The 1996 controlled flood in grand canyon: Flow, sediment transport, and geomorphic change

Abstract

The 1996 controlled flood released from Glen Canyon Dam into the Colorado River was a small magnitude, short duration event compared to pre-dam floods. The controlled flood was of lesser magnitude than a 1.25-yr recurrence, and only 10% of the predam spring snowmelt floods during the period 1922-1962 were of lower magnitude. The flood occurred unusually early: 36-38 d prior to any previous annual flood since 1922. The stage difference between the flood's peak and the recessional baseflow was smaller than in those pre-dam years of similar magnitude or annual volume. However, the controlled flood was large from the perspective of the post-dam flood regime. The flood had a recurrence of 5.1 yr for the period between 1963 and 1999 and a similar magnitude flood had not occurred in 10 yr. The sediment flux of the flood was small in relation to pre-dam floods, and the suspended sand concentration was within the historical variance for flows of similar magnitude. This flood reworked fine-grained deposits that are primarily composed of sand, but the flood caused much less reworking of coarser grained deposits. Scour primarily occurred in the offshore parts of eddies, in many eddy return-current channels, and in some parts of the main channel. Return-current channels constitute important nursery habitats for the native fishery when baseflows are low, because these channels become areas of stagnant and warmer water. The number and area of these backwaters increased greatly after the flood. Fluvial marshes were extensively scoured because these habitats occur in the low elevation centers of eddies where velocities during the flood were large. Riparian shrubs that were inundated along the banks were not scoured, however, because these shrubs occur where flood velocities were very low and where deposition of suspended sediment occurred. Some physical changes persisted for several years, but other changes, such as the area of newly formed backwaters decreased quickly. Thus, the lasting effect of this flood varied among different small-scale fluvial environments.