Lake Tirragen was situated in the Hill Country bordering Tortall's eastern border. The lake was near Fief Tirragen, which was most likely named after the lake. It was bigger than Golden Lake and Lake Naxen, making it probably the biggest lake of the country.
Lake Tirragen feeds the River Tirragen, which starts at the lake. The river deposits into the large Olorun River farther along its course. The land surrounding the lake is fertile and perfect for agriculture, due to the many rivers and bodies of water there are. The city of Whitethorn is located between the two rivers near Fief Tirragen.