In Lake Kasumigaura, artificial increases in water levels to meet the national government's plan for The Kasumigaura Development Project have been implemented since 1996. In the present study, the degree and characteristics of the decrease in emergent vegetation around the lake after the implementation of water-level control were analyzed using data obtained by governmental organizations. An analysis of the change in the width of the emergent vegetation area, which was defined as the distance from the concrete levee to the lake-side end of the emergent vegetation, revealed that the width decreased by 9.54±7.71 m (mean ± standard deviation, n = 34) between 1997 and 2010. The decrease in width was significantly positively correlated with an index of wave strength. An analysis of the pattern of decrease in vegetation area showed that areas dominated by Typha angustifolia and Zizania latifolia, which generally occupy relatively low and sheltered areas of emergent vegetation, had decreased more severely than other types of vegetation. Recently, the government has initiated a new practice for water-level management that is intended to test the compatibility of goals for water use and environmental conservation. Our results suggest that reductions in water levels should be tested as a method for harmonizing water use and vegetation conservation.