Removing a Dam and Re-Routing a River: Will Expected Benefits for Steelhead Be Realized in Carmel River, California.

摘要

The question of where to upgrade and where to decommission aging dams is currently a matter of national debate. Ecological benefits of dam removal are diverse, but a key expected benefit is improved wild fish populations and fisheries, particularly for migratory fish such as anadromous salmonids. However, in general one cannot expect with certainty how strongly or how soon such benefits will materialize after dam removal, due to inadequate data on ecological impacts, unpredictable ecosystem dynamics, or poor understanding of the processes themselves. Each dam removal is thus an experiment, and each expected benefit is an hypothesis to be tested and learned from. The scientific literature suggests that river restoration in the USA has been impeded because individual projects were not viewed as learning opportunities to help inform and refine future projects elsewhere, but in the past decade this situation has started to improve. Here we outline how to transform a large dam removal project in California into an opportunity to learn about ecological benefits for a threatened population of steelhead trout (anadromous Oncorhynchus mykiss) inhabiting one of the distinctive “episodic” type river systems of the state. The Carmel River Re-route and Dam Removal project (CRRDR), now underway near Monterey California, is the largest dam removal project ever in California, and one of the largest in the USA. The principal goals of the project emphasize ecological benefits for steelhead, by improving ecological connectivity to habitat upstream of the dam, habitat-forming processes downstream of the dam, and restoration of habitats within the former dam-site itself. Here we describe a research framework to discern which of these expected benefits are realized, on what timescale, and with what magnitude and effects on steelhead population viability. A long-established scientific concept is that viable salmonid populations need rivers with abundant habitat and natural (minimally altered) flow dynamics. However, CRRDR is expected to have modest effects on the amount of accessible habitat and on flow dynamics: The amount of accessible habitat upstream of the dam will not increase much because an existing fish ladder already provides passage for migrating adults; and flow dynamics will not change much due to the modest storage capacity of the reservoir. A broader conceptual framework for steelhead viability emphasizes not just habitat abundance and flow dynamics, but also ecological connectivity and unimpaired habitat-forming processes, and it is improvement of these two latter characteristics that is the focus of the CRRDR. The research program that we outline has applicability to dam removals whose goals are framed within this broader concept of viable salmonid populations.