Wetted channel and bar features for the Nehalem River, Oregon in 2005

The Tillamook Bay subbasins and Nehalem River basins encompass 1,369 and 2,207 respective square kilometers of northwestern Oregon and drain to the Pacific Ocean. The Tillamook, Trask, Wilson, Kilchis, and Miami Rivers flow into Tillamook Bay near the towns of Tillamook and Garibaldi. The Wilson and Trask River basins cover the largest areas (500 and 451 square kilometers, respectively) whereas the Tillamook and Kilchis Rivers encompass similar sized areas (156 and 169 square kilometers, respectively) and the Miami River the smallest area (94 square kilometers). In cooperation with the U.S. Army Corps of Engineers, the U.S. Geological Survey completed a reconnaissance-level assessment of channel condition and bed-material transport relevant to the permitting of in-stream gravel extraction along the the major alluvial portions of six river systems, including the lowermost 14.1 km of the Tillamook River, 16.3 km of the Trask River, 15.2 km of the Wilson River, 7.8 km of the Kilchis River, 11.6 km of the Miami River, and 31.4 km of the Nehalem River. To support these analyses, digital channel maps were produced to depict channel and floodplain conditions in the Tillamook Bay sub-basins and Nehalem River basin from different time periods. GIS layers defining the wetted channel and bar features and channel centerline in the study area were developed for four time periods: 1939, 1967, 2005, and 2009. For this project, the active channel was defined as area typically inundated during annual high flows, and includes the low-flow channel as well as side channels, islands, and channel-flanking gravel bars. The wetted channel and bar feature datasets were developed by digitizing from aerial photographs. Aerial photographs from 1939 and 1967 were scanned, rectified, and mosaicked for this project (See metadata for each photograph set for more information on the rectification process and resolution of each dataset). Digital orthophotographs from 2005 and 2009 are publicly available.

蒂拉穆克湾（Tillamook Bay）子流域与尼黑勒姆河（Nehalem River）盆地坐落于俄勒冈州西北部，流域面积分别达1369与2207平方公里，最终均注入太平洋。
蒂拉穆克河、特拉斯克河、威尔逊河、基尔奇斯河与迈阿密河均在蒂拉穆克镇与加里波第镇附近汇入蒂拉穆克湾。其中威尔逊河与特拉斯克河流域面积最大，分别为500与451平方公里；蒂拉穆克河与基尔奇斯河流域面积相近，分别为156与169平方公里；迈阿密河流域面积最小，仅94平方公里。
美国地质调查局（U.S. Geological Survey）与美国陆军工程兵团（U.S. Army Corps of Engineers）合作，针对六个河流系统的主要冲积河段的河道状况与推移质输运情况完成了一项踏勘级评估，相关评估服务于上述河段河道内砾石开采的许可审批工作。本次评估涉及的六个河流系统河段包括：蒂拉穆克河下游14.1公里段、特拉斯克河16.3公里段、威尔逊河15.2公里段、基尔奇斯河7.8公里段、迈阿密河11.6公里段，以及尼黑勒姆河31.4公里段。
为支撑上述分析工作，研究人员制作了数字河道地图，用于呈现蒂拉穆克湾子流域与尼黑勒姆河流域不同时期的河道与漫滩（floodplain）状况。研究针对1939年、1967年、2005年及2009年四个时期，构建了定义研究区内湿润河道、沙洲特征与河道中心线的地理信息系统（Geographic Information System，GIS）图层。
本项目将活动河道定义为年度高水位期间通常被淹没的区域，包含低流量河道、旁道、江心洲以及河道两侧的砾石沙洲。湿润河道与沙洲特征数据集通过对航空摄影照片进行数字化处理得到。本次项目使用的1939年与1967年航空摄影照片均经过扫描、校正与拼接处理（有关校正流程与各数据集分辨率的详细信息，请参见各照片集的元数据（metadata））。2005年与2009年的数字正射影像（digital orthophotographs）为公开可用资源。