MODERN CHANNEL CHANGE IN THE COLORADO RIVER SYSTEM: A MANDATE FOR RESTORATION?

The modern Colorado River system can be distinguished into three categories: (1) physically transformed, (2) essentially wild and in sediment deficit, and (3) essentially wild and in sediment surplus. The transformed parts of the system primarily occur downstream from Hoover Dam. The wild parts of the system occur in Grand Canyon and in the upper basin. The beds have degraded in sediment deficit segments; channel width has decreased in alluvial segments and increased in debris fan-dominated reaches. Channels have typically narrowed in sediment surplus segments. The different styles and magnitudes of channel present a wide range of challenges in the rehabilitation of the channel network towards pre-dam conditions. There are more options for rehabilitation of sediment surplus segments than sediment deficit segments, and the costs of rehabilitation are probably less. Since it is impossible to rehabilitate, much less restore, the entire river network, tradeoffs and priorities must be established. River scientists must develop restoration response functions for the different segments of the system so that policy makers can identify where the least cost in changing the existing infrastructure will result in the greatest environmental improvement.