Regulated Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 53

is a compromise. Operations implementing spring and fall drawdowns for lood control are also in
conlict with recreation and isheries management, where high water levels in the spring may be
preferred. The operations of a reservoir also directly impact the immediate downstream river, the
reservoir tailwater. Dams and their operations will be discussed in greater detail in a later chapter
on the physical characteristics of reservoirs, while tailwaters are the subject of a separate chapter.

3.2.1.3 What Are Dam Impacts?
Dams impact both the characteristics of the impounded system and the dam tailwaters, and the
impact varies with the design characteristics and the operation of the dam. Dams typically con-
vert systems from lowing or lotic environments to standing or lentic environments, with conse-
quent changes in the physical, chemical, and biological structures of the impounded system and its
releases. With increases in the hydraulic retention time, the magnitude and the timing of release
lows are altered. The thermal structure of the reservoir and its releases are altered. For example,
during the summer months, in many southern reservoirs the bottom waters are colder than the
surface, anoxic waters, and they have high concentrations of reduced materials, such as sulides,
metals, and nutrients. The release of bottom waters, such as from peaking hydropower facilities,
has commonly changed tailwaters from warm to very productive cold-water isheries. Dams also
isolate communities, such as the tailwater and reservoir communities. Dams impact the ability of
the system to store and transport sediments. The longer hydraulic retention time impacts seasonal
temperature variation. Other impacts of dams are discussed in greater detail in later chapters.

3.2.2 w eIrS and d IkeS
Weirs are similar to dams, but are essentially low walls built across a river for the purpose of raising
the upstream water level. Weirs are usually low-low structures and may be completely submerged
during high lows. The low over a weir is uncontrolled (Figure 3.9).
Weirs are commonly used as an alternative low measurement structure by measuring the height
of the water lowing over the weir. These weir low equations are used in estimating the low over
gated structures or spillways that act hydraulically like a weir. An example of a portable weir for
measuring low rates is provided in Figure 3.10.
Dikes are structures that protrude into a river channel, generally transverse to the low, and are
also called “groins,” “jetties,” “spurs,” “wing dams” (Biedenharn et al. 1997), or “wing dikes.”

FIGURE 3.9 Weir on the Big Sunlower River, Mississippi.