The river springs as Weeser Aa southeast of the Ankum Heights and east of Merzen in Lower Saxony. After crossing the boundary between Lower Saxony and North Rhine-Westphalia, it changes it name into Halverder Aa and near Halverde (a district of Hopsten) into Halverder-Schaler Aa. Near Schale (a district of Hopsten), it changes its name again into Schaler Aa. Finally, it discharges into the Große Aa south of Freren.