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Abstract Background Land use and development alter mudflat and wetland habitat availability, although mudflats and wetlands provide important stopover habitats for shorebirds during the spring and autumn migrations and support communities of ducks and geese during the winter months in the Republic of Korea. This study investigated land use changes around Sihwa Lake (Republic of Korea) and evaluated the effect of these changes on waterbird community characteristics. Methods We conducted a land-use-change analysis at the medium-resolution level using land cover maps for 2001, 2007, 2009, and 2014. Also, a tidal stream survey was conducted in Sihwa Lake and the surrounding reclaimed mudflats every season for 10 years (2003–2012) to identify the seasonal and interannual variations in waterbird species composition. To determine the total annual waterbird species and population counts, species diversity index, and interspecies variations, a TRIM (trends and indices for monitoring data) analysis was used. Results Wetland area decreased more than 10% while agricultural land, barren land, and grassland area increased more than 10% due to continuous reclamation activities around Sihwa Lake. Barren land later turned into agricultural land or other land use. Sixty-three species and 566,623 individuals were recorded. The number of species, population size, and species diversity index by year and by species showed decreasing trends that were more marked in spring and summer. Furthermore, seasonal and annual variations in waterbird species composition showed decreasing trends in dabbling ducks, herons, grebes, and shorebirds but diving ducks displayed increasing trends. In particular, shorebirds were reduced to a greater extent than other waterbird species because of the reduction and simplification of the intertidal zone, and shallow waters caused by reclamation and road construction. Conclusions Increased development and construction around Sihwa Lake has altered migratory shorebird populations with a general decline in species diversity and population size. The greatest decline was observed in wading birds, while diving duck populations showed increasing trends.