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<br />,
<br />
<br />20 The State of the Colorado River Ecosystem in Grand Canyon
<br />
<br /> B.
<br /> 70,000
<br /> - 60,000
<br />j j '"
<br />~ ~ ~ 50,000
<br />~ '"
<br />~ ~
<br />~ ~ ~
<br />u no
<br />~ ~ -'= 40,000
<br />~
<br />J '0
<br /> .!: 30,000
<br /> '"
<br /> en
<br /> ~ 20,000
<br /> :z.
<br /> .;;;
<br /> Cl 10,000
<br />
<br />A.
<br /> 180,000
<br /> 160,000
<br />en 140,000
<br />13
<br />~ 120,000
<br />Q;
<br />u..
<br />'" 1 00,000
<br />'"
<br />'"
<br />-'
<br />:0 80,000
<br />'"
<br />~ 60,000
<br />no
<br />-'= . I
<br />'-'
<br />.~ 40,000
<br />Cl
<br /> 20,000
<br />
<br />~
<br />"
<br />~
<br />o
<br />>
<br />~
<br />u
<br />~
<br />~
<br /><;;
<br />
<br />I I' ,
<br />,\~ t~~.,M ],1'
<br />
<br />o
<br />~
<br />~
<br />
<br />
<br />o
<br />~~~#~~~~#~#$#~~~$$
<br />
<br />September 13, 1927, flood with peak
<br />discharge of 125,000 cfs
<br />
<br />Closure of Glen Canyon Dam
<br />
<br />Dam operations
<br />constrained on
<br />August 1, 1991
<br />
<br />
<br /># ~ ~ # # # # # ~
<br />o ~ ~ 0 0 0 0 0 0
<br />
<br />Figure 1. Instantaneous discharge (A) and daily range in discharge (B) in cubic feet per second of the Colorado River at Lees Ferry
<br />(RM 0) between 1921 and 2004 (modified from Topping and others, 2003), Before construction of Glen Canyon Dam, the annual peak
<br />flow routinely exceeded 100,000 cfs. Dam operations during the period from 1963 through 1990 were characterized by daily fluctuations
<br />from typically less than 5,000 cfs to near powerplant capacity, or about 31,000 cfs, and included the record wet period of the mid-1980s,
<br />which resulted in the use of the spillways in 1983 for emergency releases exceeding about 90,000 cfs, Interim operating criteria, which
<br />constrained daily release fluctuations, began in 1991 and were followed by the modified low fluctuating flow operating alternative that
<br />was implemented as part of the Secretary of the Interior's Record of Decision (ROD) in 1996 (BHBF = beach/habitat-building flow).
<br />
<br />Status and Trends of
<br />Fine Sediment Below
<br />Glen Canyon Dam
<br />
<br />Changes in sand supply and flow regime down-
<br />stream from a dam affect the geomorphology of the
<br />downstream channel. ''''hen a dam traps sand and
<br />releases clear water, this clear water is often termed
<br />"hungry" because it still has the capacity to transport an
<br />amount of sand and gravel proportional to the flow and
<br />will erode the downstream channel and banks in order
<br />to satisfy its appetite with respect to sediment transport.
<br />On the basis of resurveys of historical cross-sections
<br />upstream from Lees Ferry, approximately 20 million tons
<br />(18 million Mg) of material-gravel and fine sediment,
<br />including sand-have been eroded from the first 15 mi
<br />(24 km) of the Colorado River downstream from the
<br />dam, an area referred to in this report as the Lees Ferry
<br />reach (Grams and others, 2004). The amount of mate-
<br />rial removed is equivalent to a 6 to lOft (2-3 m) drop in
<br />channel elevation averaged over the entire reach. Most
<br />of this sediment was removed by daily, high-release
<br />dam operations designed to scour the channel of the
<br />Colorado River below the powerplant during April-June
<br />
<br />1965 (fig. I). Daily suspended-sediment measurements
<br />made by the USGS at the Lees Ferry and Grand Canyon
<br />gaging stations indicated that these high flows in 1965
<br />eroded 4.4 million tons (4.0 million Mg) of fine sediment
<br />(mostly sand) from the Lees Ferry reach and 18 million
<br />tons (16 million Mg) of fine sediment (mostly sand) from
<br />Marble and upper Grand Canyons. Channel scour was
<br />anticipated below the dam during its design and was
<br />later needed to optimize energy generation within the
<br />operating range of the hydroelectric powerplant (Grams
<br />and others, 2004). Typical dam releases today do not
<br />result in much erosion from the Lees Ferry reach, and
<br />as a result very little fine sediment is transported down-
<br />stream to Marble and upper Grand Canyons.
<br />Despite the fact that its contributing drainage area
<br />is approximately 18 times smaller than that of the Little
<br />Colorado River, the single largest sand supplier to the
<br />reaches below Glen Canyon from 1990 through 2004
<br />was the Paria River. Farther downstream in Marble and
<br />upper Grand Canyons, the fate of fine-sediment depos-
<br />its is dependent upon the long-term balance between
<br />inputs to the system (i.e., tributary supply) and exports
<br />from the system (i.e., mainstem sediment-transport rates).
<br />Although sand inputs have been greatly reduced by the
<br />closure and operation of Glen Canyon Dam, the annual
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