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<Lakes and Reservoirs - Similarities, Differences and Importance>
How are Lakes and Reservoirs the Same and How are They Different?
Standing on the shoreline, a large lake or reservoir look much the same, and
both often contain the word “lake” in their name. Furthermore, the same
principles of biology, chemistry and physics are applicable to both types of
water-body. Indeed, it may be difficult to discern any obvious differences
between a lake and reservoir but differences there are, as well as similarities.
It is the intention of this chapter to examine some of these as well as the
implications for attempting to manage these two types of water-bodies
The science that deals with lakes and reservoirs is known as limnology, and
much of our current limnological knowledge (including that used to manage lakes
and reservoirs) was derived from studies of lakes over many decades. Although we
now have a reasonable understanding of limnological processes in lakes, we
now are only just beginning to develop a similar understanding for reservoirs.
In fact, reservoir studies initially focused on sediment loading from drainage
basins. The rationale for this was that the rate at which a reservoir filled
with sediment is a major determinant of useful operational life. Comparatively
little attention was given to the environmental and socioeconomic issues
associated with reservoir construction.
Shape and Morphometry
The shape and
form of lakes and reservoirs is determined largely by how they were formed. This
also affects some of their fundamental characteristics. Because lakes are naturally
formed, bowl-shaped depressions typically located in the central part of a drainage
basin, they usually also have a more rounded shape than reservoirs (Photo 10).
Similar to a bowl, the deepest part of a lake is usually at its center. Determining
lake water quality is typically based on measurements of water samples taken from
its deep center part, and a lake exhibits more consistent water quality characteristics
throughout its water basin than a reservoir. The shallowest part of the water
basin is usually located near the outflow channel.
a reservoir often has a shape that is fundamentally different from lakes; its
deepest part is near the dam. Moreover, because a river often has a number of
streams or tributaries draining into it, when the river is dammed, the impounded
water tends to back up into the tributaries. As a result, many reservoirs have
a characteristic dendritic shape, with the “arms” radiating outward from the main
body of the reservoir (Photo 11). In contrast, a reservoir formed by damming a
river with high banks will tend to be long and narrow. Depending on how they were
constructed, off-river storage reservoirs can have many shapes.
The dendritic shape or branching form of many reservoirs provides a much longer
shoreline than in lakes of similar volume. Reservoirs also usually have longer
drainage basins than lakes. Because of their larger drainage basins, and their
multiple tributary inputs, the inflow of water into reservoirs is more directly
tied to precipitation events in the drainage basin than it is in lakes. Further,
because the deepest parts of most reservoirs are just upstream of the dam, the
possibilities for draining the reservoir are facilitated. A lake typically
cannot be emptied; its deepest part is usually in its center.
Damming a river also inundates land previously above water. This sometimes
forces the relocation of inhabitants and wildlife living around the reservoir.
The presence of a dam downstream also allows a greater degree of control of
water-levels and volumes for reservoirs than for lakes. Constructing water
discharge structures at different levels in the dam also allows withdrawal or
discharge of water from selected depths in a reservoir. “Selective withdrawal”
has major implications for the water-mixing characteristics and increases
flushing possibilities for reservoirs.