The Haaren, which has its source in Northwest Lower Saxony on the Oldenburg/East Frisian Geest, has a length of 23.8 km and a catchment area of 115 km². Of this, 75 km² are in the district of Ammerland and around 40 km² in the area of the city of Oldenburg.[1] It is the only river in the Ammerland Geest that drains via the Hunte into the left-hand Weser catchment area. The entire water system of the Haaren is today very little natural and structurally heavily over-formed. The Haaren water eight based in Petersfehn is responsible for water maintenance.
The Haaren rises without a spring near Rastede (+19 m asl) in the district of Ammerland and, following small glacial valleys, flows through the municipality of Wiefelstede from the north-east to the south-west, before turning east in the municipality of Bad Zwischenahn near Petersfehn into lower-lying, heavily drained moor areas . Here the Haaren reaches the urban area of Oldenburg, where it cuts deep into the Geest there (inner-city Haaren lowland: +2.50 to +2.75 m above sea level). The Haaren flows into the tide-influenced Hunte in the center of the city of Oldenburg via a sluice and estuary pumping station at the traffic jam.