Source: Encyclopedia Iranica
There are three main zones in the Zagros system. The region north of a line formed by the road Qazvin, Hamadan, and Kermanshah. South-east is the area generally spoken of as Luristan. This area is characterised by intensively folded mountains in the north, opening up to the south.

Luristanhas historically been divided into Lur-i Kuchek (the small Lur), west of the Ab-i Diz, and Lur-i Bozorg (the big Lur), the Bakhtiari mountains.

The two major divisions of Lur-i Kuchek are the zones west of Kabir Kuh mountain (or west of the Saimarreh river), called Posht Kuh (behind the mountain), and Pish Kuh (in front of the mountain).

Posht Kuh is formed by two ranges, the outer which reaches a height of 1800m. forms a distinct barrier to penetration from the west. The Saimarreh river basin, dividing the peoples of Posht Kuh and Pish Kuh is a long grassy valley surrounded by high continuos ridges.

Pish Kuh to the north east of Posht Kuh, is bordered on its north-east by the Burujird valley, on its south by the alluvial plain of Khuzestan.

The Pish Kuh may also be divided into two main sub sections, one east of the Kashgan river and west of the Ab-i Diz,known as Balgariwa; and the region to the north- west of the Kashgan, the Pish Kuh itself.

Lur-i Bozorg is the region of the Bakhtiari Mountains. It is the region east of the Ab-i Diz and north-west of the Khersin river. To the south-west is Khuzestan and the Iranian Central Plateau is to the north.

The region is drained by the Karun and a large number of tributaries, which join it in the highlands; they include the Ab-i Bazoft, the Ab-i Vanak, and the Khersin.

South of the Khersin is the mountainous region of Kuhgilu. This region is more difficult and isolated than the Bakhtiari, and is characterised by rocky defiles. The region is drained by the Marun, Zohreh, and Khersin rivers. Towards the south Kuhgilu merges into Khuzestan.

Source: Encyclopedia Iranica