LAKES AND RESERVOIRS - Water Quality Engineering in Natural Systems - page 242

7

LAKES AND RESERVOIRS

7.1 INTRODUCTION Freshwater systems are classified as being either

lentic (standing) or lotic (flowing). Lentic systems, such
as lakes, reservoirs, and ponds, are more susceptible to
pollution than lotic systems because they act as sinks,
retaining pollutants, while lotic systems, such as streams
and rivers, flush pollutants downstream. More specifi-
cally, lakes and reservoirs differ from rivers and streams
in several important ways: (1) lakes rarely receive dis-
charges of organic matter large enough to cause serious
oxygen depletion; (2) lakes have significantly longer
retention times than most streams; and (3) the principal
water-quality gradients are in the vertical direction
rather than in the longitudinal direction as in streams.
Water bodies commonly referred to as ponds are smaller
and shallower than lakes, with sunlight typically pene-
trating to the bottom, where plants requiring photosyn-
thesis can grow and the water temperature varies little
throughout the water column.
Natural processes responsible for lake formation
include tectonism, volcanism, landslides, glaciation,
fluvial processes, and meteorites; natural lakes occur
most often in glaciated regions. Reservoirs are usually
dammed rivers or stream valleys fed by a major tribu-
tary. A key operational difference between natural lakes
and (human-made) reservoirs is that natural lakes tend
to have uncontrolled outflows, whereas reservoirs have
controlled outflows. Elongated and dendritic shapes are
typical of reservoirs created by damming rivers, whereas
natural lakes tend to be more circular. There is usually
more water level fluctuation and better mixing in reser-
voirs than in lakes. Pollutant loads to reservoirs are
usually greater than for lakes located in drainage basins

Lakes represent a large proportion of the world's readily
available water supply, and the importance of lakes to
a given region depends partly on their numbers and
distribution. In Scandinavia, for example, lakes occupy
almost 10% of the total land area, whereas lakes occupy
less than 1% of the total land area in China and
Argentina. It is estimated that there are 3-6 million
natural lakes in the world, covering an area of 1.6-3
million square kilometers (Jørgensen et al., 2005). The
study of lakes is an area of geophysics called limnology .
Lakes and reservoirs have a variety of uses, including
recreation, water supply, hydropower, and flood control.
Many features of natural lakes and reservoirs are similar,
and the approaches for their use and management are
also similar.
Lakes are large reservoirs of water in which currents
are driven primarily by wind. Other factors that influ-
ence the distribution of currents in lakes include the
bathymetry, density distribution, and inflow and outflow
characteristics. A typical lake is shown in Figure 7.1.
Lakes that intersect the water table and interact signifi-
cantly with groundwater are called seepage lakes , and
the water levels in these lakes fluctuate as the ground-
water table fluctuates. Lakes fed primarily by inflowing
streams are called drainage lakes . Sources of water input
to lakes include direct precipitation, stream inflow, and
groundwater inflow; outflows from lakes include evapo-
ration, stream outflow, groundwater outflow, and artifi-
cial withdrawals for such uses as water supply and
irrigation.