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)OO~ , ~L\\M\Lli 'r 'Duf ~lo3~ Application of a Dynamic Model to Assess Controls on Age...O Colorado Pikeminnow Distribution in the Middle Green River, Colorado and Utah John C. Schmidt* and Jayne Brim Box** "Department of Aquatic, Watershed, and Earth Resources, Utah State University ....USDA Forest Service, Fish and Aquatic Ecology Unit, Logan Forestry Sciences Laboratory Analysis of field data and development and application of a dynamic model indicate that the processes that control the number and distribution of age-O Colorado pikeminnow in the middle Green River are poorly un- derstood. Colorado pikeminnow are a federally endangered species endemic to the Colorado River basin that utilize backwaters during their larval stage. The present agency-mandated field sampling program for backwater habitats may be inadequate because it takes place at a time when the model predicts that most larval fish have drifted beyond the study area. The model predicts that water releases from Flaming Gorge Dam have a large potential effect on larval drift, because high releases at the time of drift greatly increase the proportion of the population transported beyond the study area to unfavorable river environments. Development of the model shows that the role of the geomorphic and hydraulic attributes that control larval drift and transport into back- waters, and that were parameterized in the model, are not well known. Resolution of these uncertainties by field study and experimentation would lead to refining this river management tool. Key Words: Colorado pikeminnow, fluvial geomorphology, Green River, dynamic model, dams. There have been numerous studies concerning the hydrology, geomorphology, and fishery of the Colorado River system, but there have been few efforts to evaluate linkages between dam operations and associated changes in stream flow, physical habitat, and populations of endangered species (Osmundson et a1. 2002). Identification of these linkages is critical, because populations respond to abiotic and biotic factors, but only some abiotic factors are controlled by dams (Sch- midt et a1. 1998). It is poor public policy to impose costly changes in dam operations when unrelated factors are the cause of the endangered status of a species. Scientifically based adaptive management programs, such as the Grand Canyon Adaptive Management Pro- gram or the Upper Colorado River Basin Recovery Im- plementation Program, typically involve extensive field measurement and monitoring programs of specific eco- system components and use statistical models to evaluate the linkages among these components. Associated changes in dam operations have not necessarily resulted in meas- urable improvement in key downstream resources (Brower, Reedy, and Yelin-Kefer 2001; Rubin et a1. 2002), because the relationship between the dam-controlled flow regime and the responding ecosystem is not well understood. The purpose of this article is to illustrate the use of a dynamic model as a complementary tool with which to evaluate processes that link dam operations with the status of endangered species. We provide this illustra- tion by describing a model that predicts the distribution of larval and age-O Colorado pikeminnow (Ptychocheilus lucius) in 225 km of the middle Green River in Colorado and Utah between 1990 and 1995. This en- dangered fish was once common throughout the Colo- rado River basin. Today, its only viable population occurs in the Green River (Gilpin 1993), although small populations occur in the upper Colorado and San Juan Rivers. Flaming Gorge Dam on the Green River creates the largest reservoir in the Colorado River basin upstream from Lake Powell. The relationship between dam opera- tions and the status of Colorado pikeminnbw is difficult to assess in the Green River. Stream flow is only partially controlled by the dam, larval and age-O fish are difficult to sample, and many biological and physical processes that determine the status of the species are poorly understood. The U.S. Fish and Wildlife Service (1992) concluded that dam operations jeopardize the continued existence of the downstream endemic endangered fishery and proposed changes in dam operations (Muth et a1. 2000). Study Area The study area extends from the confluence of the Yampa and Green Rivers in northwestern Colorado to Annals of the Association of American Geographers, 94(3), 2004, pp. 458-476 <lJ 2004 by Association of American Geographers Initial submission, November 2001; revised submission, May 2003; final acceptance, December 2003 Published by Blackwell Publishing, 350 Main Street, Malden, MA 02148, and 9600 Garsington Road, Oxford OX4 2DQ, U.K.