Mosul dam was built on the River Tigris northern part of Iraq during the period 25th January, 1981 and finished on 24th July, 1986. The foundation of the dam lies on the Fatha Formation. This formation is composed of alternating beds of marls, limestone, gypsum and clay. The beds of this formation are highly karstified. After impounding, several sinkholes developed within the vicinity of the dam site. The surface expression of the sinkholes suggests that they are caused by underground collapse.The appearance of the downstream sinkholes is most likely related to fluctuations in the tail water level of the main dam during operation of the dam and the downstream regulating reservoir. In addition, water seepage also was noticed in various areas indicating the dissolution of gypsum and anhydrite from the foundation. During the period February-August, 1986 the dissolution intensity ranged from 42 to 80 t /day.