St. Lawrence River Basin

St. Lawrence River-Great Lakes system is the second largest river network in North America in annual discharge. This chapter includes at least one river within each of the freshwater ecoregions that drain into the Great Lakes. The physiographic nature of the St. Lawrence River-Great Lakes system reflects the action of numerous glacial advances and retreats over tens of thousands of years. Three major physiographic divisions and eight physiographic provinces provide water for this system. The two principal terrestrial biomes within the St. Lawrence River-Great Lakes system are the temperate deciduous forest and the boreal forest, although grassland and savannah transition areas appear around major sections of Lake Michigan. The human impacts on rivers within the St. Lawrence basin are great. These include large dams built for navigation, hydroelectric production, and/or flood control. Most rivers in the northern St. Lawrence basin originate or flow through Precambrian Shield geologic formations, which have poor buffering capacity because of generally low alkalinity and conductivity.