The 1996 controlled flood in Grand Canyon: Flow, sediment transport, and geomorphic change

The 1996 controlled flood released from Glen Canyon Dam into the Colorado R iver 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, a nd only 10% of the pre dam spring snowmelt floods during the period 1922-19 62 were of lower magnitude. The flood occurred unusually early: 36-38 d pri or to any previous annual flood since 1922. The stage difference between th e flood's peak and the recessional baseflow was smaller than in those pre-d am years of similar magnitude or annual volume. However, the controlled flood was large from the perspective of the post-da m flood regime. The hood 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 hoods, and the suspended sand concentration was within the historical variance for flows o f similar magnitude. This flood reworked fine-grained deposits that are primarily composed of sa nd, but the flood caused much less reworking of coarser grained deposits. S cour primarily occurred in the offshore parts of eddies, in many eddy retur n-current channels, and in some parts of the main channel. Return-current c hannels constitute important nursery habitats for, the native fishery when baseflows are low, because these channels become areas of stagnant and warm er water. The number and area of these backwaters increased greatly after t he flood. Fluvial marshes were extensively scoured because these habitats o ccur in the low elevation centers of eddies where velocities during the flo od were large. Riparian shrubs that were inundated along the banks were not scoured, however, because these shrubs occur where flood velocities were v ery 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 thi s flood varied among different small-scale fluvial environments.