GEOMORPHOLOGY OF THE COLORADO RIVER IN GRAND CANYON —WHAT HAVE WE LEARNED AND WHAT DO WE NEED TO KNOW?

Geoscientists has long focused on the Colorado River in Grand Canyon and on the Canyon’s age, how it formed, and on the long Earth history that is exposed at the river’s edge. During the past 40 years, river scientists have explicitly focused on the effects of the existence and operations of Glen Canyon Dam on the downstream river, and these studies have primarily been funded by the National Park Service in the 1970s, the Bureau of Reclamation in the GCES program of the mid-1980s and early 1990s, and today’s Glen Canyon Dam Adaptive Management Program that began in the mid-1990s. During this long period of focused study, hypotheses have been developed, rejected, accepted, and revised on many topics, including:

• the geomorphic organization of the river corridor and the relative role of bedrock and debris flows as controls on river form,
• the potential of debris flows to be reworked by the post-dam stream-flow regime,
• the large-scale pattern of fine sediment erosion in relation to observations on other regulated rivers,
• the magnitude of fine sediment supply that enters the Colorado River downstream from Glen Canyon Dam,
• the roles of sediment supply and hydraulics in determining suspended sediment concentration,
• the duration of time fine sediment supplied from the Paria River is stored on the Colorado River’s bed,
• the post-dam fine sediment mass balance,
• the average response of eddy sand bars to floods and low flows,
• the magnitude of adjustment of eddy sandbar form to annual and decadal flow regimes,
• the relative role of clear water floods and hydropeaking as the dominant cause of sandbar erosion, and
• the long-term condition of Grand Canyon sand bars if only controlled floods are used and sediment augmentation is never implemented.

The body of extant applied geomorphic science has supported development and implementation of the High Flow Experiment Protocol and the reduced magnitude and rate of hydropeaking, Perhaps the fundamental question asked of the scientific community today is the same one asked 40 years ago – how long will Grand Canyon’s sandbars remain a distinctive part of the river landscape? Is the present mitigation strategy of releases of occasional clear water floods sufficient to maintain sandbars of acceptable size?