages between 1979 and 2002. It is apparent from this figure that 2002 was a low to average flow year for the Susquehanna, except for May and June. Sediment loads were even lower relative to the long-term mean, except for May.

1=1 2002 flow 1=1 2002 sed - - 79-02 avg sed -79-002 avg flow

FIGURE 10.2 Freshwater discharge and sediment load from the Susquehanna River at Conowingo Dam. Bars depict monthly averages for calendar year 2002 and lines represent long term monthly averages from 1979 to 2002. Data from USGS internet sites.

1=1 2002 flow 1=1 2002 sed - - 79-02 avg sed -79-002 avg flow

FIGURE 10.2 Freshwater discharge and sediment load from the Susquehanna River at Conowingo Dam. Bars depict monthly averages for calendar year 2002 and lines represent long term monthly averages from 1979 to 2002. Data from USGS internet sites.

The year 2001 was also quite dry with low sediment delivery. This is in marked contrast to the 1996 data presented by Sanford et al.,6 which were collected during one of the wettest years of the 1990s.

0 0

Post a comment