III.

Human Impact

Maximum discharge from the Tigris, the result of melting winter snow in the highlands and late-winter and early-spring rains, occurs from March to May, and accounts for over half of the river's annual flow, often in the form of significant floods. Construction of the Samarra Barrage in the 1950s allowed for diversion of Tigris waters to the Tharthar Depression; this assisted in flood control, and in boosting the flow of the Euphrates, but has not provided the agricultural benefits anticipated from additional irrigation. Minimum flows are experienced from August to October. In the past the Tigris carried significant quantities of silt into the Shatt Al Arab, contributing to the extension of the delta into the Persian Gulf.

Flood control projects and the construction of dams on the major tributaries of the Tigris have decreased the silt load carried to the southern river course and the Shatt Al Arab. The expansion of irrigation in southern Iraq has resulted in a deterioration in soil quality, as rising water tables, polluted waters, and decreased silt deposits have taken their toll. As water is increasingly diverted from the Euphrates river by Turkey and Syria, Iraq's dependence on the water of the Tigris will become even more critical.