36
Chemical Oceanography, 4th Edition
a resuspension of river sediments. River runoff reduces the salinity of the surface waters.
The runoff normally has seasonal fluctuations that frequently follow the local precipitation or the summer melt of snowfields and glaciers. Most of the extreme conditions in
coastal waters occur in or near river mouths or in estuaries.
Estuary can have a wide range of meanings in oceanography. The simple definition is the
tidal region at the mouth of a large river. Pritchard’s definition is “a semi- enclosed body of
coastal water having a free connection with the sea” and containing a measurable amount
of sea salt. One can classify an estuary into two major types (Pickard and Emery, 1982):
1. A positive estuary, where the salinity is less than the ocean because precipitation
and river runoff are greater than evaporation (e.g., the Baltic Sea)
2. A negative estuary, where the salinity is greater than the ocean because of evaporation greater than precipitation and river runoff (e.g., the Red and Mediterranean Seas)
It is also possible to define an estuary in terms of its shape. Three such divisions are
as follows:
1. A coastal plain estuary that results from land subsidence or a rise of sea level
flooding a river valley (e.g., the St. Lawrence and Chesapeake Bay)
2. A deep basin estuary that has a sill toward the sea that is shallower than the basin
and the outside seas (e.g., fjords of Norway and Canada)
3. A bar build estuary that has a narrow channel between the shore and a bar built
up nearby by waves and sedimentation (e.g., Mexican lagoons)
Finally, it is possible to classify an estuary based on the distribution of the water properties. Four such types are shown in Figure 1.42:
1. A vertically mixed estuary is a shallow basin where the water is well mixed. This
leads to a similar salinity from the surface to the bottom at any particular place
along the estuary. The salinity increases as the estuary flows from the head to the
mouth. The Severn River in England is an example of this type of estuary.
2. A slightly stratified estuary is also shallow, with the salinity increasing from the
head to the mouth. The water, however, is in two layers. The upper layer is a little
less saline than the deep layer. There is a net seaward flow in the upper layer and
an inward flow in the deep layer. Vertical mixing of the two layers results in the
increase of salinity from the head to mouth in both layers. The James River in the
Chesapeake Bay is an example of this type of estuary.
3. The highly stratified estuary is typical for fjords. The upper layer increases in
salinity from zero at the head to a value close to seawater at the mouth. The deep
water has an almost uniform salinity. There is a net outflow in the surface layer
and inflow in the deeper water. A very strong halocline exists between the two
layers. There is normally some vertical mixing between the two layers. The depth
of the interface is substantially the same throughout the estuary. The circulation
in this type of estuary is dependent on the sill depth, the river runoff, and the outside water density distribution. If the sill depth is too shallow, the inflow of deep
saline water will not occur. This leads to the bottom waters becoming stagnant
and perhaps devoid of oxygen. This situation occurs in Norwegian fjords but not
all deep basin estuaries. The rate of renewal is affected by the river runoff and
outside density structure (both of which can be seasonal).
Chemical Oceanography, 4th Edition
a resuspension of river sediments. River runoff reduces the salinity of the surface waters.
The runoff normally has seasonal fluctuations that frequently follow the local precipitation or the summer melt of snowfields and glaciers. Most of the extreme conditions in
coastal waters occur in or near river mouths or in estuaries.
Estuary can have a wide range of meanings in oceanography. The simple definition is the
tidal region at the mouth of a large river. Pritchard’s definition is “a semi- enclosed body of
coastal water having a free connection with the sea” and containing a measurable amount
of sea salt. One can classify an estuary into two major types (Pickard and Emery, 1982):
1. A positive estuary, where the salinity is less than the ocean because precipitation
and river runoff are greater than evaporation (e.g., the Baltic Sea)
2. A negative estuary, where the salinity is greater than the ocean because of evaporation greater than precipitation and river runoff (e.g., the Red and Mediterranean Seas)
It is also possible to define an estuary in terms of its shape. Three such divisions are
as follows:
1. A coastal plain estuary that results from land subsidence or a rise of sea level
flooding a river valley (e.g., the St. Lawrence and Chesapeake Bay)
2. A deep basin estuary that has a sill toward the sea that is shallower than the basin
and the outside seas (e.g., fjords of Norway and Canada)
3. A bar build estuary that has a narrow channel between the shore and a bar built
up nearby by waves and sedimentation (e.g., Mexican lagoons)
Finally, it is possible to classify an estuary based on the distribution of the water properties. Four such types are shown in Figure 1.42:
1. A vertically mixed estuary is a shallow basin where the water is well mixed. This
leads to a similar salinity from the surface to the bottom at any particular place
along the estuary. The salinity increases as the estuary flows from the head to the
mouth. The Severn River in England is an example of this type of estuary.
2. A slightly stratified estuary is also shallow, with the salinity increasing from the
head to the mouth. The water, however, is in two layers. The upper layer is a little
less saline than the deep layer. There is a net seaward flow in the upper layer and
an inward flow in the deep layer. Vertical mixing of the two layers results in the
increase of salinity from the head to mouth in both layers. The James River in the
Chesapeake Bay is an example of this type of estuary.
3. The highly stratified estuary is typical for fjords. The upper layer increases in
salinity from zero at the head to a value close to seawater at the mouth. The deep
water has an almost uniform salinity. There is a net outflow in the surface layer
and inflow in the deeper water. A very strong halocline exists between the two
layers. There is normally some vertical mixing between the two layers. The depth
of the interface is substantially the same throughout the estuary. The circulation
in this type of estuary is dependent on the sill depth, the river runoff, and the outside water density distribution. If the sill depth is too shallow, the inflow of deep
saline water will not occur. This leads to the bottom waters becoming stagnant
and perhaps devoid of oxygen. This situation occurs in Norwegian fjords but not
all deep basin estuaries. The rate of renewal is affected by the river runoff and
outside density structure (both of which can be seasonal).