The Alpe is a left tributary of the Aller. Its source lies between the villages of Dudensen and Nöpke, both within the town of Neustadt am Rübenberge. It initially crosses the Dudensen Moor and flows along the edge of the Lichtenmoor and past Rethem Moor. In the 1970s it lost its natural character, it was straightened and, in places, canalised. The Steimbker Dorfgraben, a drainage ditch joining it form the left is very heavily polluted (water quality class III–IV). The Alpe itself has a good overall water quality (class II = moderately polluted). Before the town of Rethem the Weiße Graben ("White Ditch") joins the Alpe along with the Wölpe. In Rethem the Alpe then discharges into the Aller.