Our study is documenting the number of species and their abundance at riverine sites throughout the Ouachita Uplands. We are examining four groups of aquatic organisms: mussels, benthic macroinvertebrates, meiofauna, and fishes. Benthic macroinvertebrates are organisms such as snails and larval insects that live on the stream bottom. Meiofauna are very small organisms, such as some worms and crustaceans, that live in the spaces between sand and gravel particles under the stream.