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I I I I I I I I I I I I I I I I I I I of factors. Larval razorback sucker enter floodplains during spring runoff, when river flows top high-water levees and inundate adjacent river bottoms. The period of connection between the river and the floodplain may vary from a few days to nearly 1 month, depending on river stage and elevation of the levee that separates the floodplain from the mainstem. A razorback sucker larvae may spend less than 1 month in the floodplain before escaping to the mainstem. Or, the fish may become trapped in the floodplain and unable to escape until the next inundating flow, which may occur 1 year later during the following spring runoff. Hence, a razorback sucker may spent 1 month, 1 year, 2 years, or possibly more time in a given floodplain. Time spent in floodplain habitats is an important consideration to razorback sucker survival and growth. Because floodplains are usually warmer and more productive than the mainstem (Grabowski and Hiebert 1989; Mabey and Shiozawa 1993), fish remaining in floodplains will have higher growth rates than fish in the mainstem. Conversely, young razorback sucker in floodplains with large numbers of nonnative predators and competitors may have low survival rates. 3.11 Survival in the Mainstem There are currently no estimates of subadult razorback sucker survival in the mainstem rivers of the Upper Colorado River Basin. Average annual survival of adult razorback sucker is estimated at about 70% (Modde et al. 1996). 3.12 Recruitment There are currently no estimates of recruitment by razorback sucker in the Upper Colorado River Basin. Average annual survival of adult razorback sucker is estimated at 70% (Modde et al. 1996); hence recruitment of approximately 30% would be expected to replace adult mortality and ensure a stable population. 4.0 THE FLOODPLAIN MODEL 4.1 Description of the Floodplain Model The interactive model developed from this project is called "FLOODPLAIN", Version 5.1. FLOODPLAIN is a user interactive mathematical model designed to predict floodplain 10