Koohrang tunnel is situated near Chelgerd city. It was constructed to annex the waters of Koohrang to that of the Zayandeh Rood river. It is said that construction of the tunnel began during the Sassanid period and approximately from the time of Shah Abbas I. At that time they planned to make a vertical crevice right at the vertical axis of Karkonan Mountain and by means of a dam which crosses the same axis, to increase the water level in Karoon River, thus leading the water through the crevice made in the mountain. The difference in distance from the peak to the river bed was measured at 300 m. The height of the dam was 50 m. and the height of the crevice 250 m. so as to allow a water flow. For this aim, fifteen years of construction work took place, and even nowadays many effects have been left which include: Shah Cut (Tarasheh). It is said that for this much of the cut which was made at the time of Shah Abbas approximately 5 million workers were required and for 15 years these workers labored at excavating free of charge. After Shah Abbas passed away, this construction activity was stopped and was forgotten. In recent decades the new tunnel of Koohrang was constructed joining the 3 water branches of the Karoon River to 3 water branches of Zayandeh Rood and therefore has created an interesting vicinity worth seeing.