Physically, Tibet may be divided into two parts, the lake 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. The region names are useful in contrasting their hydrological structures, and also in contrasting their different cultural uses which is nomadic in the lake region and agricultural in the river region. Despite its large size and mountainous nature, variation of climate across the Tibetan Plateau is more steady than abrupt. The river region has a subtropical highland climate with moderate summer rainfall averaging around per year, and daytime temperatures ranging from around in winter to in summer - though nights are as much as cooler. Rainfall decreases steadily to the west, reaching only at Leh on the edge of this region, whilst temperatures in winter become steadily colder. On the south the river region is bounded by the Himalayas, 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.