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as several INFO files of related tabular information. TIN models of the combined point
<br />coverages for each survey are formed and used to generate arc coverages of 0.5-m topographic
<br />contours and profiles at predetermined locations. Products from this study, in addition to the
<br />data base, include a map atlas of site information and reports that reflect detailed descriptions
<br />of the methodologies and analyses presented here, listings of the AML's, and FGDC (Federal
<br />Geographic Data Committee) compatible metadata developed for this project.
<br />Flow Regimes
<br />Interim flows have been in effect since August 1991 and will continue until a Record of Decision
<br />is reached for the GCD Environmental Impact Statement and a new flow regime is indicated.
<br />In the past, GCD has been operated to yield maximum hydropower revenues through the
<br />production of peaking power. Flows released from GCD have ranged between 85 and 850 m3/s
<br />(3,000 and 32,000 ft3/s) daily, depending on power demand. Interim flows limit maximum
<br />discharge to 566 m% (20,000 ft3/s) and the minimum to 142 m3/s (5,000 ft3/s), with upramp and
<br />downramp rates of 57 m%/hr (2,500 ft3/s/hr) and 42.5 m3/s/hr (1,500 ft3/s/hr), respectively. Daily
<br />change cannot exceed 142 m3/s (5,000 ft3/s). These interim flows consist of low-, medium-, and
<br />high-volume months, with low flows during the late spring and late fall and high flows during
<br />mid-summer and mid-winter.
<br />Natural flood events within the lower LCR (Little Colorado River) during January and February
<br />1993 caused a significant deviation from the lower-volume interim flow regimes along the
<br />mainstem Colorado River (fig. 9). Three flood events occurred on the LCR on January 12to 16,
<br />January 19to 23, and February 23to 26, 1993, that delivered sediment and raised flows in the
<br />mainstem Colorado to 955 m3/s (33,777 ft3/s), 783 m3/s (27,980 ft3/s), and 826 m3/s (29,450 ft3/s),
<br />respectively. During these floods, sand was deposited in nearly every eddy downstream from
<br />the LCR-Colorado confluence for at least 30 mi, to river mile 90.
<br />Results
<br />River Mile 62.4 Sandbar.-A large sandbar located within GCES/GIS site No. 5, at RM62.4,
<br />deposited during the first and largest flood event (January 12 to 16, 1993), provides an
<br />opportunity to examine sandbar development dynamics (Kaplinski et al. , 1994b). Deposition
<br />at this site was consistent with published descriptions of depositional patterns at other Grand
<br />Canyon sandbars and in flume experiments (Rubin et al, 1990; Schmidt et al., 1993). Prior to
<br />the January LCR flood, no significant deposit existed at this site. The sandbar was deposited
<br />entirely during the flood event, whereas most previously described sandbars are a composite of
<br />many different depositional packages from different years (Rubin et al., 1990; 1994). Within 6
<br />months of deposition, the entire exposed portion (above about 227 m9/s [8,000 ft3 /s] ) of the
<br />deposit had eroded to pre-flood levels. We conducted detailed topographic and bathymetric
<br />surveys of the eddy complex in April 1993, October 1993, and April 1994 and will present an
<br />analysis of depositional and erosional patterns at the site during and after the flood events.
<br />Comparison of the surveys yields information regarding deposition rates in the eddy during the
<br />flood. We used the volumetric difference of 64,644 m3 between the two surveys to estimate a
<br />minimum rate of sediment accumulation in the recirculation zone during the January 1993
<br />flood. Based on photographic and stratigraphic evidence, most of the deposition at this site
<br />occurred during the first 12 hours of the flood, perhaps even within the first several hours.
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