Physically, the Tibet AR may be divided into two parts: the lakes region in the west and north-west and the river region, which spreads out on three sides of the former on the east, south and west. Both regions receive limited amounts of rainfall as they lie in the rain shadow of the Himalayas; however, the region names are useful in contrasting their hydrological structures, and also in contrasting their different cultural uses: nomadic in the lake region and agricultural in the river region. On the south the Tibet AR is bounded by the Himalayas, and on the north by a broad mountain system. The system at no point narrows to a single range; generally there are three or four across its breadth. As a whole the system forms the watershed between rivers flowing to the Indian Ocean — the Indus, Brahmaputra and Salween and its tributaries — and the streams flowing into the undrained salt lakes to the north.