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1096 MODDE ET AL. With the closure of Flaming Gorge Dam on the Green River in 1963, the magnitude and duration of floodplain inundation has been dramatically re- duced (Flo Engineering 1996). However, because little information exists on the early life history needs of razorback suckers, few links between those needs and floodplain habitat have been es- tablished. The few juveniles captured in the upper Colorado River basin were in the main channel (Taba et al. 1965; Gutermuth et al. 1994). The purpose of this study was to examine the physical and biological characteristics of a manip- ulated floodplain wetland in the middle Green Riv- er and to describe the features that provide suitable nursery habitat for juvenile razorback suckers. The approach taken was to describe the environmental conditions and fish use and to compare larval ac- cess to floodplains with and without the effect of Flaming Gorge Dam. The specific objectives of the study were (1) to measure the environmental variables that limit or influence the survival and growth of age-0 fishes (i.e., dissolved oxygen, tem- perature, depth, and zooplankton densities) in the study wetland through the summer months, (2) to determine the access of larval razorback suckers to floodplains in general by comparing the timing of larval drift with that of average bank-full flows, and (3) to census the fishes occupying the study wetland to determine whether razorback suckers used the site. Study Area Larval razorback sucker collections were made in the middle Green River between river kilome- ters (rkm) 482 and 395 (measuring from the con- fluence with the Colorado River; Figure 1). The wetland sampled intensively was Old Charley Wash (also known as Wood's Bottom), a 147-ha floodplain depression on the Ouray National Wild- life Refuge near Ouray, Utah, where a natural wet- land was artificially partitioned into two units. His- torically, water has entered the wetland via gravity flow through an inlet canal (rkm 396) from the Green River when discharge reaches 240 m3/s and is connected to the river by flood flows over the natural levee at approximately 396 ml/s. The inlet canal was closed during this study, so that all water entering Old Charley Wash was via overbank flooding. Only the 80-ha main unit was studied. The primary vegetation in the wetland was smart- weed Polygonum sp. and sago pondweed Pota- mogeton pectinatus, with small stands of bulrushes Scirpus sp. and cattails Typha sp. around the mar- gins. The study area was drained prior to winter and filled each spring by flood flows. An outlet structure at the downstream side was closed in spring to retain water in the wetland and was opened only to drain the wetland. The outlet struc- ture included a concrete floor and walls lined with polyethylene tubing filled with gravel, allowing the efficient capture of fishes. Methods Environmental variables.-Maximum depth, subsurface water temperature, dissolved oxygen, and zooplankton densities were measured in Old Charley Wash in spring and summer 1995 and 1996. Maximum depth was determined by sub- tracting the elevation difference between the bot- tom of the wetland and the reading on a staff gauge on the outlet drain structure. Dissolved oxygen was measured just before sunrise with a Hach DREL/ 2000 Water Quality Kit in 1995 and an OxyGuard MkII meter in 1996 at a single location near the center of Old Charley Wash. Zooplankton were sampled from sites randomly selected from grids superimposed over a map of the wetland. Subsur- face temperature was taken from the first zoo- plankton sample site on each date. Data were col- lected approximately weekly while the river and wetland were connected and biweekly thereafter until the wetland was drained. Sampling dates were between 31 May and 14 September in 1995 and between 14 May and 8 July in 1996. Instan- taneous river temperatures were recorded at the U.S. Geological Survey gauge in Jensen, Utah (rkm 500). Zooplankton were collected in daylight with a plankton net 10 cm in diameter with 243-µm mesh by pulling the net just under the surface across a known distance (8 m). Volume was determined by multiplying the net diameter (10 cm) by the length of haul. Samples were taken in three different di- rections at each of three sites and preserved in a 5% solution of formalin. The total number of mi- crocrustaceans per sample was estimated as fol- lows: random grids were selected from a petri dish 14 cm in diameter with centimeter grids on the bottom; up to 10% of the available grids were counted, or a minimum of 200 zooplankters; and the total number was calculated by direct propor- tion (i.e., the number of zooplankters counted mul- tiplied by the total area of grids divided by the area counted). Relative density was estimated by dividing the total number of zooplankters collected by the water volume sampled. Larval fish collections.-Razorback sucker lar- vae were sampled in the middle Green River in