The Oker is a river in Lower Saxony, Germany, that has historically formed an important political boundary. It is a left tributary of the River Aller, 128 kilometres (80 mi) in length and runs in a generally northerly direction.

The Aller is a 215-kilometre-long (134 mi) river in the states of Saxony-Anhalt and Lower Saxony in Germany. It is a right-hand, and hence eastern, tributary of the Weser and is also its largest tributary. Its last 117 kilometres (73 mi) form the Lower Aller federal waterway. The Aller was extensively straightened, widened and, in places, dyked, during the 1960s to provide flood control of the river. In a 20-kilometre-long (12 mi) section near Gifhorn, the river meanders in its natural river bed.