Hydrology
The river is the biggest tributary of the Weser. The catchment area of the Aller covers 15,744 square kilometres (6,079 sq mi), about one third that of the Weser itself. The water flow gauge at Rethem below the Aller's confluence with the Leine registers an average volumentric flow of 114 cubic metres (4,000 cu ft) per second.
The Aller receives via the Oker about half the run off from the Harz mountains (the other half drains into the Elbe via the Saale and other tributaries). Consequently the Aller was and is frequently flooded by its tributaries. Since the commissioning of the dams in the Harz it is now possible to limit the impact, especially of seasonal floods, e.g. by delaying and slowing down the release of meltwaters on the Oker and Ecker, the Innerste and the Grane in spring. As a result the discharge of the Aller into the Weser can be largely controlled and, although the level varies, it can be kept within boundaries.
Very little of the Aller is shaded by woods (apart from the section along the Aller Canal). Direct sunlight on much of the river encourages the growth of aquatic plants and, in the absence of shady trees along its banks, the water becomes quite warm during the summer months.
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