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1 INTRODUCTION Flooded bottomlands located along the Green River are believed to be important nursery habitat for the endangered razorback sucker. The goal of the Habitat Restoration Program is to restore or enhance natural floodplain functions that support recovery of endangered fishes in the Upper Colorado River Basin. Previous investigations at Old Charlie Wash, a depression with inlet and outlet structures along Green River near Ouray, UT, suggest that razorback larvae will passively drift into floodplain areas during overbank flooding. Some of these larvae were able to survive and grow even in the presence of large numbers of non-native fishes. (Moody, USFWS, 1997). In addition, initial findings at three floodplain restoration sites, Bonanza Bridge, Stirrup, and Old Charlie Diked Bottomlands, where levee removal activities were completed prior to the 1997 runoff season, indicate that native, endangered, and non- native fishes will use restored floodplain habitats when they are made available (Levee Removal Evaluation Group, 1998). This document presents the results of floodability assessments of eight privately-owned properties along the Green, Colorado, and Gunnison rivers and discusses project goals, site descriptions, river hydrology and hydraulics, flooded bottomland hydrology, and provides recommendations and design strategies for enhancing the floodability of the target bottomlands. Project Goals and Objectives The project goal was to conduct bottomland site surveys and assess the floodability to determine the physical changes required to hydraulically reconnect the floodplain with the river and restore historic flood frequencies and durations. Specific objectives of this study were to: • Implement a cross section monitoring program in each selected reach. • Collect hydrographic data including cross section surveys, and water surface elevations throughout the runoff season to develop stage/discharge relationships and/or a hydraulic model to assess the floodability of each bottomland under existing flow regimes. • Identify flooding discharges for all the bottomland sites, pre- and post-restoration. • Perform topographic surveys in order to create mapping, estimate the area of potential flooded bottomland inundation, and identify levee removal strategies to enhance floodability. • Identify any structures, access roads and property boundaries and display them relative to a range of flood levels. • Develop area of inundation as a function of flow relationships for each site pre- and post restoration. • Relate the historical flow magnitudes, frequency and duration to existing flooded bottomland inundation. • Propose design restoration alternatives to enhance the floodability of the bottomlands to mimic historic frequencies and durations. SITE DESCRIPTIONS Green River -The Green River is the major tributary of the Colorado River with a basin draining approximately 45,700 square miles of western Colorado, southwestern Wyoming and eastern Utah. Flows in the Green River have been regulated by Flaming Gorge Reservoir since October 1962. The major tributaries to the Green River upstream of the study sites include the Yampa and Little Snake Rivers. Flows in the Green River are primarily snowmelt from the western slope of the Rocky Mountains and annually peak in late May or early June. The base flow period extends from about September to March. The hydrology of the Green River has been altered by management of reservoir releases and water resource development. 1-1