Presenter/s: Gordon Grant
Symposium Session: 2023 - 01 Exploring Deep Geologic Time and Relevance to River Processes
Topics covered: climate change, floodplain, geology, and sediment transport
Geology may or may not be destiny, but it does provide a useful and deeply informative framework for stream restoration. Specifically, most stream restoration projects occur within alluvial or semi-alluvial settings with distinctive architectures and stratigraphic histories that provide insights into the range and timing of processes driving and shaping the geomorphic structure of channels. Interpreting those landforms and deposits can be challenging, but is essential for understanding how and why streams look and behave the way they do today. Drawing on examples from both the eastern and western U.S. I will explore how landform analysis and stratigraphic interpretation can be used to provide a useful context for stream restoration. In the mid-Atlantic Piedmont, detailed stratigraphy of valley bottoms revealed that prior to European settlement, valley floors were vegetated wetlands with few single-thread channels, very different than they appear today. In western Oregon, morphology of pre-settlement streams was strongly influenced by post-glacial aggradation of valley floors; many streams are now incised into glacial outwash and fans due to reduction in sediment loads during the Holocene. Given this history, efforts to “reconnect” channels with their surrounding surfaces may be challenging. Understanding this geological history, and appreciating the role of Pleistocene processes and landforms provides a sound foundation for anticipating stream response to restoration.