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The formosan land-locked salmon is one of the representative but endangered species in Taiwan and the first-order stream of Yushen Creek in Chichiawan Watershed, Taiwan is the major habitat for the salmons. Unfortunately, the intensive water exchanges beneath the streambed caused the creek to cease flow, and the fragmented creek could not provide a sustainable environment for the rehabilitation and the population of salmon was significantly reduced. To comprehensively understand the mechanisms of surface water and groundwater interaction and mitigate the effect of fragmentation on the salmon habitat, the tracer tests and numerical models were conducted to characteristic hyporheic flux exchanges beneath the stream. Seven observation wells and seven piezometers along the stream were installed to provide long-term water levels, temperatures, and electrical conductivities of surface water and groundwater. Five tracer tests from April, 2017 to May, 2018 were conducted at different sections of reach to delineate the horizontal and vertical hyporheic flow paths and seepage flow directions through the breakthrough curves. OTIS and VS2DT were selected to simulate the concentration variations in the stream and streambed, respectively and quantify the hydraulic characteristics of hyporheic zone. The estimated storage zones and exchange coefficients by the OTIS model were used to mimic the hyporheic cells and fed into the VS2DT model to identify the hyporheic and seepage flows. The simulated results revealed that the first-order stream has frequent flux exchanges beneath a high porosity gravel streambed but this intensive interaction reduces with the existence of confluences and the high level of baseflow. The results also demonstrated the effect of topography on the flow path that water flows out of the upstream wetted channel and back into downstream wetted channel to avoid the areas of step-riffle. Finally, the hydraulic characteristics of streambed, streamflow, and morphology of stream should be considered to mitigate the fragmentation when conducting the river restoration of the first-order stream in the alpine watershed. [ABSTRACT FROM AUTHOR]

The interaction between river and streambed is considered as a fundamental role in the functioning of riparian ecosystems. The endangered Formosan landlocked salmon have been successfully rehabilitated in Yusheng Creek, the first-order stream in the alpine watershed of Chichiwan Creek, Taiwan. However, the stream fragmentation, no surface streamflow, seriously reduced the salmon population, hampering the restoration work. The utility of combining high spatial and temporal resolution of distributed temperature sensing (DTS) technology and field installation of monitoring wells were demonstrated to comprehensive understand the exchange process and mitigate the effect of fragmentation on endangered salmon. The DTS measurements was conducted on the length of the 1,250 m reach covering the fragmented reach of creek from 7-13 November 2018. The monitoring wells provided the long-term water levels and streambed temperatures to analyze the dynamic behavior of hydraulic characteristics of streambed. The results showed that the DTS enabled to detect the locations of groundwater downwelling/upwelling and hyporheic exchange along the study reach. The temporal interactive processes between stream and groundwater from continuous to ceasing flow were also detected. Due to intensive rainfalls and stream fragmentation, the spatial and temporal changes of streambed in vertical hydraulic conductivities were significant. DTS measurements demonstrated significant impact of groundwater and hyporheic inflows on reducing the maximum stream water temperature by 1-2oC in November, 2018. These results implied that the downstream stream has the potential as the thermal refuge for the salmon and the fragmentation can be mitigated through the river restoration to significantly increase the salmon population. [ABSTRACT FROM AUTHOR]