Evaluating the restoration benefits of the gueisan dam regulation changes by using modified index of stream conditions

This study used the modified index of stream condition (MISC) to evaluate the restoration benefits of different dam regulation, dam decommissioning, and dam removal of the Gueisan Dam at Nanshih River in the northern region of Taiwan. Twelve scenarios were established and analyzed. The changes of water quality, aquatic organism, and riparian vegetation in short-term period before and after the scenarios were assumed stable. The results reveal that the most influent reach in water discharge lies in the section 78 to the section 67 while the Froude numbers and habitat types lie in the section 80 to the section 78.1. Comparing the current situation, decrease 50% water bypass for hydroelectric power, dam decommissioning, and dam removal can increase the score of the hydrology sub-index of MISC. In addition, only the scenarios of dam decommissioning and dam removal can significantly raise the score of the physical habitat sub-index. According to the MISC calculations, the river condition of current situation, after decrease 50% water bypass for hydroelectric power will be detected and calculated for a better score at level C (still at a marginal condition). However, after dam decommissioning as well as after dam removal will be promoted from the original level C, a marginal condition, for a better score at level B, a good condition.