Hydropower - Environmental Impacts of Renewable Energy - page 141

DID YOU KNOW?
To find out how much water is flowing in a stream or river, USGS person-
nel have to go out and make a discharge measurement —a measurement of
streamflow at that instant in time. USGS uses the term discharge to refer to
how much water is flowing, and discharge is usually expressed in cubic feet per
second. The term streamflow can be used instead of discharge.
in lower numbers than Chinook fry and steelhead fry (Woodin, 1984). However,
Beck Associates (1989) determined that the rate of chum and pink fry stranding per
the available fry was substantially higher than for Chinook. The low numbers of pink
and chum salmon stranding is a result of the short freshwater residency; they emi-
grate to saltwater shortly after emergence, whereas Chinook and steelhead remain in
the river for months or years.
Hamilton and Buell (1976) observed extensive coho stranding in the Campbell
River (British Columbia), and coho stranding has been observed in incidental num-
bers in other studies (Olsen, 1990). The overall incidence of coho stranding is rather
low in the studies conducted to date. The likely reason for this is that coho prefer
streams for spawning and breeding, whereas the formal research and evaluation have
taken place in large and medium rivers. Juvenile coho rear for a full year in fresh-
water; thus, it is reasonable to assume that stranding would occur at rates similar to
those for Chinook and steelhead.
The total drop in stage from an episode of flow fluctuation (known as ramping
range ) affects the incidence of stranding by increasing the gravel bar area exposed.
In addition, it increases the number of side channels and potholes that become iso-
lated from surface flow (Beck Associates, 1989). Stranding increases dramatically
when flow drops below a certain water level, defined as the critical flow (Bauersfeld,
1978a; Phinney, 1974a; Thompson, 1970; Woodin, 1984). In hydropower mitigation
settlements, the critical flow is defined as the minimum operating discharge, or as an
upper end of a flow range where more restrictive operation criteria are applied. The
factors that likely account for this response have been discussed above. The expo-
sure of the lowest gradient gravel bars often occurs in a limited range of flows. The
exposure of spawning gravel from which fry are emerging may also account for the
higher incidence of stranding.
In rivers with seasonal side channels and off-channel sloughs (slews, slues), even
a natural flow reduction can trap fry and smolts. Under normal circumstances, the
natural population can sustain a small loss several times a year; however, when a
hydropower facility causes repeated flow fluctuations, these small losses can accu-
mulate to a very significant cumulative loss (Bauersfeld, 1978a).
The ramping rate is the rate of change in stage resulting from regulated dis-
charges. Unless otherwise noted, it refers to the rate of state decline. The faster the
ramping rate, the more likely fish are to be stranded (Bauersfeld, 1978a; Phinney,
1974). Ramping rates less than 1 inch per hour were needed to protect steelhead fry
on the Sultan River. Olson (1990) determined that a ramping rate of 1 inch per hour