The Aabach is a small river that runs through the Swiss cantons of Lucerne and Aargau, in the Aare River catchment area. It flows from south to north through the Seetal valley (literally lake valley).CourseThe Aabach has its origin in Lake Baldegg, which is fed by the Ron and a number of smaller streams. In the township of Mosen it empties into Lake Hallwil (the Hallwilersee). The Aabach leaves the lake between the townships of Boniswil and Seengen. Hallwyl Castle, one of the most important water-controlling castles in Switzerland, sits in the middle of the river on two artificial islands, about 700m north of the northern end of the lake.On the southern outskirts of Lenzburg an underground tunnel takes much of the flow of the Aabach under the city to rejoin the river in Niederlenz township. It was built as a relief tunnel to mitigate against flooding in the city, which had been occasionally severe in the city during the spring runoff. Further on, in the township of Möriken-Wildegg, the Aabach joins the river Bünz. About 200m downstream the Bünz flows into the river Aar.