This basin occupies 26,440 sq km north and east of Shiraz at a lowest altitude of ca. 1525 m. Its lowest part is occupied by a chloride lake, the third largest lake in Iran, composed of two parts, a northern basin known as Narges or Tashk and a southern basin known as Neyriz = (Niriz) or Bakhtegan. The two basins are not always connected and the southern basin is saltier because major freshwater input is from the north. Löffler (1956; 1957; 1959; 1968; 1981) gives details of this lake. The lake area varies between 1210 and 2400 sq km, with a maximum depth of 1.1-1.7 m and a mean depth of 0.5 m. Salinity is 13.7-101.6 gl^-1 and temperatures range from 15°C to 45°C in the shallows. The lake is reported to have dried out completely in 1871, 1933 and 1966 (Cornwallis, 1968a) and in 2000 (www.irna.com/newshtm/eng/05142727.htm, IRNA, 26 July 2000). Löffler (1993) considers that this lake may dry out permanently in the near future if abstraction of water from the Kor River for irrigation continues to grow. The drought in 2003 reduced Lake Bakhtegan to a series of puddles. Fluctuations in lake levels affect the freshwater faunas of springs, including fishes, which drain into the lake: high levels swamp the springs with water too saline for fishes to survive. Low levels, however, allow streams to connect and exchange faunas on the lake bed so they are not as isolated as they might appear.

Bobek (1963) suggests that there may have been an outflow from this basin to the Gulf at the south-east corner of the lake which was cut off at the end of the Pleistocene by alluvial fans. However Krinsley (1970) maintains that any outlet was closed by the late Pliocene.

Major rivers are the Kor (= the classical Araxes) and its tributary the Pulvar (or Sivan) (= the classical Medus) which rise in the Zagros Mountains to the north and north-west and drain to the north-west corner of Lake Tashk. These mountains are high enough (Kuh-e Dinar at 4432 m and 30°50'N, 51°35'E) to have a snow cover and thus there is a continuous flow throughout the year. However in summer water does not reach the lake because of the demands of irrigation. Drainage and irrigation canals run through the basin on the plains at the north end of the lake. Several springs feed marshes, notably the Lapu'i marshes, a wetland of 150 sq km to the north-west of the Kor-Pulvar junction, the Zarqan marshes of 4 sq km, an extension of the Lapu'i marsh (both now severely damaged by construction of a drainage canal as part of the Dorudzan or Sadd-e Daryush-e Kabir (dam) at 30°15'N, 52°20'E, a project on the Kor River), the "Gomun", "Gumoon", "Gumoo" or "Sangare" marshes of 2 sq km at the north-west corner of Lake Tashk and the Sahlabad marshes of 5 sq km on the south-east coast of Lake Bakhtegan (Cornwallis, 1968a; 1968b). The Band-e Amir or Kamjan Marshes at 29°40'N, 53°05'E are formed at the delta of the Kor River and encompassed about 100 sq km but the Daryush-e Kabir Dam severely restricts the water flow to these marshes. A dam on the Bolaghi Gorge is proposed which would affect the flow of the Pulvar but is being opposed on archaeological grounds (www.netiran.com, downloaded 4 October 2004).

The fish, Aphanius sophiae, is found in these marshes and springs, but suffers predatory attacks in an unusual way. The greater flamingo stirs up mud in its feeding and this releases H₂S, CO₂, and CH₄, suffocating the fish and making them easy prey for herons.

The Neyriz Lakes and Kamjan Marshes are a Ramsar Site (World Conservation Monitoring Centre, 1990; Khan et al., 1992) although the Kamjan Marsh area may be deleted because of drought and other factors such as rice, wheat and cotton growing and livestock grazing. The "Cheghakhur" and "Gandoman" marshes in Chahar Mahall and Bakhtiari Province will be substituted for the Kamjan Marshes as a listed Ramsar Site (Khan et al., 1992). The "Gumoon" marshes have been partially drained for irrigation and for conversion into aquaculture ponds (Khan et al., 1992).

The Ghadamghah spring-stream system at 30°15'N, 52°25'E and 1660 m altitude has been described by Esmaeili et al. (2007) and is a regional hotspot for biodiversity. The fishes present are Petroleucsicus persidis (Cyrpinidae), Cobitis linea (Cobitidae), Seminemacheilus tongiorgii, Barbatula farsica (Balitordiae), Aphanius sophiae (Cyprinodontidae) - all Iranian endemics, and Alburnus mossulensis, Capoeta aculeata and Capoeta damascina.

The Daryush-e Kabir Dam on the Kor River contains 990 million cu m of water, is 24 km long and about 9.5 km wide. Its conductivity is 363 µS compared to Lake Bakhtegan at 105,900 µS and consequently it can support a fish fauna. Band-e Amir on the Kor River is a diversion dam over 1000 years old and also provides a small reservoir habitat for fishes (Houtum-Schindler, 1891). At least three other dam sites have been proposed in this basin (Tang "Boraghi" (= Tang-e Boraq), "Tang Bulak" and "Ghaderabad" (= Qaderabad)). Surber (1969) gives some spot data on pH, total alkalinity, calcium-magnesium hardness, chlorides and free CO₂ in this area. Water is relatively hard. Concentrations of total dissolved solids vary between 202 mg/l and 436 mg/l in the rivers compared to a range of 333-6937 mg/l in the Gulf basin.

Kaftar Lake at 30°34'N, 52°47'E is at ca. 2300 m in the Zagros Mountains northeast of Shiraz. It occupies 4700 ha (500 ha in Khan et al. (1992)) and is a shallow, semi-permanent freshwater lake which can dry out completely in summer and is frozen over in winter. The annual mean water temperature is 14.4°C, the mean maximum 23.5°C and the mean minimum about 2°C (B. Jalali, pers. comm., 1999). Lake water has been proposed for irrigation usage in the past and a recently proposed earthen dam would reduce the lake area by half (Scott, 1995). It has a mixed ichthyofauna of native species and exotics. The fishes recolonise from springs and the main river entering the lake and are also stocked.

The Kor River basin also contains qanats. Some of these flank the Pulvar River, for example, and serve to bring water to fields above the incised river bed.

Pollution in this basin has been recorded by Merchant and Ronaghy (1976) where industry discharges waste untreated into surface and ground waters. Waste from a sugar mill killed 1 million fish in 1994 and a further 500,000 fish died in 1996 from industrial waste (http://www.iran-e-azad.org/english/noi/noi-83.html or News on Iran, 83, 15 November 1996). A fish kill was reported from the Pulvar River in 1978, polluted by wastes from a food factory (Coad, 1980c). Peritore (1999) and Moussavi and Saber (1999) record the Kor River receiving organic wastes from animal processing plants, ammonium and mercury from petrochemical complexes and such heavy metals as cadmium, chromium and arsenic from electronics manufacturers. Ebrahimi et al. (2008) report lead levels in Cyprinus carpio and Capoeta spp. to be less than the maximum allowable by the European Union but still of concern.

Channels started in 1981 to provide more agricultural land drain through the Kamjan Marshes to Lake Tashk and the Kharameh Marshes to Lake Bakhtegan. Much of the marsh habitat has been destroyed. The "Gumoon" Marsh has been drained for agriculture and fish ponds.

Miller (1985) reports on deforestation in this part of Iran during the fourth to second millennium B.C. Even marsh areas were probably treed before demands for charcoal and construction materials increased. The fish faunas must have adapted to increased insolation and any species sensitive to higher marsh and stream temperatures would become less common.

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