Moselle is the most populous department in Lorraine, in the northeast of France, and is named after the river Moselle, a tributary of the Rhineā¦Moselle is the most populous department in Lorraine, in the northeast of France, and is named after the river Moselle, a tributary of the Rhine, which flows through the western part of the department. It had a population of 1,046,543 in 2019. Inhabitants of the department are known as Mosellans.