What happens when you pull the partition between the two 
fluids? I'll ask you this on the same exam after the reading 
assignment, but I'll give you a hint. The heavier saltwater 
flows under the freshwater, and the freshwater flows over the 
saltwater. This process continues to the point where there is 
an equilibrium. Thereafter, this picture stands until molecular 
diffusion removes the salinity difference between the two. That 
is a very, very slow process. This picture here resembles what 
we describe as a salt-wedge estuary, which the Chesapeake is 
not. The amount of freshwater flowing towards the sea over the 
saltwater flowing towards the head of the estuary is not much 
greater than the total freshwater moving in. 
The Chesapeake Bay, if you took a typical cross-sectional 
area in the mid-section of the Bay, maybe a 100,000 square 
meters, and tried to move the Susquehanna through that you get 1 
centimeter per second average velocity across the crosssection. 
And that is about a kilometer a day, seemingly not very strong. 
Then we go out to our current meters in the estuary and find 
indeed that the estuary has mean velocities 10 times the amount 
or even 25 times that amount. What drives this robust 
circulation? 
This is a picture of what's called a partially mixed estuary 
by Pritchard's definition. We see freshwater moving toward the 
mouth of the estuary and saltwater penetrating in along the bot¬ 
tom. The Chesapeake Bay is the foremost example of this type of 
estuary. During the process of moving toward the mouth of the 
estuary, the freshwater mixes with the lower water reducing its 
freshwater component, and getting saltier and saltier as it 
moves to the sea. Likewise, the ocean water moving toward the 
head of the estuary gets fresher and fresher to the point where 
it reduces its saltwater component. 
The critical driving element in the Chesapeake Bay circula¬ 
tion is this mixing process, which we traditionally think of as 
the turbulence generated by the sloshing back and forth of the 
tidal currents over the bottom of the Bay. This picture has 
been modified somewhat lately, but mixing remains the crucial 
determinant of Bay circulation. 
Well, we've known this circulatory picture for 30 years. 
What's new? And how did we get there? 
We got there in the last 20 years by an increasing use of 
moored instrumentation. If you picked up the description of 
NOAA's circulatory survey, you will see described that they've 
conducted a large set of mooring operations over the Bay. For 
example, 61 current meters were placed on 23 moorings in June 
1980 under EPA sponsorship. 
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