The Range of Variability Approach (RVA) is used to investigate the hydrologic impacts of a diversion weir on Chou-Shui Creek, Taiwan. Thirty-two hydrologic parameters, called Indicators of Hydrologic Alteration (IHA), are used to evaluate the flow conditions before and after weir construction. One standard deviation from the mean for each of the pre-construction hydrologic parameters is set as the management target range. Under the prevailing diversion rules, large hydrologic alterations are observed, especially for low flows. The means of 19 hydrologic parameters presently fall outside of the targets and the average non-attainment rate for the 32 indicators is 73.2%. Increasing the instream flow release or reducing diversions could mitigate the hydrologic impacts of weir construction. Increasing the instream flow to 40 m3/s and reducing monthly water demands by variable percentages significantly improves the altered flow conditions. Under the proposed water release and diversion scheme, 29 hydrologic parameters will fall within the management targets and the average non-attainment rate will be reduced to 35.6%, much closer to the pre-construction value of 25.3%. Restoring the natural flow variability is expected to promote the natural stream biota.