[1] It starts down from its source near the village of Winkel, in the Jura mountains, with a resurgence near Ligsdorf, turns around Ferrette on its east side, and then runs northward through Alsace, flowing parallel to the Rhine.

As the Ill nears the city of Mulhouse, most of its flow is diverted into a discharge channel leading to the Doller, protecting the historical center of the town from floods.

Flowing through the city of Strasbourg, the river forms part of the 17th-century fortifications and passes through a series of locks and channels in the picturesque old town, including the Petite France quarter, where its waters were once used to power mills and tanneries.

[2] Navigation through the section of the central part of this section, through Petite France, is restricted to small pleasure craft in the downstream direction only; upstream traffic and commercial traffic must use an indirect route from the Canal de la Marne au Rhin to the Canal du Rhone au Rhine via the Port of Strasbourg.

Passenger trip boats use this section in the opposite direction, completing their loop via the Canal du Faux-Rempart that is closed to all other traffic.