Have large scale river plumes and coastal circulation patterns been fundamentally altered by historic estuary outlet modifications?

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Buoyant river plumes, the result of the light fresh water discharged by rivers onto the continental shelf, are ubiquitous features in the world's coastal oceans. River plumes are vital components of the coastal circulation and biogeochemical cycles, serving as essential conduits to the coastal ocean for terrigenous carbon, nutrients, and other terrestrial constituents. Numerous theoretical, laboratory, and numerical studies point out the importance of the depth and width of the estuary mouth in setting bulk plume properties such as the cross-shore plume extent and coastal current transport. Additionally, the initial plume structure and subsequent response to external forcing (winds, waves, ambient shelf circulation) is largely set by the outflow. Since the late 1880's, the Lower Columbia River Estuary (LCRE) has experienced continual engineering modifications and the construction of jetties and the dredging of navigation channels at the mouth of the Columbia River that have altered both outlet depth and width by nearly 50%. The historical and modern LCRE/Columbia River Plume (CRP) systems provide a unique and novel testbed for examining the fundamental physical processes governing the coupling between the estuary, plume and coastal ocean while providing valuable insight and improve the collective understanding of the human impacts on the LCRE/CRP system. The project addresses how human-imposed changes to the river flow, geometry, morphology, and channel orientation of the LCRE over the last 130 years have impacted, or altered, the dominant physical processes that control the mixing, structure, and transports over the entire coupled river-estuary-coastal ocean system. Because plumes are a primary link in the land/ocean exchange of sediments, nutrients, and other scalars, a better understanding of historical changes to plume dynamics will broaden our understanding of long-term change to coastal ecosystems. The position and structure of the CRP additionally has important implications for reproductive strategies for the Columbia River salmon populations. The results of the project will be disseminated to local and regional stakeholders and managers at the Columbia River Estuary Conference where PIs will work to develop beneficial interdisciplinary collaborations. The proposal supports two early-career scientists and graduate student.The main hypothesis of the project is that basic plume characteristics - hydraulic control at the outlet, mid-field plume size, stratification, plume entrainment rates, and interaction with upwelling winds - have been fundamentally altered. The existing historical and modern numerical models of the Lower Columbia River Estuary, which have been developed and calibrated using 19th and 21st century bathymetry and tide data, will be used. Sensitivity studies will alter forcing parameters such as river flow, tides, and wind thru their historical range, and results will be analyzed using non-dimensional numbers and thru conservation of mass/momentum/energy approaches. Because nearly all major river outlets in the US (and the world) have been modified, the proposed case-study will form a template for understanding changes in other locations; specifically, since the Columbia River system straddles a large parameter space in terms of tidal amplitudes and river flow, the proposed research will cover the boundary conditions (stratified and partially stratified conditions; currents; geometry) found at other major river outlets. Moreover, the historical changes have been so large that they will allow clear contrasts (historic vs. modern) to emerge. Arguably, the Columbia River Plume is the best location in the U.S. for such a study. The effects of anthropogenic changes are also important in the context of climate change: historical changes to river flow and depth provide a template for understanding future changes due to sea-level rise and altered hydrological patterns. The proposed research will be a critical step in advancing the knowledge of the human-induced impacts on the Oregon/Washington coast while simultaneously addressing fundamental questions on the role of outflow controls on plume structure and transports as it spreads and evolves on the continental shelf.