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<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. <br />