(Lough Derg), a lake in Ireland in the Shannon River Valley. Area, 129 sq km; depth, up to 24 m; water level, 33 m. The primarily low banks are steep and rocky in the south and southeast. The lake is used as a reservoir for a hydroelectric power station on the Shannon and serves as a link in the waterway from the upper reaches of the river to the Atlantic Ocean. The city of Killaloe lies on the southern end of the lake.