Thingvallavatn(thēng`gvät'lävä'tən), Icelandic Þingvallavatn, lake, c.35 sq mi (90 sq km), SW Iceland, E of Reykjavík. It is Iceland's largest lake. On its north shore is historic Thingvellir.
a lake in southwestern Iceland, the largest lake in the country. Thingvallavatn has an area of 82.6 sq km and is 114 m deep at its deepest point. It is located at an elevation of 103 m in a tectonic basin in a volcanic plateau and is dotted with volcanic islands. Trout and salmon are caught in the lake. A national park is located at Thingvellir on the northern shore of Thingvallavatn. On the Sog River, which drains Thingvallavatn and empties into the Atlantic Ocean, there is a hydroelectric system that supplies the city of Reykjavik with electricity.