9665

• UCREFRP Catalog Number: 9665
• Author: Schmidt, J. C. and J. B. Box.
• Title: Controls on the Longitudinal Distribution of Age-0 Colorado Pikeminnow in the Middle Green River, Colorado and Utah - Draft Report.
• USFW Year: 2001.
• USFW - Doc Type: Logan, UT.
• Copyright Material: NO

11/14/01 draft report, Schmidt and Box

Even with these photographs, we faced the difficulty of how to characterize shoreline
complexity at discharges that were higher than the base flows at the time of photography. Two
geomorphic surrogates were used to estimate complexity for higher discharges than base flow: the
shoreline defined by the contact of wet and dry sand and the shoreline at bankfull stage (Fig. 6).
Shoreline tracings of the river's edge and surrogates were digitized, and complexity was calculated
for each measured reach. For purposes of this analysis, we assumed no change in river
geomorphology since 1963, although channel narrowing is known to have occurred (Allred and
Schmidt 1999, Grams and Schmidt in press).
The shape of the relationship between discharge and shoreline complexity was constrained
in the following way. We assumed that complexity is 2 when discharge is approximately zero. We
also knew the complexity at base flow and bankfull discharge. Over-bank floods are not relevant to
the modeling exercise, because larval drift occurs during flood recession. The magnitude of
bankfull stage was estimated to be the discharge of the 2-yr recurrence flood (Allred and Schmidt,
1999; Grams and Schmidt, in press). We assumed that the discharge associated with the contact
between wet and dry sand was 33 percent of the difference between base flow and bankfull
discharge. These assumptions and constraints led to a relationship that is unimodal (Fig. 7 A), and
the shape of this relationship is the same as that measured by Rakowski (1997) in a detailed study
of a l.6-km reach near Ouray. We assumed that the shoreline complexity was the same between
0.8 and 1.0 times the bankfull discharge. Additional data for our study area would improve our
characterization of this relationship, but we can not conceive of how the unimodal shape of the
relationship would change.
We assumed that there is a direct relationship between shoreline complexity, and the
proportion of the total length of main current bordered by shear zones; more complex shorelines
have a higher proportion of shear zones bordering the main current (Fig. 7B). Thus, each reach had
a value for the proportional length of shear zones for.each day of model simulation.
Bioloey
We paramaterized the movement of larvae across shear zones. In each reach, the number of
laryae entering backwaters was calculated as
a - b [cr + (1-cr)(l-NA)] (2)
where a _equals the number of larval fish entering backwaters, b equals the proportion of the
boundary of the main channel flow that is comprised of shear zones, cr equals an assumed
proportion of the main channel flux of neutrally-buoyant particles that move across shear zones,
and NA is the proportion of the total larval fish population that has no ability to swim. We used a
value of 0.6 for cr in our models; we describe the impact of using other vales of cr below.

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