Geographically, Kiev belongs to the Polesia ecological zone (a part of the European mixed woods). However, the city’s unique landscape distinguishes it from the surrounding region. Kiev is located on both sides of the Dnieper River, which flows south through the city towards the Black Sea. The older right-bank (western) part of the city is represented by numerous woody hills, ravines and small rivers. It is a part of the larger Dnieper Upland adjoining the western bank of the Dnieper in its mid-flow. Kiev expanded to the Dnieper's lowland left bank (to the east) only in the twentieth century. Significant areas of the left-bank Dnieper valley were artificially sand-deposited, and are protected by dams. The Dnieper River forms a branching system of tributaries, isles, and harbors within the city limits. The city is adjoined by the mouth of the Desna River and the Kiev Reservoir in the north, and the Kaniv Reservoir in the south. Both the Dnieper and Desna rivers are navigable at Kiev, although regulated by the reservoir shipping locks and limited by winter freeze-over.