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

distribution of larvae. We use the model to predict the effects of Flaming Gorge Dam on the
distri bution of larval fish. We also evaluate the accuracy of model predictions, the adequacy of field
measurements, and comment on the present understanding of the processes that determine larval
fish distribution in the middle Green River. Our results demonstrate that the spatial distribution of
critical nursery habitat partly results from species adaptation to the unique attributes of the
hydrology and geomorphology of this part of the Colorado River basin. Dam operations have the
potential to eliminate these unique attributes.
Background
Many attributes and functions of lotic ecosystems are partly controlled by the geomorphic
characteristics of the host stream and its floodplain (Power et al. 1988, 1995). These geomorphic
characteristics are in turn determined by the hydrology and sediment transport of the stream, as well
as by the lithology and structure of the geologic formations within which the channel/floodplain
system is established. The influence of geologic characteristics on channels and floodplains, and
ultimately on lotic ecosystems, primarily arises because lithology and structure control valley width,
the linkage between hillslopes and channel, and the location and characteristics of tributary inflow.
These controls are prominent in small and intermediate-size watersheds in mountainous terrain
(Gregory etal. 1991; Grant and Swanson 1995), but geologic control may also occur where large
rivers cross mountains or high plateaus.
Few large river systems in North America are more dominated by their surrounding
geology than the Colorado River and its major tributaries, the Green, San Juan, and Gila Rivers.
These streams cross more geologic barriers than any other in North America (Hunt 1969). Narrow
valleys occur where the surrounding rocks are relatively hard, and wide valleys occur where the
surrounding rocks are erodible (Fig. 2). There are reaches of the middle Green River with wide
alluvial valleys and low-gradient gravel or sand-bed channels, and there are narrow canyons with
gravel or sand beds or with rapids and large eddies immediately downstream. The downstream
changes in channel form are not progressive and do not resemble those on which the river
o continuum concept (Vannote et al. 1980) is based. _
) v r '_ The,middle Green River begins at the confluence of two co-equal branches, the upper Green
'\ V/,- \7 River and the Yampa Ri ver; th~Illean annual flow and annual floods of th~~e~ranches are similar
I 0' ; \. (Grams and Schmidt in pres~)/ Average annual stream flow of the middle Green River increased
v "
1 about 70 percent and ~us~nded-s~diment l~ increased by about 120 percent downstr~ from
i the Duchesne and White Rivers pnor to extensl ve water development (loms et al. 1965). _
A unique assemblage of native fish species has developed within this geologically-isolated
basin, and the fishery of the Colorado River basin has the highest degree of endemism (74% of all
species) of any large basin in the United States (Williams et al. 1989). Prior to extensive water

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