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12 <br />flows, water temperature, and other physical variables (Tyus and Karp 1990; Modde and <br />Wick 1997). Prior to spawning, flows of a particular magnitude are also thought to be <br />necessary to remove fine materials from gravel and cobble spawning bars (Wick 1997). <br />Subsequently, flows must provide nursery habitat for juveniles in backwaters and <br />floodplain areas to obtain recruitment in a given year (Modde et al. 1996; Wick 1997; <br />Modde 2001). Interannually, extreme high flows, although possibly associated with <br />limited recruitment, are required to redistribute sediment, cycle nutrients, reduce non- <br />native plant encroachment, and maintain channel connections with the floodplain <br />(Stanford 1994). <br />There have been extensive geomorphological studies (e.g., Andrews 1986; <br />Andrews and Nelson 1989; Lyons and Pucherelli 1992; Harvey et al. 1993; Grams and <br />Schmidt 1999) and biological studies focusing on the razorback sucker (e.g., Tyus 1987; <br />Tyus and Karp 1990; Modde et al. 1996; Muth et al. 1998; Modde et al. 2001) conducted <br />in the Green River that have contributed vast amounts of information to our <br />understanding of the physical processes at work in the river and the biology of the <br />razorback sucker. The substantial amount of knowledge gained on the life history, <br />population demographics, and habits of the razorback sucker is notable because many of <br />these studies were initiated after the fish was very rare and federally protected. The goal <br />of the current study is to assess relations between physical variables and potential <br />reproductive bottlenecks for the razorback sucker by investigating and comparing <br />spawning areas in the Green and Yampa Rivers. Specifically, our objectives are to: (1) <br />describe long term trends in hydrology that relate to reproduction of razorback sucker, <br />especially access to floodplain habitats; (2) characterize physical conditions (i.e.,