716 MODELING OF ESTUARINE WATER QUALITY
on the tidal mixing. Naturally, in a river flow dominated estu-
ary, the water quality in the estuary is most similar to that
of the river, whereas in the tidal mixing dominated estuary,
its water quality is more like that of the off-shore waters.
The other factor greatly influencing the hydraulic regime in
an estuary is the physiography of the estuary which is very
greatly dependent on the origin of the estuary and the subse-
quent natural events which have taken place in geologic time
and man-made events in contemporary time to modify the
original shape of the estuary.Origins of EstuariesFrom a geomorphological standpoint, there are four primary
subdivisions of estuaries: (1) drowned river valleys; (2) fjord
type estuaries; (3) bar-built estuaries; and (4) estuaries pro-
duced by tectonic processes (Pritchard, 1967). Each of these
types of estuaries is characterized by the fact that at some
point in geologic time, it has been inundated with ocean
water due to the rise in the sea level. During the last glacial
stage, sea level was about 450 feet below its present level,
and the shorelines of the continent were at or near the present
continental slopes. Within the last 50,000 years, the sea level
has risen from that stage to the present with the last changes
in sea level occurring about 3,000 years ago (Russell, 1967).
As the name implies, drowned river valley estuaries are
river valleys found along a coastline with a relatively wide
coastal plain, which were inundated with ocean water as
the sea level rose. The Chesapeake Bay is a prime example
of this type of estuary. During the last glacial period, the
Susquehanna River reached the ocean about 180 kilometers
seaward of the present shoreline; the York River and the
other rivers now entering the bay to the north of the York
were then tributaries of the Susquehanna River. The rise in
sea level flooded the valleys of these rivers to form the pres-
ent Chesapeake Bay system. The drowned river valleys, or
as they are more commonly called, coastal plain estuaries,
extend up river to a point approximately where the floor of
the river rises above sea level. This is also the point at which
a major change in water quality occurs from the ocean and
estuary type water quality to that of the river. This geograph-
ical point may be downstream from parts of the river which
are still influenced by the oscillation of the tidal currents.
The fjord type estuary is that formed by glaciers. These
estuaries are generally U-shaped in cross section, and
they frequently have a shallow sill formed by terminal gla-
cial deposits at their mouths. The basins inside these sills
are often quite deep, reaching depths of some 300 or 400
meters. Most fjords have rivers entering at the head and
exhibit estuary features in the upper water layers. The sill
depths in Norwegian fjords are often so shallow that the
estuarine features develop from the surface to the sill depth
while the deeper basin waters remain stagnant for prolonged
periods.
Bar-built estuaries are those formed in an offshore area
where sand is deposited as a sand island and sand pit built
above sea level, and they extend between the headlands in
a chain broken by one or more inlets. Such bays often occurin areas where the land is emerging geologically. The area
enclosed by the barrier beaches is generally parallel to the
coast line. Frequently, more than one river enters the estu-
ary, though the total drainage area feeding a bar-built estuary
is seldom large. The lower valleys of such rivers have fre-
quently been drowned by the rising sea level, and hence the
bar-built estuary might be considered as a composite system,
part being an outer embayment partially enclosed by the bar-
rier beaches, and part being a drowned river valley or valleys.
Tidal action is usually considerably reduced in such estuar-
ies. These systems are usually shallow, and the wind provides
the important mixing mechanism (Pritchard, 1967). Several
of the North Carolina estuaries and most of those along the
Texas Gulf Coast are examples of this type of estuary.
Estuaries produced by tectonic processes are those
formed by faulting or by local subsidence, and they usually
have an excess supply of freshwater inflow. San Francisco
Bay is an example of such an estuary.Circulation in EstuariesOther than the physiography of estuaries, the dominant
physical processes associated with movement of water and
mixing in an estuary are the wind, tides, and the inflow of
river water. Extensive analysis of these processes has been
presented in Fischer et al. (1979), Fisher (1981), Thomann
and Mueller (1987), and others. The composite actions of
these processes produce a variable interaction or interfacing
of fresh water from the river and salt water from the ocean.
Because these two sources of water have very different den-
sities, the less dense fresh river water will tend to float on top
of the dense salt water, and the extent that the two types of
water mix is dependent on the strength of the mixing mecha-
nisms. In an estuary with no tides or wind and a steady river
inflow, the fresh water inflow would ride on top of the salt
water from sea level in the estuary or river bed to the ocean.
Because in a real system friction is present, the fresh water
will force sea water some distance downstream from the sea
level point in the river and the interface between the salt and
fresh water layers will tilt downward in the upstream direc-
tion in a wedge shape. The friction between the layers will
also cause an exchange of water from one layer to another,
generally from the salt water, or “salt wedge,” to the fresh
water. The amount of exchange depends strongly on the
mixing mechanisms, wind, tides, and river inflow.
In a wind dominated estuary, wind provides most of the
energy for moving and mixing the water. In a tide domi-
nated estuary, turbulence associates with the tidal currents
results in mixing between the salt and fresh water, which
in turn produces the density gradients associated with the
non-tidal circulation pattern. In a river dominated estuary,
such as the Mississippi River estuary, water movement is
predominantly related to riverflow and mixing is caused
mostly by the breaking of unstable interfacial waves at the
upper boundary between the fresh river water and the salt
water from the ocean.
In an estuary in which a salt wedge occurs distinctly
the river flow completely dominates the circulation. TheC013_006_r03.indd 716C013_006_r03.indd 716 11/18/2005 12:49:00 PM11/18/2005 12:49:00 PM