Hydropower - Environmental Impacts of Renewable Energy - page 136

FIGURE 4.15 Coho salmon typically found in Lake Washington, Seattle, WA.

FIGURE 4.16 Steelhead typically found in Lake Washington, Seattle, WA.

over long periods of time (weeks, months, or seasons) resulting from the storage of
water, irrigation diversions, municipal diversions, or the reactions of flow between
dams and powerhouses. These changes in net flow usually change the availability
of fish habitat and thus change the fish production potential of a river. Flow alter-
ations are evaluated by studying the fish habitat requirements and estimating the
changes in habitat area at different flows using a hydraulic model. The Instream Flow
Incremental Methodology (IFIM) (Bovee, 1982) has become a standard method for
estimating habitat changes resulting from flow alterations. The IFIM that was devel-
oped under the guidance of the U.S. Fish and Wildlife Service (USFWS) is a pro-
cess utilizing various technical methodologies to evaluate changes in the amount
of estimated usable habitat for various species or groups of species as flow changes
(Stalnaker et al., 1995). The IFIM methodology is routinely used to facilitate nega-
tion of instream flow requirements, usually minimum flow requirements that meet
the habitat needs of economically important or threatened fish species.
Flow fluctuations are unnaturally rapid changes in the flow over periods of min-
utes, hours, and days. Flow fluctuations can be immediately lethal or have indirect
and delayed biological effects. Flow fluctuations can be measured either by changes
in low , which is the volume of water passing a specific river transect, or by changes
in stage , which is the water surface elevation or gauge height. Both units are needed
to understand the problem, and the terms are used interchangeably in this text.
Hydrologists and engineers require flow measurements for many applications; how-
ever, the biological impact of flow fluctuations is best measured by stage. These two
units do not have a simple functional relationship, thus rating tables or rating curves
are used to define the flow at each stage for a specific river transect.
Flows in unregulated rivers respond to changes in precipitation and snow
melt. West of the Cascade Range, the peak flows occur from heavy rain storms in
November, December, and January. A lesser but more sustained peak occurs from a
combination of rain and snow melt in the spring. The lowest flows coincide with the