A COMPREHENSIVE HISTORY OF BED DEGRADATION AND CHANNEL ADJUSTMENT FOR THE COLORADO RIVER WITHIN GLEN CANYON NATIONAL RECREATION AREA DOWNSTREAM FROM GLEN CANYON DAM

The 25-km reach of the Colorado River downstream from Glen Canyon Dam in Glen Canyon National Recreation Area is a classic example of a channel whose bed has degraded and armored in response to flow and sediment regulation caused by a dam. Although the evacuation of bed sediment from this reach was originally documented nearly 30 yrs ago, the complete array of channel adjustments has never been described. This study integrates abundant historical data and recent measurements in a comprehensive analysis of channel change.

Bed degradation began when the cofferdam was installed in 1959, but the greatest proportion of degradation occurred in 1965 when the U. S. Bureau of Reclamation intentionally released high flows from the dam in a series of pulses intended to scour the reach downstream from the dam. This event evacuated a reach-average of 2.6 m of sediment from the center of the channel. Continued erosion occurred in the downstream half of the study area during emergency releases of the mid-1980s. Small adjustments in bed elevation occurred in the 1990s. The total volume of bed sediment evacuated from the study reach is equivalent to one-third the pre-dam annual sediment load and is two orders of magnitude greater than estimated post-dam sediment inputs to the reach. The average size of bed material increased from 0.2 mm in 1956 to over 20 mm in 1999.

The magnitude of degradation in riffles decreases systematically with distance downstream from the dam, while the magnitude of degradation in pools is variable and extends farther downstream. The alluvial deposits in Glen Canyon include a complex suite of pre- and post-dam fine- and coarse-grained deposits that occur at multiple elevations above the active channel. Many deposits are pre-dam relicts, no longer inundated in the post-dam flow regime. These include fine-sediment deposits and gravel and cobble bars. Some of the pre-dam deposits have eroded episodically in the post-dam period. Narrow fine-grained post-dam flood deposits occur discontinously throughout the reach. Both pre- and post-dam deposits are colonized and stabilized by invasive riparian vegetation, resulting in a 6% net decrease in bankfull channel width.