Citation:

Faulkner, Barton R., J. Renee Brooks, D. Keenan, AND Kenneth J. Forshay. Temperature Decrease along Hyporheic Pathlines in a Large River Riparian Zone. ECOHYDROLOGY. Wiley Interscience, Malden, MA, 13(1):e2160, (2020). https://doi.org/10.1002/eco.2160

Impact/Purpose:

The Clean Water Act calls for the restoration and maintenance of the ``chemical, physical, and biological integrity of the Nation's waters.'' This manifests itself in the Pacific Northwest, through the goal to protect fresh water habitat of salmon and trout. Key to meeting this is protecting and restoring the thermal regime that provides the cold water critical to the survival of these fish. Resource managers are challenged to implement restoration practices that enhance the development of cooler temperature environments which can be used by salmonids to cope with warmer temperatures in summer. This study used observations of temperature decrease along hyporheic pathlines in three islands of differing size, shape, and vegetation characteristics, along a portion of the Willamette River, Oregon. It is the only study the authors are aware of that uses monitoring wells with loggers along known pathlines in large hyporheic zones. Findings included that larger islands with mature vegetation may not be able to support temperature reduction due to forest interception and high infiltration, however, smaller islands may support temperature decrease of up to 7 degrees Celsius in 600 m flow distance. The smallest and least mature island and the medium sized, irregularly shaped island had thermal Péclet numbers of 2.7 and 6.5, respectively. Mixing with shallow groundwater and cooling during infiltration were inferred to be the primary mechanisms of temperature reduction, and this finding was supported by stable isotope sampling in monitoring wells. This information should prove useful to restoration planners, and should be a substantial contribution to understanding the effect of hyporheic zones on temperature reduction in large rivers.

Description:

Hyporheic zones contribute to lower temperatures in many rivers, creating a longitudinal heterogeneous array of thermal refuges. In this study, we had the unique opportunity to show temperature reduction along actual hyporheic zone pathlines in a large river system that contribute to the maintenance of refuges through discharge in off-channel habitats. Temperature was monitored in a dense network of wells that were located along pathlines in small islands, from a calibrated groundwater flow model. Temperature along one 600 m pathline was reduced about 7 degrees Celsius. Among three islands that were adjacent to the river, the northern two showed exponential decrease in temperature with distance, with fitted thermal Péclet numbers of 2.7 and 6.5, while the southern island showed no significant decrease. We suggest this is due to the higher infiltration rate in the wet season in this larger, more mature island, which suppresses hyporheic flow in the wet season. Stable isotope sampling showed that values of δ2H were higher in areas where we observed lower temperatures. The overall relationship of δ2H versus temperature was significant with a slope of -0.329. This implies that lower temperatures are associated with water that has had contact with deeper groundwater or that lower temperatures have been affected by local rainfall infiltration, or water that has entered the hyporheic zone in winter. These findings are important because they allow estimation of the temperature benefit that may be achieved in similar geomorphic settings, providing implications for riparian restoration.

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