The Arlau rises southeast of Sollwitt in North Frisia. It divides the Goesharde region into North and South Goesharde. Initially it flows a few kilometres south and runs along the municipal boundary of Behrendorf and Bondelum. About 3 km southeast of Behrendorf it changes course and heads in a westerly direction. The Arlau then flows south of Viöl and north of Arlewatt, crosses the Hattstedt Marsh and finally flows into the North Sea at the Beltringharder Koog at the Holm Sluice.