<br />,
<br />
<br />Influence of Glen Canyon Dam Operations on Downstream Sand Resources
<br />
<br />19
<br />
<br />Effects of Dam Operations on
<br />Flow Frequency and Duration
<br />
<br />Changes in the flow regime of the Colorado River
<br />since construction of Glen Canyon Dam have also been
<br />dramatic in terms of seasonal variability, as well as in
<br />terms of daily fluctuations that occur because of "peak-
<br />ing" hydroelectric power generation. Dam operations
<br />have altered seasonal variability by eliminating long-
<br />duration flood flows that occurred during the spring
<br />snowmelt and short-duration flood flows that occurred
<br />during the late summer and early fall thunderstorm
<br />season, as well as the very low flows that occurred dur-
<br />ing summel; fall, and winter. With regard to the highest
<br />flows, dam operations have reduced the 2-yr recurrence
<br />interval flood (i.e., the flood that occurs every other
<br />veal' on average) from 85,000 cubic feet per second
<br />(cfs) during the predam period to 31,500 cfs during the
<br />postdam period. In the predam era, discharge exceeded
<br />9,000 cfs only 44.3% of the time, while in the postdam
<br />era this percentage has gradually increased by decade,
<br />from 52.7% in the 1960s to 82.6% in the 1990s. This
<br />decrease in the duration of low flows has important
<br />implications for sediment transport because Topping and
<br />others (2000b) showed that flows less than about 9,000
<br />cfs result in accumulation of tributary sand inputs in
<br />the ~vIarble Canyon and Grand Canyon reaches of the
<br />river, whereas flows above this generally lead to transport
<br />of new sand inputs through these reaches or erosion of
<br />sand from these reaches.
<br />Dam operations have introduced large daily varia-
<br />tions in discharge to generate hydroelectric power that
<br />tracks daily peaks in demand throughout the \IVestern
<br />United States. Also, because peak energy demand varies
<br />seasonallv in the \Vest with peak demand occurring in
<br />, ,
<br />midsummer and winter, the month-to-month flow pattern
<br />related to dam operation is substantially different from
<br />natural, predam, seasonal patterns. Highest discharges in
<br />the river now occur during the two seasons when predam
<br />discharge had typically been the lowest, midsummer and
<br />winter. Furthermore, daily patterns of flow in the river
<br />have been altered by dam operations. For example, dur-
<br />ing the pre dam period the median daily range in dis-
<br />charge was only 524 cfs, whereas in the postdam era the
<br />median daily range increased to 8,580 cfs, a value greater
<br />than the predam median discharge. Before dam opera-
<br />tion, the daily range in discharge exceeded 10,000 cfs
<br />only about I % of all days; postdam, the daily discharge
<br />range exceeded 10,000 cfs on 43% of all days.
<br />
<br />Initially, operation of the dam's powerplant was
<br />characterized mostly by unconstrained daily fluctua-
<br />tions that were designed to optimize electrical generation
<br />around peak daily demand, which had patterns that also
<br />varied on a monthly timescale related to seasonal changes
<br />in energy demand. From 1963 through 1991, these oper-
<br />ations typically caused the Colorado River's discharge to
<br />fluctuate on a daily basis from less than 5,000 cfs to near
<br />powerplant capacity of about 31,000 cfs. These so-c~lled
<br />"no action" daily operations (because they were consId-
<br />ered the no action alternative in the EIS) were first altered
<br />in 1990 to facilitate experimental release patterns imple-
<br />mented through July 1991 as part of field investigations.
<br />associated with the EIS on dam operations. The expen-
<br />mental flows of 1990-91 were then followed by "interim
<br />operating criteria" from August 1991 until October
<br />1996 when SecretalY of the Interior Bruce Babbitt
<br />, .
<br />implemented current Record of Decis.ion d~m ?p.eratIons.
<br />Implementation of the interim operatll1g cntena ll1 I ~91,
<br />as well as the tvILFF in 1996, constrained the change ll1
<br />discharge over any 24-h period to 5,000; 6,000; or 8,000
<br />cfs, depending on the monthly volume-release schedule
<br />specified in the annual operating plan for the Colorado
<br />River Storage Project. The flow histOlY of the Colorado
<br />River into Grand Canyon as measured at the Lees Ferry
<br />gaging station is shown in figure 1. These flo:v data
<br />illustrate a transformation of the Colorado RIver from a
<br />fluvial ecosystem with significant seasonal variability in
<br />the predam era to a postdam river ecosystem with little
<br />seasonal variability and substantial daily fluctuations.
<br />Another important aspect of the MLFF operation
<br />is the schedule of monthly release volumes in relation to
<br />the seasonality of sediment inputs. Because of energy
<br />demand and hydropower economics, monthly release
<br />volumes are highest during months with high demand,
<br />including those in late summer. Historically, however, the
<br />late summer months were characterized by low mains tern
<br />flows and the highest tributary inputs, leading to sediment
<br />accumulation during the predam era. Postdam, high
<br />summer releases coincide with tributary inputs, leading
<br />to rapid export instead of accumulation. Therefore, not
<br />only has the sand supply been drastically reduced th:o~gh
<br />the impoundment of Lake Powell, but the seasonal tlmll1g
<br />of low and high flows has also been both highly com-
<br />pressed and significantly shifted to late.r periods of .the
<br />year that coincide with tributary sand ll1put.s. The ll1for-
<br />mation in this section was taken from Toppll1g and others
<br />(2003); readers with further interest in the Colorado
<br />River's hydrology, both before and after the dam was
<br />closed, should consult this report.
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