Environmental Engineering Reference
In-Depth Information
DID YOU KNOW?
To measure instantaneous discharge, USGS hydrologic technicians stand in
the stream, on a bridge, or in a boat and measure the depth and how fast the
water is moving at many places across the stream. By doing this many times
and at many stream stages, over the years the USGS has developed a relation
between stream state and discharge.
was adequate to protect steelhead fry; however, the ramping rate was measured at a
confined river transect, whereas the stranding was observed on lower gradient bars
farther downstream. Thus, the effective ramping rate at these bars was less than 1
inch per hour.
Although many hydropower mitigation settlements specify ramping rates, some
research has indicated that ramping rates cannot always protect fish from stranding.
Woodin (1984) determined that
any
daytime ramping stranded Chinook fry. Beck
Associates (1989) could not find any correlation between ramping and the incidence
of pothole trapping, nor was there any correlation between the ramping rate and
steelhead fry stranding during the summer. In both cases, stranding occurred regard-
less of the ramping rate.
Small fry are highly vulnerable to stranding and are present in streams only at
certain times of the year. Chinook, coho, pink, and chum fry emerge during late win-
ter and early spring, while steelhead emerge in late spring through early fall (Olson,
1989). Fingerlings, smolts, and adults are vulnerable to stranding in other seasons;
however, less restrictive ramping criteria are often sufficient to protect them.
For at least some species, the incidence of stranding is influenced by the time
of day. Chinook fry are less dependent on substrate for cover at night and thus are
less vulnerable to stranding at night (Woodin, 1984). Two studies (Olson, 1990;
Stober et al., 1982) concluded that steelhead fry are less vulnerable during the day,
presumably because this species feeds during the day. However, two other studies
(Beck Associates, 1989; Monk, 1989) found no difference in the rate of steelhead fry
stranding relative to day and night.
Salmonids respire using their gills and do not survive out of water for more than
10 minutes; thus, beaching is always fatal. Juvenile salmonids trapped in side chan-
nels and potholes can survive for hours, days, or under favorable circumstances for
months (Hunter, 1992). However, many trapped fish die from predation, temperature
shock, or oxygen depletion. Survivors that are rescued by higher flows are probably
in poorer condition than fish in the free-flowing channel.
Some observations suggest that a highly stable flow regime for a week or more prior
to a flow fluctuation will increase the incidence of fry stranding (Phinney, 1974b).
Two hypotheses might explain this observation. One hypothesis states that after long
periods of stable flow more fry are available for stranding. In other words, a major
flow reduction after a week of stable flows strands seven daily cohorts of emerging fry
at once, rather than one cohort when fluctuations occur daily. An alternative hypoth-
esis is that juveniles become accustomed to residing and feeding along the margins
