Zones and Shapes in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 317

11

Zones and Shapes in
Lakes and Reservoirs

11.1 INTRODUCTION
Lakes and reservoirs vary widely in shape, size, and origin. Their size is often characterized by
their depth, surface area, and volume. However, these metrics are not constant, but can vary both
spatially and temporally, particularly for some reservoirs. For reservoirs, the dam and dam design,
as translated into reservoir operations, impact the characteristics of, and transport within, the res-
ervoir. Other metrics are needed to characterize the physical differences between and within lakes
and reservoirs as they impact their physical, biological, and chemical characteristics.

11.2 LAKE ZONATION AND NOMENCLATURE
The size and physical characteristics of lakes and reservoirs impact their chemical and biological
characteristics. Depth, for example, can inluence a variety of other characteristics, such as the pro-
pensity to mix as a function of wind (Figure 11.1). Light and light availability is another characteris-
tic that can be used to establish zones. Light striking the water surface decreases exponentially with
depth as a function of the properties of light, water, and the materials dissolved or suspended in it
(see Chapter 12). The zone that light penetrates is referred to as the photic zone, the bottom of which
is usually taken as the depth at which the light is 1% of that striking the surface. The zone below the
photic zone is referred to as the aphotic zone. The photic zone is obviously the zone in which plants
can grow, and at the bottom of the photic zone is the compensation point, below which respiration
dominates. So, in the dark and usually colder zone, also called the profundal zone, decomposition
processes dominate, which result in reductions in oxygen and increases in reduced materials (e.g.,
reduced forms of iron and other metals).
If the depth of a lake or a reservoir, or portions of a lake or a reservoir, is shallow enough that
the light penetrates to the bottom (the photic zone extends to the bottom), then rooted aquatic veg-
etation or bottom algae may survive. The zone in which this occurs is the littoral zone. The extent
and the impact of the littoral zone vary widely. In many small and shallow lakes and reservoirs,
the littoral zone represents a large fraction of their surface area. Steep-sided lakes, such as those
formed in caldera, or reservoirs created in steep valleys may have little or no littoral zone. Littoral
currents are usually parallel to the shoreline and are strongly inluenced by the shoreline shape. The
shallow depth and wind mixing often cause complete vertical mixing of the littoral zone. Aquatic
plants may also impact transport. Transpiration by aquatic plants may represent a large water loss.
Synthesis by aquatic plants often serves as a major source of organic materials. The littoral zone
often provides an essential habitat for maintaining productive isheries.
The offshore region where emergent plants cannot grow is the pelagic or limnetic zone. This
is the zone where plants are dominated by loating or planktonic species and the bottom is free of
vegetation. In this zone, there are also often vertical variations in temperatures and water quality.
Since light (heat) transfer is restricted to the surface (photic) zone, often a temperature gradient
is established between a well-mixed, warm, productive, surface zone (the epilimnion ) and the dark,
colder hypolimnion . The zone of transition separating these two zones is the metalimnion .
None of these zones are static, and may vary seasonally and spatially. For example, neither the
factors impacting light penetration nor the depth is constant. In natural lakes, the depth can vary