The objectives of this study were to determine the long-term changes in water chemistry, physical habitat, and fish fauna/composition before and after construction of the Sejong Weir in Geum River, South Korea, during 2007–2014, and to evaluate the integrated ecological health based on chemical water quality, physical habitat, and biotic integrity using a star plot integration approach. As an indicator of water quality, the mean total phosphorus (TP) in the surface water decreased 2.6-fold after weir construction, indicative of P sedimentation from the water column due to structural changes from a lotic to a lentic system. The chlorophyll-a concentration decreased in response to P levels. Considering total nitrogen (TN), more than 91.7% of the total observations after weir construction (n = 96) had a TN:TP ratio >17, indicating potential P limitations on algal growth. The log-transformed TN:TP ratio had a weak positive correlation (p<0.005, R² = 0.085, n = 96) with TN and a strong negative correlation (p<0.001, R² = 0.575, n = 96) with TP. This indicated that the N:P ratio was regulated mainly by variations in P rather than N. After weir construction, the population of the lotic-type fish Zacco platypus decreased 13-fold, and the population of the lentic-type fish Pseudorasbora parva increased 3-fold. Overall, the integrated ecological health assessments revealed large degradations of >3-fold in biotic health, 2-fold in physical habitat health, and >5-fold in chemical health after weir construction.