The Kleine Aa rises in East Westphalia 7 km north of the border to Hesse, between Bad Wunnenberg to the north-northwest and Marsberg in the east on the border between Brilon Plateau to the west and Sintfeld to the north.

Its water comes from a small wetland, located approximately 650 m northeast of the Totenkopf (502.6 m above sea level.

A few hundred meters northeast of the source of the Kleine Aa is the source of the Karpke ( "Cold Siepen"), not far south are the northern boundary of the nature park Diemelsee and the headwaters of Dütlingsbach.

Finally, the Kleine Aa flows approximately 3 km south of Fürstenberg on 345.7 m elevation into the southeastern branch of the Aabachstausee, which is traversed by the Aabach.

Since establishment of this small reservoir the last almost 800 m flow path of the stream to its former confluence with the Aabach are flooded by waters of the dam.