|
<br />Computation and Analysis of the
<br />Instantaneous-Discharge Record for the
<br />Colorado River at Lees Ferry, Arizona-
<br />May 8, 1921, through September 3D, 2000
<br />
<br />ByDavid J. Topping, John C. Schmidt, and L.E. Vierra, Jr.
<br />
<br />ABSTRACT
<br />
<br />A gaging station has been operated by the U.S.
<br />Geological Survey at Lees Felry, Arizona, since May
<br />8, 1921, In March 1963, Glen Canyon Darn was
<br />closed 15.5 miles upstream, cutting off the upstream
<br />sediment supply and regulating the discharge of the
<br />Colorado River at Lees Ferry for the first time in
<br />history, To evaluate the pre-darn variability in the
<br />hydrology of the Colorado River, and to detennine
<br />the effect of the operation of Glen Canyon Darn on
<br />the downstream hydrology of the river, a continuous
<br />record of the instantaneous discharge of the river at
<br />Lees Ferry was constructed and analyzed for the
<br />entire period of record between May 8, 192 I, and
<br />September 30, 2000. This effort involved retrieval
<br />from the Federal Records Centers and then synthesis
<br />of all the raw historical data collected by the U,S,
<br />Geological Survey at Lees Ferry, As part of this
<br />process, the peak discharges of the two largest
<br />historical floods at Lees Ferry, the 1884 and 192 I
<br />floods, were reanalyzed and recomputed. This
<br />reanalysis indicates that the peak discharge of the
<br />1884 flood was 21O,000:t30,000 cubic feet per
<br />second (ft3/s), and the peak discharge of the 1921
<br />flood was 170,000:t20,000 ft3 Is, These values are
<br />indistinguishable from the peak discharges of these
<br />floods originally estimated or published by the U.S.
<br />Geological Survey, but are substantially less than the
<br />currently accepted peak discharges of these t100ds.
<br />The entire continuous record of instantaneous
<br />discharge of the Colorado River at Lees Felry can
<br />
<br />now be requested from the U.S. Geological Survey
<br />Grand Canyon Monitoring and Research Center,
<br />Flagstaff, Arizona, and is also available electronically
<br />at http://www.gcrnrc.gov.This record is perhaps the
<br />longest (almost 80 years) high-resolution (mostly
<br />15- to 30-minute precision) times series of river
<br />discharge available, Analyses of these data, therefore,
<br />provide an unparalleled characterization of both the
<br />natural variability in the discharge of a liver and the
<br />effects of dam operations on a river.
<br />Following the construction and quality-control
<br />checks of the continuous record of instantaneous
<br />discharge, analyses of flow duration, sub-daily flow
<br />variability, and flood frequency were conducted on
<br />the pre- and post -dam parts of the record. These
<br />analyses indicate that although the discharge of the
<br />Colorado River varied substantially prior to the
<br />closure of Glen Canyon Dam in 1963, operation of
<br />the dam has caused changes in dischar'ge that are
<br />more extreme than the pre-dam natural variability.
<br />Operation of the dam has eliminated flood flows and
<br />base flows, and thereby has effectively "flattened" the
<br />annual hydrograph, Prior to closure of the dam, the
<br />discharge of the Colorado River at Lees Ferry was
<br />lower than 7,980 ft3/s half of the time, Discharges
<br />lower than about 9,000 ft3 Is were impm1ant for the
<br />seasonal accumulation and storage of sand in the pre-
<br />dam river downstream from Lees Ferry, The cun'ent
<br />operating plan for Glen Canyon Dam no longer
<br />allows sustained discharges lower than 8,000 ft3/s to
<br />be released, Thus, closure of the dam has not only cut
<br />off the upstream supply of sediment, but operation of
<br />
<br />D2?51
<br />
<br />Abstract 1
<br />
|