The principal feature of this basin is the Zayandeh River which rises in the Zagros Mountains east of Zard Kuh at 4548 m (32°22'N, 50°04'E) and flows east for about 300 km to its terminal basin, the Batlaq-e Gavkhuni at 32°20'N, 52°47'E, a salt marsh with a salinity of 315‰ (Löffler, 1961) and an average depth of about 1 m (www.netiran.com/php/artp.php?id=1615, downloaded 19 July 2004). The salt marsh can dry up in summer. Wetlands associated with the terminal basin are a Ramsar Site of 43,000 ha (or 37,000 ha; sources vary as does the size of the marsh seasonally and annually). Associated marshes at the river delta and along its banks are fresh to brackish. These marshes are fed by flooding and by irrigation canals but dry up in late spring or early summer. Flooded areas often freeze over in winter. There is little natural marsh vegetation and flooding occurs over degraded steppe and cultivated land. Water is diverted for irrigation and for domestic and industrial uses. It  receives pollution from Esfahan and other urban sources. Esfahan is a major oasis city on the Zayandeh at 32°40'N, 51°38'E with a population over 1 million.

The Zayandeh basin encompasses about 30,480 sq km and is connected to the upper Karun River basin (which drains to the Persian Gulf) by the Kuhrang Tunnel constructed in 1953 although first proposed in the early sixteenth century (Fitt, 1953; Afifi, 1966; IRNA, 5 February 2002). Two additional tunnels are under construction (Stoltz, 2002). A hydroelectric dam at Godar-e Langar (also known as Karun-4) would also supply piped water to Esfahan 300 km away if it is completed (Whitley and Gallagher, 1995). Dams have deleterious effects on a riverine fish fauna and are often stocked with exotic species. The upper Karun has not been well explored for endemic taxa. Mean annual flow of the Zayandeh is estimated at 1.2-1.45 billion cu m, used mostly for agriculture but an increase in population and industry has necessitated dam construction (Shah Abbas Kabir or Sadd-e Zayandeh Rud, capacity 1450 million cu m) and diversion schemes. There is also the Hana Dam on the Hana River at Semirom with a height of 35 m and a capacity of 45 million cu m (http://netiran.com/news/IRNA/html931003IRGG04.html) and the Izadkhast dam to the southwest of the Batlaq-e Gavkhuni (www.irna.com/newshtm/eng/12003142.htm, IRNA, 2 July 2000). As well as man-made diversions, the upper Zayandeh basin has captured headwaters from systems tributary to the Persian Gulf. The Shah Abbas dam has reduced the natural flood flows downstream and little water now enters the salt desert.

Plans have been made to transfer Zayandeh River water from the Band-e Cham-e Asseman to Yazd's Shahneh Reservoir by pipeline over a distance of 375 km (Hamshahri, Tehran, 629:5, 22 February 1995). 78 million cu m of water will be transferred annually and this will decrease the habitat for fishes in the Zayandeh River basin.

Spring flow is at least 1700 cu m per second, but this drops to 28 cu m per second in autumn (Oberlander, 1968b). Discharge peaks in April with low values in September-October and decreases dramatically downstream after abstraction, evaporation and infiltration (Beaumont, 1981). The Zayandeh can be forded on foot at Esfahan in summer and Buckingham (1829) reported it to be dry. It dried again in 2000, 2001 and 2003 under drought conditions, partly through water abstraction upstream for irrigation and partly through aqueducts to other desert cities (Rafsanjan and Yazd) not in the Esfahan basin (Anonymous, 2001b; Foltz, 2002; newspaper reports). The river is polluted by city sewage, local wastes dumped directly into the river, and industrial wastes (Moghadam, 1976; Al-Hashimi, 1987; Tehran Times, 15 September 1997). 172,000 cu m of industrial pollutants enter the river daily. Pollutants include phosphorus, nitrogen, lead, nickel, zinc, organic substances, iron, manganese, oil products, mineral and organic dyes and the sewage from villages with a population of 900,000 people. Nadim (1977) found the highest mercury levels in fish were 0.19 mg/kg. As the acceptable limit was 0.5 mg/kg, mercury contamination in fish was not considered a problem. The flow is 1.45 billion cu m annually of which 1.1 billion cu m is used for agriculture, 150 million cu m for industry and the remainder is used as drinking water. The basin has a high demand for water supplies and has been under stress in this regard for the last 50 years. It will be unable to meet water demands in less than 15 years (Salemi and Heydari, 2006).

Ouseley (1819-1823) noted numerous small "bleak" and caught several carp-like fish up to 12-14 inches long (ca. 30-36 cm) in the deeper waters around the bridges over the Zayandeh at Esfahan.

The Batlaq-e Gavkhuni and marshes on the lower Zayandeh are a Ramsar Site, the lake occupying 12,000 ha, permanent marsh 1000 ha and temporary marsh 30,000 ha (World Conservation Monitoring Centre, 1990) or 47,000 ha (Mehrabi, 2004). It lies at 1470 m and has an average depth of 1 m. The Batlaq (= salt lake or marsh, gavkhuni = cowshed because cattle are put out to pasture in the marshes) is fishless but the marshes have a freshwater character depending on the input from the Zayandeh River. The substrate is silt and mud. Much of the marsh has been converted for agriculture. Flooded areas may freeze over in winter. The salt lake is said not to dry out completely (Mehrabi, 2004) although flows were down to 10-100l/s in the dry years 2000-2002 and the lake was dried out (Esteky, 2006).

As with all plateau basins, this one also has springs and qanats which contain fishes. Surber (1969) gives some data on total alkalinity and calcium-magnesium hardness in this basin and characterises it as moderately hard.

Fish farms have been developed in Esfahan Province (Tehran Times, 31 October 1999). Thirteen cold water and 10 warm water fish farms are expected to yield 490 t of fish, rising to 18 cold water and 15 warm water farms by the year 2000 with a yield of 760 t.

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