This basin is flanked by the Alborz Mountains to the north and the Zagros Mountains to the west. On the east is the vast expanse of the Kavir basin and on the south such ranges as the Kuh-e Karkas at 3899 m (33°27'N, 51°48'E). The basin encloses about 87,600 sq km.

A small sump near Arak (34°05'N, 49°41'E) is included as part of this basin as it is not separated by any major landform. A second salt lake is the Howz-e Soltan by the Tehran-Qom road and this lies in the same depression as the much larger Namak Lake south of Tehran. The lowest part of this basin is at 765 m and is covered by water in spring but this generally evaporates by the middle of summer.

The proximity of the capital, Tehran, to the rivers of this basin and its rapid growth in population and industry has led to many water diversionary schemes (Anonymous, 2003). A proposed dam northwest of Tehran would be the largest man-made lake in the country and the Middle East (sic) (Nouri et al., 2005). Much of this basin lies in Markazi or Central Province which has 42 dams of varying sizes. The Abbasabad Embankment Dam in Khomein, for example, is 36 m high, has a crest of 260 m and has a reservoir of 25,000 ha (IRNA, 3 February 1999).

The principal river in the west draining the Alborz south towards the Namak Lake is the Karaj River. Average temperatures of the Karaj River at the dam site before construction ranged from 2.5°C in January to 16.4°C in August (Nümann, 1966). Rieben (1954) and Hariri (1966) give details of surface and ground water in this river basin. The Amir Kabir Dam on the Karaj contains 205 million cu m of water and feeds through pipelines to Tehran. The reservoir has an area of 4 sq km at high water, 1.1 sq km at low water. Vladykov (1964) and Nümann (1966, 1969) give some details on the limnology of this reservoir, particularly temperature regimes. The Karaj has a discharge of 124 cu m per second in spring but this falls to 4.2 cu m per second in autumn. 55.6% of the annual discharge occurs during spring. There is no vegetation because of the steep rock sides and water fluctuations. Nümann (1966) recommended stocking the Karaj reservoir with Coregonus sp., Sander lucioperca, Acanthobrama terraesanctae (a Levantine species) and cichlids from Israel as environmental conditions and plankton levels were suitable. Nadim (1977) found the highest mercury levels in fish from the Karaj were 0.05 mg/kg. As the acceptable limit was 0.5 mg/kg, mercury contamination in fish was not considered a problem.

The Abhar River and its tributaries drain the land west of Tehran and south of Qazvin (36°16'N, 50°00'E). Its headwaters approach those of the Zanjan River, a Caspian Sea tributary. The course of the Abhar is about 350 km from its headwaters to the terminal sump. The lower part of this river is known as the Shur and is salty. Sewage and untreated factory wastes, as much as 40,000 cu m, flowed into the streams around the city of Qazvin although waste-water and sewage treatment plants are offsetting this problem (http://netiran.com/news/IRNA/html/941220IRGG05.html).

Other rivers draining the Alborz are much shorter. The Jajrud (Jaj, Jaji or Jaje River) to the east of Tehran is dammed at Latian (95 million cu m) for the Tehran water supply also. The Jajrud discharge is 60.5 cu m per second in spring and 1.5 cu m per second in autumn. Nümann (1966) reports fish kills in the thousands for Capoeta buhsei on turbid spring floods of this river. The Band Ali Khan River flows from the Khasrang Mountain (as does the the Jajrud which it receives) and its branches on the Varamin Plain are used for irrigation. Much of this river is polluted from wastes in the Jajrud and Tehran's sewage floodway (Rohani, 2004). The Lar River, a Caspian Sea tributary, was scheduled for diversion via a massive tunnel into the Jajrud (Marwick and Germond, 1975a; 1975b). This would affect flow in the Heraz River of the Caspian Sea basin and plans to offset this involved weirs and canal construction no doubt with the usual deleterious effects on fishes. These major projects are a far cry from the days in the twentieth century when Tehran depended solely on qanats for its water supply (Rieben, 1954).

