FLUVIAL GEOMORPHOLOGY OF THE ENTIAT RIVER, WA, AND IMPLICATIONS FOR STREAM RESTORATION

In the Entiat subbasin of the Upper Columbia River Basin, human-induced changes to channel processes are believed to have historically reduced the quality and availability of aquatic habitat (CCCD 2004). The current study was undertaken to provide a long-term context and understanding of geomorphic processes along 26 miles of the Entiat River and to determine the extent that channel processes have been altered during the historical period. Results were utilized to identify areas impacted by human activities and opportunities for protection or restoration of channel and floodplain processes. While some previous work states that human modifications are largely responsible for habitat conditions in the Entiat River, this study demonstrates that channel morphology on a broad scale is influenced by geologic features rather than historically constructed features in the channel. This conclusion is illustrated by the change in stream gradient at geologic features, the presence of alluvial surfaces that are 1,000 years or older adjacent to the stream channel and few detectable impacts to channel pattern from anthropogenic activities. In the upper 10 miles of the study reach, channel morphology can be directly related to the position of alluvial fans along the valley margins and the downvalley glacial limit during the Last Glacial Maximum (LGM). Alluvial fans from side tributaries create steep channel gradients and confine the river to a single-thread channel morphology with low sinuosity. The channel constrictions are preceded by low gradient meandering reaches, which show extensive lateral channel migration in response to a wide floodplain and the local base level control of the alluvial fans. Lower gradients near the glacial limit (about river mile 16) are likely caused by deep glacial scour during the Late Pleistocene and subsequent infilling with glacial and post- glacial sediment. In the lower 16 miles, channel gradient is steep and uniform, controlled by the base level of the Columbia River and in-channel bedrock as well as laterally by gravelly stream banks of glacial outwash terraces and high floodplain and terrace deposits that were determined to be 1,000-2,000 years old and 3,000-4,000 years old, respectively. While most of the channel characteristics are controlled by geologic features and formed by naturally occurring geomorphic processes, localized reaches within the study area do show channel changes that are associated with anthropogenic modifications such as channel straightening, levees, bank protection, and large woody debris and vegetation clearing. The channel changes are variable and include decreased sinuosity, increased stream gradient, channel incision, channel widening and bank undercutting. Reaches with these documented channel changes present the most opportunities to improve the quantity and quality of physical channel characteristics important for salmonids.