The effects of interim flow operations from Glen Canyon Dam on Colorado River sand bars in the Grand Canyon, Arizona

Discharges from Glen Canyon Dam (GCD) affect the geomorphology and stability of downstream alluvial sediment deposits. To protect downstream resources, the US DOI mandated interim flow criteria (IFC) on 1 August, 1991. The IFC consist of reduced daily fluctuations (226--566 m3/s) and reduced ramping rates (42.5--57 m3/s/hr), the primary objective of which is to maintain sediment storage in the river system by minimizing sediment transport. This study was initiated to determine the effectiveness of the IFC in achieving this objective. The authors examined whether reduced daily fluctuations lead to subaerial sand bar erosion and increased subaqueous sediment storage. They collected and analyzed topographic and bathymetric survey data from sand bars throughout the Colorado River corridor in Sept/Oct, 1991 and in Oct/Nov, 1992 to compare changes in sand bar morphology. They examined changing topography due to GCD operation in what they termed the hydrologically active zone (HAZ), that portion of the sand bar exposed to daily dam operations (142--900 m3/s stage elevations). Volumes within the HAZ and profiles across this zone were generated from these sediment deposits. Their preliminary results show that, in general, erosion of sediment at higher bar elevations was coincident with deposition along lower parts of the bar platform. The observed response to IFC elevation in order to maintain sediment deposits for Colorado River corridor bio-diversity (e.g., fisheries habitats). 88% of sand bars that showed significant volume gain were preceded by significant volume loss, implying that antecedent conditions are an important factor in sand bar response to GCD operations. Sediment transport capacity was reduced as evidenced by increased sediment storage in recirculation zones and sediment infilling of eddy return channels. The authors conclude that IFC are achieving their primary objective of maintaining sediment storage within the river corridor