Weir at the Altmühlüberleiter

You can cycle along it beautifully

The approx. 8.7 km long transfer line connects the Altmühlsee with the Kleiner Brombachsee, which is about 4 m below. The centerpiece is a tunnel approx. 2.7 km long. It passes under the ridge to the east of Gunzenhausen, which separates the water catchment areas of the Danube and Rhine (the main European watershed).

The Altmühlüberleiter was built between 1972 and 1981. The East section was completed in the fall of 1977. The punch of the tunnel took place on February 13, 1978. In 1985, the canal went into operation. Before the construction of the tunnel, the subsoil was explored using drill holes and a test tunnel. After that, a milling-boring machine drilled the actual tunnel.
de.wikipedia.org/wiki/Altmühlüberleiter