In-river weir effects on the alteration of flow regime and regarding structural stream habitat

Abstract

In-river hydraulic structures may negatively impact aquatic ecosystems, including changing river flow regimes and the habitat quality of endemic fish habitats. This study integrates the wavelet analysis and the time-series exceedance probability method to quantify the degree of evolution in the river flow regime caused by weirs and determine the critical time scale most affected in the environmental flow. Hydrodynamic and habitat models were conducted to analyze the influence of the variant flow characteristics between the dry and wet seasons on the habitat structure. Four endemic fishes were selected as the indicator species to represent different habitat preferences and reveal the spatiotemporal changes in habitat quality. The flow difference between dry and wet seasons became more extensive after the weir construction, indicating the alternative flow regime downstream. The annual deviation of daily flow dramatically raised, showing the flow rates decline in the dry season and enlarge in the wet season. The findings exhibit that the downstream flow amplitude changes regardless of time scale after constructing the weir. The flow data from the 2-days scale to the 1-week scale were most adjusted, meaning that these periods to meet the environmental flow decline significantly. The critical environmental flow corresponding to lower flow events was apparently influenced in the dry season, in which the Q75 and Q90 displayed the highest degradation in fish habitat quality. In addition, the simulation results of the habitat model found that glide is the primary habitat type. The total water area and the habitat diversity were reduced, while the habitat quality also had a lowered tendency after the weir operation. The habitat run was most degraded, and the riffle was also significantly negatively impacted. Regarding the endemic fish species,F. lacustresuffered the most harmful effects on suitable-habitat quality and quantity from the weir. This study suggests considering natural flow regimes at the planning and design phases of weir construction and rehabilitating these essential habitat patterns to mitigate the downstream ecosystem impacts.