Althora's geography would make tunnels impossible. Althora has a three river surrond so there is no way to get out of Althora with out crossing one of the rivers, the swamp, or a lake. Granted there is land around the city but to leave the lands you have to cross a river. Althora is also a northern city having both nice summers and bitter winters (due to the lake).
Really there can be 2 answers 1. is real world and 2. is only in DnD
Remember is says that they came upon a river not dry river bed.