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3.0 Scientific Basis And Underlying Principles 3-16 April 2004 3.7.5 Reconnection Of Floodplain To Main Channel Reconnection of a floodplain to the main river channel is critical to completion of the "reset theory" cycle of inundation and desiccation. Reconnection allows the 1 or 2-year old razorback sucker to escape to the river where they can mature and reproduce. Observations of hatchery razorback sucker indicate that age-l fish will not leave a floodplain during reconnection (Personal Communication, Tim Modde, U.S. Fish and Wildlife Service; Kevin Christopherson, Utah Division of Wildlife Resources). Similar observations have been made for fish 1 to 2 years of age, although these conclusions are preliminary. These observations indicate that young razorback sucker will remain in sheltered floodplains through their first 1-2 years of life, which is consistent with the floodplain management strategy fundamental to this Plan. Recent studies of hatchery razorback sucker released in floodplains also show that survival in floodplains in the first month is low (<5%), but little or no survival is presumed in the main river channel (Christopherson and Birchell2002; Birchell and Christopherson 2002). Survival in floodplains after the first month is greatly increased, but it is believed that razorback sucker must be over about 90 mm TL (about 6 months old at low growth rate) and preferably over 230 mm TL (about 17 months old at low growth rate) to survive in the main channel. Fish entrained in a floodplain depression that do not escape to the main channel during a flow connection will become stranded until the following runoff cycle. Given that floodplain connections during spring runoff are typically less than 1 week, the best survival strategy for razorback sucker is believed to be a 24-month residence in a productive floodplain that allows the fish to reach sufficient size for mainstem survival and to escape predators. Until self- sustaining populations become established and multiple spawning sites and floodplains are used by wild fish, it may be necessary to manually transfer fish from floodplains to the main channel when river flows are insufficient to connect floodplains and fish are old enough for mainstem survival. Levee breaches at key floodplains will increase the frequency of connection with the mainstem and inundation in all but dry and moderately dry years. The hydrologic cycle of the Green River Subbasin typically consists of periods of 3-5 years of wet and moderately wet years followed by periods of dry and moderately dry years (Muth et al. 2000). The "reset theory" is based on this hydrologic cycle in which connection of most floodplains occurs annually in wet years and desiccation (i.e., reset) occurs in intervening dry years. This cycle will also occur in average years with fewer floodplain sites expected to be connected annually. This strategy also recognizes that magnitude, duration, and possibly frequency of inundation will vary among floodplain sites with river stage, and emphasizes the importance of all sites for overall recovery of the endangered fish species. 3.7.6 Desiccation To Reset Floodplain The key to the "reset theory" is periodic desiccation of the floodplain to reset or kill all remaining fish and reset productivity. Ideally, floodplains should desiccate every fourth year to