From there, after flowing some 20 kilometres (12 mi) in a northwestern direction, it leaves Belgium and enters the Netherlands at Cottessen in the Vaals municipality.

After a further 38 kilometres (24 mi) in a west-northwestern direction through the southernmost part of Limburg, which in its turn is the most southern province of the Netherlands, it flows into the Meuse, north of the city of Maastricht.

This pollution is a result of former zinc and lead mining activities near the Belgian towns of Kelmis (La Calamine) and Plombières (formerly named Bleiberg).

Although mine factory buildings have been demolished, old railway dikes still are abundantly present in the area.

They provide ideal hiking routes, that are known for the presence of so-called "zinc flora" (zinc-tolerant plants, such as yellow calamine violet and Viola calaminaria).