Demonstration of fiber optic distributed temperature sensing to differentiate\ud cold water refuge between ground water inflows and hyporheic exchange

Graduation date: 2009Recent developments in Distributed Temperature Sensing (DTS) have allowed\ud new insight into the surface-to-ground water interaction. The continuous temperature\ud measurement by the DTS allows for cool water inflows to be located during warm\ud summer months. These cool water inflows can then be differentiated between ground\ud water and hyporheic exchange. The cool water inflows are important to many species\ud that inhabit the rivers of the Western United States. As temperatures increase due to\ud climate change the importance of cool water inflows will grow.\ud This study used fiber optic cable and the DTS system to measure the\ud temperature in 2km of the Walla Walla River, near Milton-Freewater, Oregon. The\ud temperature data was split into nighttime and daytime averages. The correlation\ud between these two data sets was calculated to reveal when the daytime and nighttime\ud data followed the same cooling pattern (evidence of ground water inflow) or when the\ud nighttime temperature increased, but the daytime temperature decreased (evidence of\ud hyporheic exchange). Nine areas of apparent ground water inflow and nine areas of\ud expected hyporheic exchange were identified. The average quantity of inflow at each\ud ground water site was calculated as 0.04 m3/s. The computed average hyporheic\ud exchange depth along the 2000 m channel was 0.32 m. The located cold water inflows\ud were compared to salmon and trout location from a US Fish and Wildlife Survey.\ud This comparison showed that the cool mid-day inflows (either ground water or\ud hyporheic exchange locations) made up 1/3 of the study reach length and had\ud approximately 60% of the cold water fish. Half of the cool water inflow sites did not\ud have fish present during the survey which suggests that while influent cool water is a\ud major factor for fish congregation, it is not the sole factor