Abstract: Opening Plenary Session  2:45 pm (BACK)

Challenges and Opportunities in Restoring Rivers of the Southwestern United States and Northern Mexico

John C. Schmidt
Department of Watershed Sciences, Utah State University
Grand Canyon Monitoring and Research Center
U. S. Geological Survey 

In contrast to the Pacific Northwest, the rivers of the American southwest and northern Mexico naturally have modest to small total annual stream flow but once had very large fine sediment loads. Only the Rocky Mountain headwaters are low transport, gravel bed streams similar to those abundant in the Northwest. Whereas the main-stem rivers of the Northwest have large reservoirs but exert minimal to modest flood control, fewer dams in the Rio Grande and Colorado River networks exert substantial flood control and facilitate large off-channel withdrawals.

The abundant natural supply of fine sediment in the Southwest is difficult for river managers to ignore. There is substantial emphasis placed on the sediment mass balance context of river rehabilitation. Immediately downstream from all dams, rivers have been perturbed into fine sediment mass balance deficit, and the length of deficit river segments can be 100s of kilometers in places. Elsewhere, mass balances have been perturbed into fine sediment surplus, and channels have substantially aggraded. In either case, the habitat of the native fish has been substantially perturbed, because the native fish of the region evolved life history strategies that took advantage of the natural variable flow regimes, large season sediment loads, temperature regimes, and distinctive geomorphology of these rivers.

The approaches and challenges of rehabilitating rivers perturbed into mass balance deficit and into mass balance surplus are very different. In the former case, management of sparse sediment supplies focuses on targeted flow regimes and short duration flood release to redistribute fine sediment temporarily stored on the channel bed to the channel margins. In the latter case, a mix of channel excavation and riparian vegetation control are typically implemented. However, the root cause of the perturbations, too little sediment in the case of deficit and too little water in the case of surplus are rarely considered because of their high financial and political cost. Today, climate change and continued growth in human use of water is challenging the existing engineering infrastructure and political agreements that allocate water among states and countries. Today’s nexus of impending water shortage, reconsideration of water distribution agreements, and limited financial resources presents an opportunity to reconsider traditional approaches to river rehabilitation and to target those segments where significant improvement in river conditions is possible.