The Namak Lake receives the Qareh Su (Gharechay), which flows north of Qom, and the Qom River from the Zagros Mountains. Discharge of both these rivers is about 312 cu m per second in flood falling to about 4 cu m per second in October (Oberlander, 1968b). The Qareh Su exceeds 400 km in length. The Qom River has captured headwater streams of Persian Gulf drainage. The Golpayegan River near Golpayegan has a storage reservoir, the Sadd-e Shah Esma`il. Borowicka (1958) gives some early figures on siltation and irrigation requirements. The Haroon Canal had diverted water for irrigation from the Golpayegan River for over 1000 years, and during the summer and fall all river water entered this canal. The Ghadir or Qadir Dam near Saveh has a volume of 290 million cu m of water. The 15th Khordad Dam is located 80 km south of Qom on the Qom-Delijan road (http://netiran.com/news/IranNews/html/95030718INPL.html). The Khandab Diversionary Dam is near Arak (http://netiran.com/news/IRNA/html/951217IRGG09.html).

Egglishaw (1980) gives some details on the water quality and environment of rivers and streams of this basin. Imandel et al. (1978) recorded ground water pollution by detergents in Tehran, where there was no method of sewage disposal other than discharge to wells and seepage pits. Södergren et al. (1978) reported on pollution with organochlorines in the Karaj and Latian reservoirs. Capoeta buhsei, Oncorhynchus mykiss, Alburnoides bipunctatus, Coregonus sp. had accumulated the DDT metabolite p,p^'-DDE, particularly in the Latian Reservoir. Direct removal of plants for fuel and laying bare the roots of such thorny plants as "giavan" for extracting gum tragacanth leading to plant loss has caused soil loss by erosion, gullying and affected recharge of groundwater. Poor farming practices on steep slopes has also led to the loss of topsoil such that runoff is too fast for infiltration of rain and snow (Rieben, 1954). These factors causing silting of reservoirs, added silt input to rivers and reduced groundwater recharge with consequent reduction in spring and qanat flows, all detrimental to fish habitats. Some areas of southern Tehran receive 300 kg/ha/yr of sulphate ions as acid rain which lowers river pH and has effects on the fish fauna (Salahi Kojoure, 1997). An effluent leak from a power station in the Vian area of Hamedan sent 40-50,000 litres of furnace oil into 1 km of river in the Qareh Su basin (Iran Network 1, Persian TV, 1730 GMT, 2 January 2000). Monavari and Mardani (2007) record the effects of sewage from fish culture ponds in this basin on water quality in the Jajrud, most factors being within acceptable limits except coliform bacteria.

Qanats are still a major feature of this basin. Alamouti (1966) records 260 qanats producing 99 million cu m per year on the Varamin Plain (35°20'N, 51°39'E), 220 qanats producing 161 million cu m per year on the Karaj Plain and 600 qanats producing 200 million cu m per year on the Qazvin Plain. However numerous pump wells have led to the drying of qanats and a complex irrigation system has reduced groundwater recharge (Beaumont, 1974). Alibekov (1994) gives a Russian account of qanats in Central Asia and also refers to those around Tehran in the Namak basin.

The Qazvin area has more than 20 aquarium fish farms producing over 2 million fish (www.tehrantimes.com, downloaded 28 July 2004). Waters in this area drain also to the Caspian Sea and there may be potential for escapes of exotics.

Berg (1940) refers this basin to his Tehran District of the Iranian Province. He notes that some drainages are close to those of the Caspian Sea basin and that the fauna may be of quite recent origin, rather than the Pliocene advocated by Derzhavin (1934) for Salmo trutta. Saadati (1977) considered that the fish fauna of this basin was not derived from movements through a large freshwater lake connecting all the tributaries. Some species came from the Caspian Sea basin and others from the Esfahan and Tigris River basins. The basin may also have served as a "filter-bridge" allowing such species as Capoeta aculeata, Capoeta capoeta and the progenitor of Capoeta fusca to reach the Dasht-e Kavir basin.

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© Brian W. Coad (www.briancoad.com)