Our study aimed to identify patterns of temporal variation and changes in the structure of the community of Chironomidae larvae in two rivers in the Upper Paraná River floodplain after the construction of a reservoir  upstream (Porto Primavera). Samples were taken with a Petersen grab,  and were obtained between 2000 and 2007. Chironomidae larvae were  identified down to the lowest taxonomic level possible. The high richness  of Chironomidae observed in the Paraná and Ivinhema Rivers (100 morphospecies) in comparison to the world average of rivers of the same  size (44 species) emphasizes the importance of these habitats for the  maintenance of biodiversity. The composition and density of Chironomidae in the years 2000 and 2001 differed from the other years. This period was characterized by extreme changes in the Paraná River flow caused by the  closing of the Porto Primavera Dam, which added to a severe dry period in late 2001. The different compositions of morphospecies and the higher similarities in subsequent years are indicative of the recovery and  adaptation of the community. In spite of the changes in the composition of morphospecies, diversity was maintained and the community continued to respond to fluctuations in the hydrometric level.