Russian Plain, plain and series of broad river basins in eastern Europe (including western Russia). It extends over nearly 1,500,000 square miles (4,000,000 square km) and averages about 560 feet (170 meters) in height. The plain is subdivided into a number of distinct regions, including the Valday Hills; the Central Russian Uplands; the Volga Uplands; and the Dnieper River, Black Sea, and Caspian Sea lowlands.