each tributary has something to reveal concerning the transport 
of water and salt in an estuary. An especially interesting tri¬ 
butary is the Patapsco River — Baltimore Harbor System, where a 
remarkable three-layer circulation was discovered. The Patapsco 
River cannot provide sufficient flow to drive a classical two- 
layer circulation. The fresher upper layers of the main stem of 
the Chesapeake Bay therefore move into the Harbor, mixing as 
they go with the saltier waters below. The salty lower layers 
of the Bay also move into the Harbor along the dredged shipping 
channel. What happens when these two currents move in? There 
has to be an outflow and it occurs at mid-depth. This has been 
inferred by Pritchard and Carpenter many years ago and until 
recently hasn't had a direct measurement. 
Here are flow measurements revealing the three-layer struc¬ 
ture. Six current meters are employed to resolve the remarkably 
small scales of this profile. 
Bob Biggs mentioned the importance of the continental 
shelf. Until told that we ought to pay attention to the sea 
level at the continental shelf, we were always ignoring that and 
treating the continental shelf as a large reservoir and source 
of high salinity water. But now we've become very interested in 
the source waters on the continental shelf realizing that the 
continental shelf sea level can drive motions in the estuary, 
especially very low frequency. 
We've also studied where the Chesapeake Bay water goes when 
it exits the continental shelf. At times it moves all the way 
to Cape Hatteras during high outflow and to the Gulf Stream. 
I mentioned numerical modeling. Dr. Shenn-Yu Chao at the 
University of Maryland is developing a computer model, a mathe¬ 
matical description, of the circulation of the Bay and the inner 
continental shelf. Of interest is the outflow from the Bay, 
which can move rapidly down the coast toward Cape Hatteras, and 
ultimately, become entrained in the Gulf Stream. This is the 
upper layer model predictions. In spite of the simplistic geo¬ 
metry, the picture is a remarkably accurate description of the 
Chesapeake Bay outflow plume, as observed recently in our micro¬ 
bial exchanges coupling in coastal Atlantic systems experiments. 
The flow of water into the Chesapeake Bay in the lower layer in¬ 
tensifies along the coast off Virginia Beach. 
Modeling is especially helpful in the attempt to assess 
long-term trends. Bob Biggs referred to the lack of long-term 
data sets on the Chesapeake Bay. The EPA Chesapeake Bay Program 
could only develop a 30-year record on dissolved oxygen of the 
Bay. Such a record length is uncomfortably short to assess 
39 
