From HODP Wiki

The River is technically the nearby Charles River but usually refers to the nine upperclassmen Houses located around Harvard Square and between Harvard Yard and the Charles River. These Houses are sorted into three neighborhoods: River Central or Harvard Square Houses, are Adams, Lowell, and Quincy; River East is Dunster, Leverett, and Mather; and River West is Eliot, Kirkland, and Winthrop.