Environmental Engineering Reference
In-Depth Information
FIGURE 6.9
Bathymetric maps of Belton Reservoir, Texas (left), and Flathead Lake, Mon-
tana (right). The reservoir is dendritic and shallow, with the deepest portion near the dam
(lower right). Flathead Lake was formed by a combination of tectonic and glacial processes
and is deep with a regular shoreline and shallow outlet (lower left).
how quickly the biota can be washed out, and the general influence of trib-
utaries entering a lake. For example, Lake Tahoe has a 700-year residence
time (Table 6.2) so it is very sensitive to nutrient pollution (see Chapter 17).
Another important aspect of morphometry is the irregularity or degree of
convolution of the shore. An index used to quantify this is called the
shore-
line development
(
D
L
). This index compares the minimum possible circum-
ference of the lake, given a specific surface area (i.e., a perfect circle), to the
actual circumference of the lake and its surface area. A value of 1 for shore-
line development is a perfect circle and a larger value means that the shoreline
is highly dissected. High shoreline development is generally related to small
values of mean depth and mode of formation, and it is indicative of a high de-
gree of watershed influence. Shoreline development is calculated as follows:
L
2
D
L
A
0
Where
L
is the length of the shore, and
A
0
is the surface area of the lake.
Lakes with high shoreline development are often naturally productive rel-
ative to those with low shoreline development (Example 6.1). The idea of a
dissected shoreline resulting in increased productivity leads to consideration
of the watershed of a lake as a determinant of productivity. A lake with a rel-
atively large watershed will have much land from which nutrients can be
washed. Such a lake is likely to be more productive than a lake with a small
watershed. Land-use practices also play a major role in determining nutrient
inputs.
Reservoirs are common features of today's landscape, so understand-
ing how they vary from natural lakes is important. Damming can form
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