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Karkheh (kär´kĕ), ancient Choaspes, river, c.350 mi (560 km) long, rising in the Zagros Mts., W Iran, and flowing S into the Khuzestan lowland, where it forms a swamp bordering the Tigris River. An ancient storage dam on the river at Shush made Khuzestan one of the most prosperous agricultural regions of SW Asia until the system fell into disrepair and the irrigated area reverted to desert. The area has been reclaimed as part of the Khuzestan project.