The riverbottom forest community of St. Mary River, Lee Creek and Belly River in southwest Alberta, Canada is a unique ecological entity characterized by poplar species having their major Alberta distribution along these streams. Stands in the community are dominated by three tree species, six shrub species and nine herb species. Establishment of the community is dependent on climate and substrate; destruction is the result of progressive lateral stream-flow erosion. Soils are sandy loams above gravel, with pH values of 7.7 to 8.0 and soluble salt concentration of 176 to 458 parts per million. Trees in mature stands averaged 23.0 centimeters in diameter and 40 years in age; maximum tree age was 250 years. The vascular flora consists of 294 species of which 44 are woody and 250 herbaceous. One species (Prunus nigra Ait.) new to Alberta and range extensions for 13 species are cited. There are no true community endemic species. Recreational and livestock raising uses are present community modifiers. Fire is not important in current forest dynamics.