8110

• UCREFRP Catalog Number: 8110
• Author: FLO Engineering, I.
• Title: Green River Flooded Bottomlands Investigation Ouray Wildlife Refuge and Canyonlands National Park, Utah-Final Report.
• USFW Year: 1996.
• USFW - Doc Type: Breckenridge, CO.
• Copyright Material: NO

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Executive Summary

Historically, many of the bottomlands along the Green River were frequently flooded by spring peak flows.
During the 16 year period from 1947 to 1962, flows in excess of 22,000 cfs occurred 11 times including 7
consecutive years from 1947 to 1953. This virtually ensured that some bottomlands were flooded in the majority of
years in the reach from Jensen to Ouray, Utah. The frequency, magnitude and duration of flooding has been curtailed
as a result of upstream water resource development. lbis includes both the flow regulation of the Green River due to
the operation of Flaming Gorge Dam and the cumulative impacts of a numerous water development projects
throughout the Green River basin. A decrease in sediment supply to this reach has been documented in conjunction
with the reduced peak flows. The river has responded to reduced flood levels and frequency with adjustments in
channel morphology, a smaller width-to-depth ratio and vegetation encroachment on sand bars. The decrease in the
frequency of bottomlands flooding has been compounded by the construction of levees along some reaches of the
river.

Two study reaches were selected for the investigation of bottomlands flooding and backwater inundation in
the Green River. The fust reach was located in the vicinity of Ouray National Wildlife Refuge, near Vernal, Utah and
the second reach was located in Canyonlands National Park near Anderson Bottom. At Ouray, the vast, frequently
flooded, historical bottomlands were the subject of the investigation. In Canyonlands, the focus of the investigation
was the backwaters created by side canyon channels during high river flows. An objective of the study was to
determine the area of inundation of flooded bottomlands as a function of discharge. The project consisted of
hydrographic data collection at the two sites during the 1995 high flow season, an analysis of USGS stream gaging
data for the period of record at the Green River gages at Jensen and Green River, Utah, and a hydraulic simulation of
flood levels using the Corps of Engineers (COE) HEC-2 water surface proftle model.

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The project goal was to determine river flow magnitude, frequency and duration or physical changes that
might restore or enhance natural flooding ofbottomlands at the two selected sites on the Green River. The primary
objectives of this study were to:

Implement a cross section monitoring program in each selected reach.
· Collect hydrographic data including cross section surveys, discharge and bed material samples for three
flow periods: 1) at base flow prior to the rising limb of the seasonal hydro graph; 2) near the spring
runoff peak flow; and 3) at or near base flow in the late summer-early fall.
Calibrate channel roughness values.
Identify existing bankfull discharge throughout the two study reaches.
Estimate the area of flooded bottomland inundation as function of discharge.
Relate the historical flow magnitudes, frequency and duration to the bankfull discharge and flooded
bottomland inundation.
· Recommend restoration activities and flows related to flooding bottomlands.

Each of these objectives were met. In addition to these objectives, a review of pertinent literature was undertaken, a
qualitative assessment of the channel morpholo gy trends was conducted and an analysis of the channel response to
1995 high flows was completed.

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The analysis of the channel morphology indicated that the Ouray and Canyonlands reaches had undergone
many changes with diverse hydrologic results. Both channels had experienced some channel narrowing, sand bar
stabilization and attachment to the banks and encroachment by vegetation within the active channel. Several large
sand bars within the study reach are now so densely vegetated with mature willows and tamarisk that inundation
results in sediment deposition and vertical accretion of the bars. These bars are now stable, permanent features within
the channel. These channel morphology changes occurred because of the reduction in the magnitude and frequency
of the peak discharges and the decrease in sediment supply to the reaches. The decrease in the mean annual peak
discharge from the pre-1963 period of record to the post-l 963 data bases were: at the Jensen gage: 24,000 to 17,400
cfs and at the Green River gage: 32,700 to 22,300 cfs. For the fust 29 years ofrecord, until 1930, the mean annual