It is about 23 kilometres (14 mi) long and up to 5 metres (16 ft) wide, and flows from north to south through the district of Gifhorn and the city of Wolfsburg.

In the Middle Ages the valley of the Kleine Aller created a natural, north-south oriented border between two regions.

The Kleine Aller has been able to carve out a valley during the course of its existence which, in its middle reaches, is about 10 m deep and about 1,000 m wide in which a eutrophic peat fen has become established.

In earlier times the valley was only used as grassland due to frequent flooding; today the river meadows are mainly used for arable farming.

This is due to the fact that the waterway receives the discharge from five sewage works and that the nitrogen content in these installations has not been reduced sufficiently.