Interpretation of Sidescan-sonar Imagery in the John Day Reservoir

A two-week field operation was conducted in the John Day Reservoir on the Columbia River to image the floor of the pool, to measure the distribution and thickness of post-impoundment sediment, and to verify these geophysical data with video photography and bottom sediment samples. The field program was a cooperative effort between the USGS Coastal and Marine Geology Team of the Geologic Division and the USGS Columbia River Research Laboratory of the Biological Resources Division. The data collection was completed aboard the R/V ESTERO during September 13-27, 2000. The interest in sediment accumulation in the reservoir was two-fold. First, it was unknown how effective this reservoir was as a sediment trap to material that otherwise would have been transported down-river to the estuary and eventually to the ocean. The recent erosion of beaches along the Washington coast has been attributed to a decreased contribution of sediment from the Columbia River to the coastal system due to the damming of the river. Second, sediment accumulation on the floors of reservoirs along the Columbia River has been suggested to be diminishing salmon spawning grounds. The extent of changes in habitat since construction of the John Day Dam, however, had not been documented. Common data sets were needed to address both of these questions, and for these reasons this geophysical and sampling program was undertaken.本次野外作业为期两周，在哥伦比亚河约翰·代水库开展，旨在对水库池底进行成像，测定蓄水后沉积物的分布与厚度，并通过摄影摄像与底泥采样对所获取的地球物理数据进行验证。本次野外项目由美国地质调查局（USGS）地质分部海岸与海洋地质工作组，与美国地质调查局（USGS）生物资源分部哥伦比亚河研究实验室合作完成。数据采集工作于2000年9月13日至27日在ESTERO号科考船（R/V ESTERO）上完成。 针对该水库沉积物堆积情况的研究兼具双重意义：其一，该水库作为沉积物捕集器的实际效能尚不明确，若未修筑大坝，这些沉积物本应被顺流输运至下游河口并最终汇入海洋。此前华盛顿沿岸海滩的侵蚀现象，被归因于哥伦比亚河筑坝导致其向海岸系统输入的沉积物量减少。其二，有研究指出，哥伦比亚河沿岸水库池底的沉积物堆积正在压缩鲑鱼的产卵栖息地。然而，自约翰·代大坝建成以来，该栖息地的变化程度尚未被记录在案。为解答上述两个核心问题，亟需统一的数据集支撑，因此本项地球物理与采样研究计划得以实施。