The Current Magazine Fall 2016 - Page 20

By DR. ROB LUSARDI

CalTrout/UC Davis Wild &

Coldwater Fish Research Lead

WHAT THE SCIENCE SAYS

The construction of large dams and subsequent water supply and distribution implications in the western United States has been heralded as one of the greatest engineering accomplishments of the 20th century. The era of large dam building, however, has ceased. Instead, we are likely on the precipice of a new and equally impressive era: the era of dam removal. This may be the case particularly where dams (both large and small) are no longer economically viable, yet continue to have considerable ecological consequences. Dam removal, however, is in its infancy, and, as such, so is the science. In ‘1000 dams down and counting’, O’connor et al. (2015) examine the general trends in the numbers of dams being removed throughout the United States (Figure 1) and track the science behind those removals. In particular, the authors examine recent literature regarding the geomorphic and biological response to dam removal and the potential for unintended consequences.

The key controls on a river’s geomorphic response to dam removal are mostly related to (1) the volume of sediment trapped behind a dam (sediment supply), (2) the ability for flow to move that sediment (i.e., transport capacity), (3) sediment grain size (coarse vs. fine sediment), and (4) the rate at which a dam is removed (Grant and Lewis 2015). Sediment captured behind a dam can be particularly problematic during removal. For instance, approximately 28 million cubic yards of sediment was trapped behind Glines Canyon Dam on the Elwha River prior to removal. A tiered removal approach over several months allowed the river channel to continually adjust, carefully balancing the river’s sediment load with current hydrological conditions. In turn, this elicited a series of downstream channel adjustments. Similarly, dam removal will also facilitate upstream (within river) channel development as the change in elevation above and below the prior dam crest encourages downcutting and further downstream sediment transport. In their review, O’connor et al. (2015) found that rivers are extremely resilient to dam removal, often responding quickly to changes in sediment supply.

In this column we

highlight important

scientific publications,

by CalTrout staff and

others, that expand upon

our understanding of the management or science regarding trout, steelhead and salmon in California.