The Aller is a river in Germany (Lower Saxony, Saxony-Anhalt). It is 263 km long. The catchment area measures 15,742.9 km². It rises in the Magdeburger Börde highlands and flows in a broad valley along the southern edge of the Lüneburger Heide. Where it flows into the Weser from the right.
It reaches its highest water levels in spring. The average flow at the mouth is 126 m³/s. The river is navigable and water transport is possible for 117 km from the mouth. Where the towns of Celle and Verden are situated on the river. Important tributaries include: the left – Oker, Fuhse, Leine and the right – Ise, Örtze, Böhme.
The river is the largest tributary of the Vezera. The river basin covers an area of 15 744 km², which is approximately one third of the area of the Weser itself. Approximately half of the runoff from the Harz Mountains reaches the Aller via the Oker. The Aller was and is therefore frequently flooded by its tributaries.
The most important tributaries, which enter the river from its left (south) side towards the Harz, are the Oker at Müden, the Fuhse at Celle and the Leine at Schwarmstedt. If the Leine is counted as a source stream of the Aller, it has a total length of 346 km. Smaller rivers join from the right and drain mainly the Lüneburg mountains. Worthy of mention are the Kleine Aller quite close to Weyhausen, the Ise in Gifhorn, the Lachte east of Celle (near Lachtehausen), the Örtze near Winesen (Aller), the Meiße near Hodenhagen and the Böhme near Rethem.
