The drainage basins of Iran are shown in the Figure. The delimitation of these basins is somewhat arbitrary. Iran is a mountainous country and much of it is desert. There are thousands of small springs and streams with no present or recent connection to other water bodies. Practical considerations require a large scale and I have divided the country into 19 major basins based on field work, maps, fish distributions, history of research, works on hydrography and areas deemed important for an understanding of zoogeography. The basins are as follows:-

Exorheic Basins:-   Gulf    Hormozgan    Makran    Tigris River

There are two main types of basin, exorheic where the rivers and lakes drain to the sea and endorheic, where rivers drain to an internal basin such as a lake, or are lost in the desert, and have no connection with the sea. The exorheic basins all fringe the southern part of Iran. The bulk of the basins, in number (15) and area (about 78.1% of Iran), are endorheic. These plateau basins lie at an average altitude of 800 m, alternating with mountains ridges at an average of 2000 m. The salt lakes and flats of these basins are fed primarily by groundwater rather than rain (Issar, 1967) and water is lost by evaporation. Wolfart (1987) makes the valuable point that Quaternary environments in the closed or endorheic basins of arid Southwest Asia often have marine and brackish fossils. These are not evidence of marine invasions but of the increasing salinity derived from the mineral content of rainwater. As the water evaporates it leaves behind the minerals and over ten thousand years or less a saline environment develops.

www.bibliothecapersica.com/articlenavigation/index.html, under drainage, downloaded 24 December 2004 gives four main drainages for Iran as follows:-
 

Drainage	Area (sq km)	%
Caspian Sea	193,161	11.9
Lake Orumiyeh	54,747	3.4
Persian Gulf	335,864	21.9
Interior	1,626,520	61.8
Total	2,210,292	100

with the interior drainages as follows:-

Van der Leeden (1975) summarises water resources of Iran with discharges of principal rivers at various recording stations, lists of major dams and reservoirs, and resources and demand. www.bibliothecapersica.com/articlenavigation/index.html, under ab (= water), downloaded 24 December 2004 also lists major dams and gives a general overview of hydrology and has descriptions of various rivers under their names. McLachlan (1988) also considers water resources in Iran. Some of the earlier dam projects are described by Justin and Taleghani (1955). Later dam projects can be located by a search at "Netiran.com". Prior to the Islamic Revolution 13 dams had been built in Iran but the five-year development plan (1990-1995) designed 110 dams of which 22 were under construction in 1993. 60 dams have been constructed after the 1979 revolution (IRNA, 31 August 1998).

"Aquastat" from the Food and Agriculture Organization, Rome (www.fao.org/ag/agl/aglw/aquastat/iran.htm) gives an overview of Iranian water resources and water abstraction and is updated at intervals. The total domestic, industrial and agricultural water abstraction was estimated at 70 km³ in 1993, 51% of the renewable water resources. Annual abstraction from aquifers (57 km³) is more than the estimated safe yield of 46 km³. An additional 39 km³ is used annually, 20 km³ for electricity production, 11 km³ for flood control and 2 km³ for control and thence environmental protection of downstream parts of rivers, the remainder being surplus. The increasing demands will have serious effects on the water supply and hence the fish fauna. Nikravesh (1997) estimates, based on water consumption and population growth, that Iran will be added to the U.N. list of countries facing water shortages in the year 2025.

Kuros (1943) gives accounts of historical water resources and the problems of water supplies in Iran. Lambton (1953) gives an account of the allocation of water resources in Iran for irrigation. This latter work is important for an understanding of restrictions on fish habitats, e.g. in qanats, reservoirs, rivers and springs. Beaumont (1981) reviews management of water resources in the Middle East and places the Iranian resources in a wider context. Anonymous (1961c) and Beaumont (1974) outline water resource development in Iran, the construction of dams, abstraction for irrigation by traditional and modern means, and the demands of industry and domestic consumers of water. All these affect the habitat of fishes, often in deleterious ways. Noori (1966) describes the hydrology of surface water in Iran. Pirnia (1951), Anonymous (1961c) and Beaumont (1973b) give accounts of the river regimes in Iran with discharges and runoffs at various recording stations. Peak discharges occur in March to May because of snowmelt. Very low flows occur in summer because of the lack of precipitation, and because of abstraction for irrigation, and flow is mostly from groundwater sources. Most rivers are really streams for much of the year as minimum flows for principal rivers are 0.16-451 cu m/sec, average about 36 cu m/sec. The Caspian rivers are the only ones which lack a distinctive annual rhythm and show flows closely related to precipitation throughout the year. The areas with the largest runoff values are in the northern and central Zagros Mountains and in the Alborz Mountains while lowest runoff values per unit area are found around the deserts in central Iran. In the Zagros and Alborz, annual runoff values can attain more than 300,000 cu m per sq km. Löffler (1956; 1961) studied the limnology of several of the major basins within Iran. The Ramsar Convention on Wetlands has a report on the Islamic Republic of Iran (No. 37, at www.ramsar.org/ram_rpt_37e.htm, downloaded 4 May 2001).

Peritore (1999) gives a general overview of ecological conditions and attitudes to the environment in Iran. Zohary (1963) gives a general account of the vegetation of Iran. A general description of Iran, its structure and drainage can be found in Harrison et al. (1945), Neumann (1953), Fisher (1968) and Krinsley (1970). Water policy development is summarised in Aminipouri (2002). A description of natural areas in Iran, including a list of National Parks and Protected Rivers, can be found in Zehzad et al. (2002). The Protected rivers are the Jajrud and Karaj in the Namak Lake basin, and the Chalus, Sardab, Lar and Haraz rivers of the Caspian Sea basin.

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