Graduate Thesis Or Dissertation

Heat fluxes in the hyporheic zone of a gravel bar on the Willamette River, Oregon

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  • Increased water temperatures and thermal loading due to anthropogenic inputs has been shown to negatively impact the lifecycles of aquatic organisms in riverine systems (Poole and Berman 2001; Hannah, Malcolm et al. 2004; Quinn, Gagne et al. 2004). The studies enclosed in this thesis evaluate and quantify the heat fluxes within the hyporheic zone of a young unvegetated gravel bar on the Willamette River. The first study evaluated the application of the fiber optic distributed temperature sensing (DTS) system for thermal characterization of the shallow subsurface. The second study utilized data collected via the fiber optic DTS system, combined with data collected by thermistor probes, to calculate an energy balance for the hyporheic zone of the gravel bar. Lessons learned from the first study were that the fiber optic DTS system proved to be a flexible, high data-yielding, temperature monitoring system, but was not without limitations. The system flexibility allowed for the simultaneous temperature measurement of a 180 m² area (river and alcove combined sampling area) and multiple 2 m vertical temperature profiles with vertical resolutions of 0.022 m. Measurements made at the finest resolution of the DTS unit resulted in an increased amount of measurement variability when compared to a more traditional temperature measurement technique. An additional limitation was that the measurement sensor, the fiber optic cable, was inherently very fragile. Our recommendation for future studies is that special considerations must be taken to minimize damage to the fiber cable and signal loss. Nonetheless, the fiber optic DTS system provided an unprecedented view into the shallow subsurface. In the second study, we observed the amplitude of the thermal signal from the river decrease along a hyporheic flow path, while the net hyporheic temperature increased. Contrary to what other studies had found, no hyporheic cooling was observed. Using a simple layer control volume approach, we determined that ground-surface heating was a primary source of heat, via vadose zone storage and transport, for the hyporheic zone. The overlying conclusion of this thesis is that young, unvegetated, gravel bars on the Willamette River similar to the one examined in these studies, may increase the temperature within the hyporheic zone. Therefore, the geometry and the development of vegetation on gravel bars should be considered with regard to river engineering and planning.
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