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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 effectively.

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.

In contrast, 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.

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