The length of the coast of mainland Wales is about , and adding to this the coasts of the Isle of Anglesey and Holy Island, the total is about . Cardigan Bay is the largest bay in the country and Bala Lake (Llyn Tegid) the largest lake at . Other large lakes include Llyn Trawsfynydd at , Lake Vyrnwy at , Llyn Brenig at , Llyn Celyn at and Llyn Alaw at . Bala Lake lies in a glacial valley blocked by a terminal moraine, but the other lakes are reservoirs created by impounding rivers, to provide drinking water, hydroelectric schemes or flood defences, and many are also used recreationally.