The Schwarzbach does not have its own source, but arises at the Mönchbruch hunting lodge, directly on the federal highway 486, from the confluence of the Gundbach and Geräthsbach. The significantly longer Gundbach has its source as Hengstbach south of Dietzenbach-Hexenberg in a swampy area close to the forest. Its source is not found and is difficult to reach. The Schwarzbach flows after its origin from Gundbach and Geräthsbach through the Treburer Unterwald and the pond forest before the stream bed leads through Nauheim. In Nauheim there is a level measuring point of the Hessian State Office for Environment and Geology (HLUG). The level zero point is at 85.91 m above sea level. NN. Between Nauheim and Groß-Gerau, after crossing under the state road L 3482 (the former B 42), the Hegbach flows into the Schwarzbach from the left and east, 1000 meters further west followed by the Mühlbach coming from Darmstadt-Arheilgen and flowing through Groß-Gerau . The Landgraben, which rises between Griesheim and Wolfskehlen, flows southwest of Trebur into the Schwarzbach. To the south of Trebur, the Schwarzbach then flows further west towards the Rhine. At Astheim it then bends north and then flows south of Ginsheim into the Ginsheimer Altrhein - a branch of the Rhine. The Old Rhine is connected to the Rhine again after 1.5 km.
To the south-east of Ginsheim there is a flood barrier / pumping station, which is closed when the Rhine floods and pumps the water that has accumulated in the Schwarzbach over the dike.