The Begna watercourse system receives its water from the southern part of the Jotunheimen, Gol and Hemsedal mountains ranges. The water collects in the Aurdalsfjord before flowing into the river Begna to Hønesfoss and the Tyrifjord. At Hønefoss, Begna has an average annual flow of 90 m3/s. The total catchment area of the Aurdalsfjord is just over 2,600 square kilometres.

Åbjøra is a 60-km long tributary of the Begna watercourse system. Åbjøra includes the lakes Helin, Flyvatn and Storevatn, which stretch across several municipalities in Oppland and Buskerud. From there, the water flows down to the Tisleifjord and Bløytjern lake. Bløytjern is the intake reservoir of Åbjøra Power Plant, which is owned by Skagerak Kraft. The outlet from Åbjøra goes into the Aurdalsfjord/Begna, from which the water continues to Bagn Power Plant. Åbjøra’s catchment area is around 800 square kilometres in size. Nearly all of it is more than 800 metres above sea level, with the highest point at 1,900 metres.

The first industrial use of Begna watercourse system was related to the wood processing businesses around the watercourse. Since wood processing is a power-intensive industry, proximity to the watercourse was crucial for timber floating and as a source of power. Saw mills came first, followed by pulp mills and paper factories.

*The figures refer to plants where Skagerak Kraft holds an ownership interest