Environmental Engineering Reference
In-Depth Information
Juvenile salmonids are more vulnerable to stranding than adults. Salmonid fry
that have just absorbed the yolk sac and have recently emerged from the gravel are
by far the most vulnerable. They are poor swimmers and settle along shallow mar-
gins of rivers (Phinney, 1974a; Woodin, 1984), where they seek refuge from currents
and larger fish. Once Chinook attain the size of 50 to 60 mm in length, vulnerabil-
ity drops substantially. For steelhead, vulnerability drops significantly when the fry
reach 40 mm (Beck Associates, 1989). Larger juveniles are more inclined to inhabit
pools, glides, overhanging banks, and midchannel substrates, where they are less
vulnerable to stranding; however, many juveniles still inhabit shoreline areas and
remain vulnerable to stranding until they emigrate to saltwater (Hamilton and Buell,
1976). Adult stranding as a result of hydropower fluctuations has been documented
(Hamilton and Buell, 1976).
The river channel configuration is a major factor in the incidence of stranding. A
river channel with many side channels, potholes, and low gradient bars will have a
much greater incidence of stranding than a river confined to a single channel with
steep banks. Large numbers of small fry die from beaching on gravel bars when
unnatural flow fluctuations occur (Phillips, 1969; Phinney, 1974a; Woodin, 1984).
Bauersfeld (1978a) observed beaching primarily on bars with slopes less than 4%.
Beck Associates (1989) determined that beaching occurred primarily on bars with
slopes less than 5%. Under laboratory conditions, Monk (1989) determined that
Chinook fry stranded in significantly larger numbers on 1.8% slopes than on 5.1%
slopes, but the results were not significant for steelhead. Stranding on steep gravel
bars (>5% slope) has not been thoroughly studied.
Long side channels with intermittent flows are notorious for trapping juvenile
fish. Substantial trapping can occur even with unregulated flows (Hunter, 1992). Side
channels are valuable rearing habitats, and juveniles of several species prefer side
channels over the main channel; however, unnatural fluctuations will repeatedly trap
fish, eventually killing some or all of them (Hamilton and Buell, 1976; Olson, 1990;
Witty and Thompson, 1974; Woodin, 1984). Side channels can trap substantial num-
bers of fingerlings and smolts (up to 150 cm) as well as fry.
As water recedes from river margins, juvenile salmonids may become trapped in
deep pools called
potholes
(Woodin, 1984). River potholes are formed at high flows
from scouring (a process called
corrosion
; see
Fig u re 4.17
) around boulders and
rootwads and where opposing flows meet. Potholes may remain watered for hours or
months depending on the depth of the pothole and the river stage. Beck Associates
(1989) extensively studied pothole stranding in the Skagit River (Washington State).
Among the conclusions were that (1) only a small fraction of the potholes in a river
channel posed a threat to fish if fluctuations are limited in range, (2) the incidence
of stranding is independent of the rate of stage decrease, and (3) the incidence of
stranding is inversely related to the depth of water over the top of each pothole at the
start of the decline in flow.
Most documented observations of stranding have occurred on gravel; however,
stranding has also occurred in mud (Becker et al., 1981) and vegetation (Phillips,
1969; Satterthwaite, 1987). Under laboratory conditions, Monk (1989) found sig-
nificantly different rates of stranding on different types of gravel. In fact, substrate
was statistically the most significant factor contributing to the stranding of Chinook
