Introduction to Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 276

FIGURE 9.1 Lake Louise, Alberta, Canada. (Photograph by J.L. Martin.)

causes complete mixing over their depth and width, and rapid downstream transport. The deeper
standing water in lakes tends to move water quality constituents and contaminants more slowly and
to stratify vertically, which retards vertical mixing.
Lakes and reservoirs tend to store water for relatively long periods. The length of time is often
quantiied using the hydraulic residence (or retention) time, or the average time that a water particle
resides within a lake or impoundment (one deinition of which is volume divided by low; Martin
and McCutcheon 1999). Hydraulic residence times can vary from days to weeks in run-of-the-river
reservoirs in which outlows are not controlled and be up to years in larger reservoirs. The run-of-
the-river reservoir is a relatively shallow reservoir on a larger river in which velocities are higher
than in most reservoirs. In contrast, mean residence times in the Great Lakes range from 3 years for
Lake Erie to 180 years for Lake Superior (Chapra 2008).
The depth and hydraulic residence times largely inluence the mixing and water quality charac-
teristics of lakes and reservoirs. An increased storage or residence time allows for internal cycling
and matter originating within the lake or reservoir (autochthonous materials) to have increased
importance relative to materials originating outside and carried into the lake or reservoir (allochtho-
nous materials). Internal productivity and decay often vary over seasons or years, so that variations
in transport and mixing occurring over similar time periods become important. Variations with
depth are also often more important in lakes and reservoirs than in rivers. Light does not penetrate
to the bottom of many lakes and reservoirs so that heat exchange and productivity are limited to the
surface layers. Many lakes and reservoirs do not completely mix vertically during periods of the
year and large vertical gradients in the temperature, density, and water quality often result.
For rivers, transport is usually dominated by longitudinal advection, and material introduced
into them is often rapidly mixed over their width and depth. In contrast, lakes and reservoirs may
or may not have well-deined currents or a single direction of low. The pattern of lows and mixing
in a lake or a reservoir is affected by bathymetry, thermal structure, inlows, outlows, and wind
mixing. Unlike in rivers, horizontal transport and mixing are often more rapid than those occurring
vertically.

9.3 BRIEF HISTORY OF LIMNOLOGY
The study of inland waters, such as lakes and reservoirs, is referred to as limnology, from limnee
meaning pool, marsh, or lake, so limnology is the study of lakes. This ield of study often encom-
passes rivers and streams as well, but it commonly focuses on lakes.