The Drell River flows through the Eastern Lands and starts at the border between Galla and Scanra. It flows south and deposits into the Great Inland Sea, where it forms a delta that makes up most of Tyra. This delta is perhaps why Tyra is mostly swampland. It marks the eastern border of Tortall with Tusaine. Erosion from the river has created the Drell River Valley, of which Tortall owns one side and Tusaine the other. The river flows through the Tusaine Mountains, which also help make up the border.
The land rights of the River Drell were the cause of much dispute between Tortall and its neighboring country to the east, Tusaine. Land disputes over the Drell River Valley were the cause of the Tusaine War—which besmirched King Roald I's mostly peaceful reign.
The Tortallans won the war, and forced Tusaine to swear never to claim rights to the Valley again in the treaty.