Dez is a river that originates from the middle Zagros mountain range (in the south of Lorestan province). In these heights, which are considered as rainy areas of the country, most of the rainfall in autumn and winter is snow and their watering from late winter to late spring provides most of the annual surface water of this basin.

Sezar  and Bakhtiari are the two main tributaries of the Dez River. The Sezar  River flows in the northernmost part of the Dez Basin and consists of three tributaries: Marbare, Tireh and Sabzeh. The joining of several waterways, including the Azna River in the Aligudarz region, forms the Marbareh River, which flows westward towards the city of Dorud in Lorestan Province. The other tributary of the Sezar River is called the Tireh River, which consists of the tributaries of Kalhorud, Silakhor, Absardeh, and Biatun. The source of the Tireh river is from Boroujerd region and then this river flows in the east direction towards Dorud and in this place, the two rivers of Tireh and Marbareh join. From this point on, the direction of water flow is generally from north to south and southwest. 20 km south of Dorud, the Sabzeh River, which originates from the southern and western slopes of Oshtronkooh, is connected to this stream and the Sezar River is formed. About 25 km southwest of this point, the Vask River, a short distance from the left bank, joins the Sezar River and continues in a southwesterly direction. The small Sorkhab River also joins the Sezar River from the right bank.

Bakhtiari River is the second main tributary of Dez River, which joins Sezar River 40 km south of Sorkhab River. The Bakhtiari River originates from the southern heights of Oshtronkooh and initially flows as a river in the general direction from northwest to southeast. It flows southeast to northwest, it synchronizes. Then, after receiving water from the smaller tributaries, the river changes direction to the west and in this part it is known as Zalaki. It then gathers with a relatively large tributary that joins it from the north and joins the Sezar River. From now on, it is the Dez River that flows.

Dez river is blocked by Dez dam after entering Khuzestan province. This dam has formed a large lake in Shahyoun area near the city of Dezful, which has become one of the tourist attractions of this area.

After crossing the dam, the Dez River enters a long and deep gorge known as Chal Kandy. Access to this gorge is possible by boat from the city of Dezful. During spring and summer, when this region has a warm climate, it becomes a suitable place for recreation for the people of the province.

The Dez River eventually enters the Persian Gulf by joining the Karun River.

Ecosystem

There are special plant species in the catchment area of ​​this river from the entrance to Khuzestan plain to the confluence with Karun river. In the upstream areas of Dez, due to the existence of high mountains and proximity to Karun and Karkheh branches, there are many similarities in terms of species. Zagros forests are spread in this region and can be seen in abundance around riverbeds.