The Aller rises in the state of Saxony-Anhalt in the west of the region known as the Magdeburg Börde near Seehausen and north of Oschersleben (Bode).
It has several source streams that run down the northeastern side of the Hohes Holz within the municipal boundaries of Wormsdorf (part of Gehringsdorf), Ovelgünne (part of Siegersleben) and Eggenstedt.
The Eggenstedt tributary is the most southerly of the source streams.
The nearest large centres of population in the source region are Helmstedt, about 20 kilometres (12 mi) northwest, and Magdeburg around 25 kilometres (16 mi) east.

Initially, the Aller flows rather like a canalised brook in a northwestern direction through low hills and intensively farmed arable fields.
As it does, it passes the Flechtingen Hills to the east and another range of hills to the west that stretches from the Lappwald over the Hohes Holz as far as Oschersleben (Bode).
The river passes the villages of Eilsleben and Weferlingen.
After about 60 river kilometres (37 river miles), it reaches Oebisfelde and the southern edge of the Drömling.
After crossing the state border between Saxony-Anhalt and Lower Saxony, the Aller bends sharply southwest near Grafhorst.
Here, the terrain is already only about 55 metres (180 ft) above sea level.
Between here and its mouth, 150 kilometres (93 mi) away, its elevation drops by only about 40 metres (130 ft), so that the speed of the current is considerably reduced in Lower Saxony.
Near Grafhorst, the Aller meets the Breslau-Magdeburg-Bremen glacial valley and then, for the most part, follows it.
From here on, the river mainly runs through grassland.

Near Wolfsburg-Wendschott, the Aller passes under the Mittelland Canal in a culvert where there is a barrier that is impassable to fish.
The Aller swings northwest again from Wolfsburg.
In the area of the town it crosses the Allerpark and flows past the Allersee before flowing past the Barnbruch.


South of Weyhausen, the Aller Canal branches off the Aller.
The canal was built between 1860 and 1863 and is about 18 kilometres (11 mi) long.
It was supposed to drain the long-standing floods that were greatly feared at that time.
The canal runs parallel to, and generally a few miles south of, the Aller, rejoining it near Brenneckenbrück (west of Gifhorn).
The canal runs south past Gifhorn, whilst the Aller flows through the town.
This 20-kilometre (12 mi) long section, which parallels the Aller Canal, is the only section of the Aller that meanders in its natural river channel.
Elsewhere, the Aller has been straightened and widened along its entire length from source to mouth.
East of Gifhorn near Osloß it crosses under the Elbe Lateral Canal, which is carried over it on an aqueduct.
After that it runs via Gifhorn to Müden, where it merges with the Oker.
The Aller rises in the state of Saxony-Anhalt in the west of the region known as the Magdeburg Börde near Seehausen and north of Oschersleben (Bode). It has several source streams that run down the northeastern side of the Hohes Holz within the municipal boundaries of Wormsdorf (part of Gehringsdorf), Ovelgünne (part of Siegersleben) and Eggenstedt. The Eggenstedt tributary is the most southerly of the source streams. The nearest large centres of population in the source region are Helmstedt, about 20 kilometres (12 mi) northwest, and Magdeburg around 25 kilometres (16 mi) east.
Initially, the Aller flows rather like a canalised brook in a northwestern direction through low hills and intensively farmed arable fields. As it does, it passes the Flechtingen Hills to the east and another range of hills to the west that stretches from the Lappwald over the Hohes Holz as far as Oschersleben (Bode). The river passes the villages of Eilsleben and Weferlingen. After about 60 river kilometres (37 river miles), it reaches Oebisfelde and the southern edge of the Drömling. After crossing the state border between Saxony-Anhalt and Lower Saxony, the Aller bends sharply southwest near Grafhorst. Here, the terrain is already only about 55 metres (180 ft) above sea level. Between here and its mouth, 150 kilometres (93 mi) away, its elevation drops by only about 40 metres (130 ft), so that the speed of the current is considerably reduced in Lower Saxony. Near Grafhorst, the Aller meets the Breslau-Magdeburg-Bremen glacial valley and then, for the most part, follows it. From here on, the river mainly runs through grassland.
Near Wolfsburg-Wendschott, the Aller passes under the Mittelland Canal in a culvert where there is a barrier that is impassable to fish. The Aller swings northwest again from Wolfsburg. In the area of the town it crosses the Allerpark and flows past the Allersee before flowing past the Barnbruch.

South of Weyhausen, the Aller Canal branches off the Aller. The canal was built between 1860 and 1863 and is about 18 kilometres (11 mi) long. It was supposed to drain the long-standing floods that were greatly feared at that time. The canal runs parallel to, and generally a few miles south of, the Aller, rejoining it near Brenneckenbrück (west of Gifhorn). The canal runs south past Gifhorn, whilst the Aller flows through the town. This 20-kilometre (12 mi) long section, which parallels the Aller Canal, is the only section of the Aller that meanders in its natural river channel. Elsewhere, the Aller has been straightened and widened along its entire length from source to mouth. East of Gifhorn near Osloß it crosses under the Elbe Lateral Canal, which is carried over it on an aqueduct. After that it runs via Gifhorn to Müden, where it merges with the Oker.
Aller (Germany)