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onln31 TRIP REPORT LOCATION: Green River, Utah DATES: May 12 to 14,1992 AUTHORS: M.D. Harvey and R.A. Mussetter, RCE, Inc. RCE Project No. 91-714'01. SUBMITTED TO: Mr. Ray Tenney, P.E. Colorado River Water Conservation District DATE: December 3, 1992 INTRODUCTION Between May 12 and 14, 1992, Drs. Mike Harvey and Bob Mussetter of Resource Coflsultants & Engineers, Inc. (RCE) In the company of Individuals from the U.S. Fish and Wildlife Service (USFWS), the Colorado River Water Conservation District (CRWCD) and the U.S. Bureau of Reclamation (USBR) conducted l1i water-bome field Inspection of the Green River from the Town of Green River, Utah at River Mile (RM) 120 to Unknown Bottoms at RM 28. During the field Inspection the discharge of the Green River was about 9,500 ets. The primary objective of the trip was to provide an Interdisciplinary overview of this reach of the Green River In terms of historical and present usage of habitat by threatened and endangered native species, eSJl8Clally the Colorado Squawllsh. Specific areas of Interest were: 1) the effects of Tamarisk (Tamarlx chlneuli;lI) on channel morphology and capacity within the reach, 2) the effects ::! water storage projects on the frequency of overbank flows within the reach, and 3) the consequences of both areas of Interest on potential within and out-of-channel (bottomlands) habitat utilization by larval and juvenile stages of the endangered and threatened species. Larval stages are distributed downstream from spawning sites by the mechanism of passive drift and therefore their dlspersement Into overbank habitats Is depenclent on a temporal concurrence of larval presence (a function of drift rate) and greater than bankfull discharge at a location where there Is a floodplain of sufficient width to provide the low velocity, shallow flow conditions required by the larval stages. TRIP OBSERVATIONS 1) Tamarisk Grafs (1978) study of the effects of the Invasion of tamarisk on the morphology of the Green River within Canyonlands National Park Indicated that the establishment of dense stands of the plant along the riparian margins of the river had caused channel width to narrow by about 27 percent between 1915 and 1951. Establishment of the vegetation was probably assisted by a period of drought that lasted into the 194Os. Subsequent high flows in the 1950s did not appear to remove the tamarisk. In general terms, establishment of dense stands of riparian vegetation increases the hydraulic resistance so that overbank flows tend to deposit their sediment loads more rapidly thereby accelerating the process of natural levee formation. With an effective Increase In the height of the bank as a result of natural levee formation, the channel capacity increases and the frequency of overbank flows Is diminished. On the Green River, Graf (1978) concluded that prior to colonization of the channel margins by tamarisk, overbank flooding was infrequent and occurred only during the highest floods. He argued that channel constriction and reduced erodablllty of the banks due to tamarisk colonization Increased the frequency of overbank flooding, such that flooding occurred even at modest discharges. Eventually, If the increased frequency of overbank flows caused natural levee formation, the frequency of overbank flows must have decreased through time. 1 Resource Consultants & Engineers, Inc.