9315 ‎(2)‎

• UCREFRP Catalog Number: 9315
• Author: Harvey, M. D. and R. A. Mussetter.
• Title: Evaluation of the Required Frequency of Bar Forming Events and 2-Dimensional Hydrodynamic Modeling at Colorado Squawfish Spawning Sites in the Lower Yampa Canyon, Colorado.
• USFW Year: 1996.
• USFW - Doc Type: Fort Collins.
• Copyright Material: NO

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1. INTRODUCTION -~ct.:.J~-rC-- ~ '" ;i.- t,.-{M J..
1.1. Background ~, ~tr-'S" c1 ~cJYro/' r k... k'JO ?
Interdisciplinary studies of the Lower Yampa River Canyon in Dinosaur National Monument
funded by the Colorado River Water Conservation District (CRWCD) have resulted in the development
of a physical process-biological response model (PRM) to explain the flow requirements for the formation
of Colorado squawfish spawning habitat (Harvey et aI., 1993; Mussetter and Harvey, 1994), The model
indicates that the spawning bars are formed by sediment deposition as a result of local backwater )~t;. 5~ L":
conditions at high flows when bed material is mobilized in upstream non-backwatered reaches, and ~ '
incipient motion conditions are exceeded, In contrast, spawning habitat (tertiary bars) is formed during
recessional flows when the bars are dissected as a result of locally increased hydraulic gradients due to
reduced downstream tailwater, During low to moderate flows, when the primary bar is being dissected,
the bar acts as a hydraulic control, causing deposition of sand in the upstream pool. The combined
effects of bar dissection and sand deposition in the pool provide the mechanism for flushing of fines and
development of a clean cobble substrate that is required for successful spawning, '7
,
The PRM that was initially developed based on data in the lower Yampa Canyon was
subsequently tested and validated at two other spawning sites where the macro-scale features of the
rivers were very different. The validation sites were located within the Lower Yampa Canyon at Mathers
Hole (Mussetter and Harvey, 1994), and at Three Fords West at the lower end of Desolation Canyon on
the Green River (Harvey and Mussetter, 1994). The PRM provides a rational basis for evaluating the flow
requirements for spawning habitat, and demonstrates that contrary to previously held beliefs (Tyus and 7
Karp, 1989; O'Brien, 1984), high discharges are not required for formation of the localized spawning .
habitat. ~ LL.--r--- (Y-.L !'-"P-1 r d ~ ~-tL lolA.....r-o 7
However, a critical and as yet unanswered question is the required frequency of the high
discharge bar-forming or bar-maintaining events (Harvey et aI., 1993). Constant dissection of the primary
bar without re-deposition of sediment as a result of higher discharges will eventually lead to conditions
where spawning habitat is not created. The branch and' chute channels formed by bar dissection will
widen until the combination of coarsening of the surface bed material (armoring) and reduced unit
discharge caused by channel widening will prevent reworking of the primary and secondary bars. To date,
there are insufficient data to address the issue of the required frequency of bar rejuvenating events.
1.2. Project Objectives
The primary objective of the work conducted in the Lower Yampa River during the 1995 field
season was to begin to address the issue of the required frequency of bar building events at the known I~,. ~ :(.If~
Colorado squawfish spawning sites located at Mathers Hole (RM 17.5) and Cleopatras Couch (RM 16.5) '*'g1~
(Figure 1.1). The specific objectives of the 1995 field work, which was conducted under low flow
conditions (about 500 cfs) following a 1995 peak discharge of about 18,300 cfs, and a runoff season

1,1

Mussetter Engineering. Inc.