-

A limnological study of the Aghasht and Kordan rivers, that originate from the southern plains of the Elborz mountains was carried out during a period of 12 months in 1995-96. This study was conducted on four study stations located between 5 km north of Aghasht village and the Tehran - Qazvin freeway.
The results obtained from the investigation of the physical and chemical factors (water and air temperature,
water velocity, inputs to rivers, dissolved oxygen, pH, Sulphates, chlorides, etc.) show that the environmental and chemical state of these rivers is suitable and free from acute pollution.
The results of the biological investigations indicate that tbe benthic invertebrates inhabiting these' rivers belong to 4 phyla, 11 classsest 30 families and more than 35 genera. Among them the larvae of aquattic insects
were the most abundant (98.68%) and showed the highest biomass.
This study revealed that the diversity, distribution and abundance of benthic organisms differed in the four
study stations and also during the four seasons. The biological diversity, biomass and final productive potential (fish) in stations 1 and 2 were very high, especially during the warm months of the year.
On the basis of this study', the Aghasht and Kordan rivers lie in the Oligosaprobic - {J - Mesosaprobic zone.