Environmental Engineering Reference
In-Depth Information
9 Introduction to Lakes
and Reservoirs
Geomorphology and Classiication
A lake is the landscape's most beautiful and expressive feature. It is the earth's eye looking
into which the beholder measures the depth of his own nature.
Henry David Thoreau
9.1 WHAT IS A LAKE, OR A RESERVOIR?
A lake (see Figure 9.1) is generally taken to be a body of water surrounded by land, but with a free
surface (open to the atmosphere). But, what is a lake, versus a reservoir, or versus a pond? Or, for
that matter, what is a lake versus a wetland or a river? If a river lows into a reservoir, where does
the river end and the reservoir begin? And, does this matter from a regulatory perspective? Or from
an ecological perspective? How about if you wanted to name waterbodies or complete a survey of
them, how would you determine which is which?
One commonly used distinction between a lake and a reservoir is that a lake is a “natural”
body of water, while a reservoir is man-made. A large number of natural lakes are derived from
a variety of natural processes (e.g., wind and glacial erosion, as discussed in a later section) and
their source or origin often has a profound impact on their characteristics. Similarly, reservoirs
are constructed for a variety of purposes (recreation, hydropower, etc.), which also has a profound
impact on their characteristics. The origin of lakes (and reservoirs) is of more than casual interest
(Wetzel 2001), since it affects their physical, chemical, and biological characteristics.
What is the difference between a lake (or a reservoir) and a pond (natural or man-made)? Often,
the distinction is based on their size and/or depth, but there is no precise and accepted distinction.
From a regulatory perspective, theoretically it may not make any difference since they are both
“waters of the United States” and are thus subject to the same water quality criteria. In practice
though, the distinction may be important. Rivers are often distinguished from lakes and reser-
voirs in that they are vertically well mixed. But, as discussed later, that distinction is not always
true. Similarly, wetlands and lakes may have many common characteristics. So, as with rivers and
streams (and brooks and becks, creeks and cricks, see Chapter 2), distinctions are typically some-
what arbitrary and no precise deinition is available. However, while that may be true, there is a
difference from a limnological or ecological perspective and that difference is important.
9.2 GENERAL CHARACTERISTICS
As noted previously, a primary difference between rivers and lakes or reservoirs is in their speed
of water movement, or currents. Rivers and streams are lotic (from lotus , meaning washed). Lotic
systems are characterized by running water, as opposed to lakes and reservoirs, which are lentic
(from lenis , meaning calm) and are characterized by standing water. While the lowing nature of
rivers dominates their transport and mixing processes, the standing nature of lakes and reservoirs
results in other processes assuming an increased importance. The lowing nature of rivers normally
 
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