Connectivity & Socio-Hydrogeomorphic Vulnerability of the Oregon Coast

Abstract:

Human and natural systems are highly connected along the Oregon coast. Small, mountainous rivers drain the OR Coast Range, transporting sediment to estuaries, a fraction of which accumulates in salt marshes providing flood protection. Variability in streamflow regime thus drives marsh morphology but in poorly understood ways. From a social standpoint, human land-use practices and policy decision-making, primarily logging practices, also has substantial impacts on fluvial suspended sediment and estuarine morphology, but the relationships must be elucidated. As a transdisciplinary team of scientists, we assess the complex relationships between human land use (especially logging), hydrology (stream flow regime and discharge record), and salt marsh sediment accumulation over the last century for two OR watersheds (Nehalem and Alsea), chosen to provide the greatest parameter breath. We also evaluate physical and social vulnerability of the two estuarine communities to loss of salt marsh and flood protection. Preliminary results of aerial photograph analysis indicate that the Alsea salt marshes have been drowning while Nehalem salt marshes have expanded, fastest in the first half of the 20 th century. Alsea River is flasher than the Nehalem, indicating that short-lived storms may result in less down-stream sediment accumulation. Further, intensive logging and splash damming of the late 19th to early 20th century within Nehalem likely resulted in excess sediment transport to and accumulation in salt marshes. Policies aimed at forest conservation, such as the 1970s Oregon’s

Forest Practices Act and the 1993 Northwest Forest Plan, reduced sediment supply and may have resulted in the apparent slowing of salt marsh accumulation within the last ~30 years in both marshes. Interestingly, changes in canopy cover from 1990 to 2012 show Nehalem experienced a decrease in cover, while Alsea cover grew possibly related to land ownership. The vulnerability analysis indicates that Alsea experiences the greatest physical vulnerability to loss of salt marsh, though Nehalem displays that greatest economic and demographic vulnerability. Ultimately, our research displays the complex connectivity of coastal socio-hydrogeomorphic systems, underscoring the importance of full system approaches to vulnerability assessments.