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<br />Downstream effects of Flam n
<br />on the Green River, Colorad
<br />
<br />
<br />E. D. ANDREWS U.S Geological Survey, Water Resources Division. Box 25046 MS 413, Denver Federal Center, Denver, Colorado 80225
<br />
<br />Ill1d~ M-6 /1 cg' (0
<br />
<br />ABSTRACT
<br />
<br />The Green River is one of the principal
<br />tributaries in the Colorado River basin and
<br />drains 44,700 mi2 in Wyoming, Colorado,
<br />and Utah. Since October 1962, flows of the
<br />Green River have been regulated by Flaming
<br />Gorge Reservoir, which is located 412 river
<br />miles upstream from its confluence with the
<br />- Colorado River. Mean annual runoff has not
<br />peen affected by the reservoir. The duration
<br />!, of the relatively large discharges that trans-
<br />port most of the annual sediment load, how-
<br />enr, has decreased significantly. As a result,
<br />) the mean anoual sediment discharge has de-
<br />Lcreased by 54% to 3.21 x 106 tons from 6.92 x
<br />106 tons 3li1ie Jensen gage located 105 river
<br />miles downstream from the reservoir and by
<br />48% to 8.83 x 106 tons from J7.0 x J06tons at
<br />the Green River, Utah, gage located 290 river
<br />hililes downstream from the rl'servoir. Sedi-
<br />ment supply to the channel eCluals the annual
<br />transport within a relatively short distance,
<br />IGH river miles, downstream from the reser-
<br />, voir. Downstream from river mile 166, the
<br />supply of sediment from upstream plus tribu-
<br />tary inflow exceeds the transport of sediment
<br />by -5.4 x J06 tons per year on an average.
<br />The quasi-equillbrium that appears to have
<br />existed prior to the reservoir no longer occurs
<br />along a mlljority of the Green River.
<br />- In response to the reduced peak dis-
<br />charges, tbe bankfull channel width of the
<br />Green River has decreased by -10%. Ad-
<br />justment of the channel to decreased peak
<br />flows and altered sediment loads is nowhere
<br />complete. At present, it appears that a cen- .
<br />tury or more will be required for the Green
<br />niver to adjust to the effects of Flaming
<br />Gorge Reservoir.
<br />.
<br />INTRODUCTION
<br />
<br />Alluvial channels adjust over a period of
<br />years, so that the sediment supplied to the chan-
<br />nel is transported with the available discharge.
<br />When there is no net accumulation or depletion
<br />
<br />of sediment in the bed, banks, or flood plain, the
<br />average hydraulic characteristics width, depth,
<br />velocity, roughness, slope, and channel pattern,
<br />through a reach of channel at a given discharge,
<br />will be nearly constant. Such river channels are
<br />in quasi-equilibrium. Although this condition
<br />may not be exact, the uniform elevation of flood
<br />plains along many rivers indicates that quasi-
<br />equilibrium is approached for period, of several
<br />decades to centuries. An appreciable and per-
<br />sistent change in the water discharge, sediment
<br />load, or sediment size will cause a disequilib-
<br />rium between the quantity of sediment supplied
<br />to the reach and the quantity transported out of
<br />the reach. Thus, the hydraulic characteristics
<br />will readjust, so as to attain a new quasi-equilib-
<br />rium.
<br />The quasi-equilibrium adjustment of a river
<br />channel located downstream from a reservoir
<br />typically will be altered to a substantial degree
<br />by the storage of sediment in the reservoir and
<br />the decrease of river discharge, especially the
<br />peak flows. The nature of the disequilibrium will
<br />vary longitudinally downstream in magnitude,
<br />direction (sediment surplus or deficit), and dura-
<br />tion. Williams and Wolman (1984) described
<br />the complex channel changes that have olXurred
<br />downstream of 21 dams. Degradation of the riv-
<br />erbed immediately downstream of a reservoir
<br />has been the most commonly studied channel
<br />impact (for example, see Hathaway, 1948;
<br />Komura and Simons, 1967; and Petts, 1979).
<br />These investigations usually have been limited to
<br />a reach extending only a few hundred channel
<br />widths downstream from the dam. Investiga-
<br />tions into the downstream effects of reservoirs
<br />have rarely considered channel adjustments that
<br />might occur downstream from the confluence of
<br />the first major tributary, although there is evi-
<br />dence that the changes arc quite significant.
<br />Lawson (1925) described extensive channel ag-
<br />gradation in a reach of the Rio Grande begin-
<br />ning > 100 mi downstream from Elephant Butte
<br />Reservoir. Degradation and aggradation of the
<br />Missouri River channel have become serious
<br />problems in the nearly 800-mi reach between
<br />the last downstream reservoir and the river
<br />
<br />Geological Society or America Bulletin, v. 97, p. 1012-1023, 10 figs., 3lables, August 1986.
<br />
<br />1012
<br />
<br />mouth. Sayre and Kelllledy (1978) attributed
<br />this channel disequilibrium, in part, to res-
<br />ervoirs.
<br />The opportunity to study the downstream ef.
<br />fects of reservoirs has been limited by two pri-
<br />mary difficulties. In most alluvial rivers, the
<br />mean annual sediment discharge is small com-
<br />pared to the quantit)' of sediment stored within a
<br />reach of a few hundred chanllel widths in length.
<br />Consequently, the annual sediment deficit
<br />caused by reservoir storage also is small com-
<br />pared to the volume of sediment available for
<br />transport. Major channel adjustments may occur
<br />only after an appreciable change in quantity of
<br />sediment within the channel. 'l11is condition
<br />may require several decades to develop, depend-
<br />ing upon the distance downstream from the res-
<br />ervoir. Reservoirs with storage greater than
<br />1,000,000 acre-feet have been built only within
<br />the past 50 yr or so. The vast majority have been
<br />constructed since 1950. The far-downstream ef-
<br />fects of these reservoirs upon channel equilib-
<br />rium are only now becoming evident.
<br />The second difficulty is closely related. Com-
<br />prehensive, long-term records of river flows and
<br />sediment transport at several locations down-
<br />stream from the reservoirs, as well as on major
<br />tributaries, usually do not exist (Petts and
<br />Lewin, 1979). Thus, the information required to
<br />describe the characteristics of river flows and
<br />sediment transport prior to a reservoir and the
<br />change since regulation is usually unavailable.
<br />Several of the longest records of daily sedi-
<br />ment transport that exist for North American
<br />rivers have been collected at gaging stations in
<br />the Colorado River basin. As of 1983, the
<br />length of most of these records of sediment
<br />transport was almost 40 yr. Large reservoirs
<br />were constructed on each of the threl;, major
<br />headwater tributaries during the early 1960s.
<br />The pre- and post-reservoir periods of record
<br />therefore are now -20 yr. 111is investigation
<br />considered the downstream effects of Flaming
<br />Gorge Reservoir on the Green River.
<br />Long-term channel change is of particular in-
<br />terest for the Green River because of the impacts
<br />it may have on the survival of several species of
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