MITIGATING REGULATED FLOWS THROUGH CHANNEL AND FLOODPLAIN RESTORATION DOWNSTREAM OF DAMS: A SUCCESS STORY FROM THE CLE ELUM RIVER, WA

摘要

The downstream impacts large dams have by altering natural flow regimes and cutting off sediment and wood supply are well known and cause severe adverse effects to in-stream and floodplain habitat. One of the most significant impacts is disconnection of a river from its floodplain and secondary perennial and ephemeral channels. Other impacts include the loss of processes such as natural bank erosion and wood recruitment that directly influence channel morphology and habitat. These impacts are rarely addressed with an in-stream flow assessment. As such the importance of physical structure, particularly on flow regimes, can be missed. Sending the same flow down a simplified channel versus a morphologically complex channel results in two very different ecological systems. Attempts to mitigate these impacts are challenging and typically involve attempting to naturalize flow regimes as much as possible, particularly with regards to sediment transport and channel altering flows; and instigating programs to augment gravel and wood supply. But for many systems it may not be possible to implement flows sufficient to restore the desired habitat. We present a case study downstream of the Cle Elum Dam in central Washington State where we implemented a habitat restoration project that demonstrates the potential for mitigating regulated flows. The Cle Elum Dam is a large earthen dam operated by the US Bureau of Reclamation (BOR) for irrigation needs in the Yakima Valley. Flows are regulated under a highly controlled management system called flip-flop in which periods of naturally low flow during the summer are artificially high and periods of naturally high flows are artificially low. The river has experienced a reduction in sinuosity and complexity downstream of the dam. To rehabilitate habitat we identified an opportunity to reconnect flow to an abandoned meander channel that could create over 1.5 km of perennial side channel habitat, improve hyporheic exchange and dramatically increase complex cover, pools and spawning areas available to salmonids. No modification of flows was possible, so to achieve these goals we had to focus on sustainable channel modifications. Using two strategically placed engineered logjams (ELJs) and minor grading we successfully achieved the project goals and we have been able to document how the ELJ structures have induced positive changes in the main channel (pools, bars, and cover) in addition to more than doubling the length of channel available to salmoinds. The design approach was well received by regulatory agencies who have been impressed with the results. Our approach is expected to provide decades of reliable, positive increasing benefits such as: fine sediment accumulation behind the ELJs in the mainstem; increased sinuosity of the mainstem; perennial flows in the side channel complex; initiation of channel forming processes and additional wood recruitment and habitat formation in the side channel. The project life expectancy is predicted to be more than a 100 years since trees are expected to mature on the ground over the ELJs. The project demonstrates the tremendous potential of recognizing the geomorphic opportunities of a site and the principal factors linked to habitat creation. It is applicable in most alluvial channels that have at least some portion of its floodplain intact and can be applied in any region. The Cle Elum project could be further enhanced by introducing a more process-based flow regime, so the addition of large roughness elements in a channel could be combined with naturalized flows.