Rivers are dynamic systems that have the potential to support various compositions of planktonic communities throughout the river and between different rivers. It is understood that the dominant plankton taxa within a river varies spatially. However, it is unclear how the composition of plankton body sizes varies spatially though their variation reflects changes in the riverine food web. Water samples were collected in June and July 2024 at seventeen sites across four rivers in the Middle Missouri River Basin. Individual plankton body sizes were measured to the nearest picogram using an automated cell counter. A generalized linear model was used to compare the individual size distribution (ISD) between sampled planktonic communities. Despite clear differences in taxonomic make-up, the size distribution was remarkably stable between the four rivers with ISD lambdas averaging -1.95 +/- 0.45. In July 2024, the communities in three rivers slightly shifted to higher frequencies of smaller individuals than in June. The similarities between planktonic communities show the apparent stability of planktonic food web, despite taxonomic variation.