THE PONDS PROJECT - Sustainable Conservation

shallow riffles that potentially may become too shallow for fish passage. Describing this
method, Thompson states,
To determine the flow to recommend for passage in a given stream, the
shallow bars most critical to passage of adult fish are located and a linear
transect marked which follows the shallowest course from bank to bank.
At each of several flows, the total width and longest continuous portion of
the transect meeting minimum depth and maximum velocity criteria are
measured. For each transect the flow is selected which meets the criteria
on at least 25 percent of the total transect width and a continuous portion
equaling at least 10 percent of its total width. The results averaged from
all transects is the minimum flow we have recommended for passage. I
might caution that the relationship between flow conditions on the transect
and the relative ability of fish to pass has not been evaluated.
Thompson (1972) provides criteria for minimum depths and maximum velocities for
successful upstream passage of adult steelhead (0.6 ft and 8 ft/s), coho salmon (0.6 ft and
8 ft/s), and Chinook salmon (0.8 ft and 8 ft/s). However, his criteria were not evaluated,
although they have often been adopted in fish passage studies. Bovee (1982) suggested
that additional depth may be necessary for fish to migrate extended distances. Discussing
Thompson’s minimum depth criteria, Bovee (1982) states,
The investigator should temper this criterion by the number and length of
crossings the fish must make. Fish that encounter very few passage
barriers can probably negotiate some fairly shallow water. The same
species moving up a stream with many passage bars may arrive at the
spawning area in poor condition if the passage depths are minimal.
Thompson’s technique for identifying fish passage flows could be a practical approach
for determining whether the February median, or some higher flow, is needed
downstream from sites where CFII exceeds 10 percent. Assessments of this kind should
include all reaches where fish passage may be potentially affected by the proposed
project. When applying Thompson’s method, stage-discharge relations at cross-sectional
transects should be developed based on empirical measurements at three or more
alternative flows that span the likely range of flows meeting passage criteria. Mean
column velocities across the study transects should be recorded at the study flows, and
each transect should be photographed at each study flows with reference objects in the
stream (e.g., persons, yardsticks, etc.) to document stream depths and velocities.
In addition to documenting flows facilitating passage at shallow riffles, a qualified
fishery biologist should map the location of cascades, falls, debris jams or other fish
passage obstacles or barriers in reaches downstream from the diversion site. At some
falls or steep cascades, fish passage conditions may be favorable over a small range of
flows. The biologist should photo document, and measure depths and velocities at these
potential barriers to fish migration at each of the alternative study flows.
58