Rivers Hvita and Sog change names after their confluence and are called River Olfusa the rest of the way (25 km) to the ocean.  This river is the most voluminous of the country, 373 m³/sec. at Selfoss, where it is 25 m wide and 9 m deep.  Beyond Selfoss the river widens considerably until its spreads to 5 km in the estuaries.  The tides can be traced 10 km inland.  The river’s outlet is relatively narrow between the reefs Oseyrarnes in the east and Hafnarskeid in the west, where the river was bridged in 1988 (360 m.).  The first bridge at Selfoss was built in 1891.  It collapsed in 1944 and a new one was opened in 1946 (132.25 m.)

The flow of the river is relatively even the whole year round because of the great quantities of spring water added to it by many tributaries (i.e. Sog, Bruara, Tungufljot).  The discharge area of River Olfusa is 5,760 km² (65 l/sec. per km²).  Sometimes the course gets flooded, especially during heavy rain and hight temperatures.  Sometimes it dwindles when the tempeature is extremely low in the upper reaches.

River Olfusa has no fords and had to be crossed on ferries or by having the horses swim across. There were several ferry points during the centuries.  The greates accident happened in 1627, when 10 people drowned.

Olfusa is a prolific salmon river.  The salmon is both netted in the estuaries and frequented by anglers.  Seals enter it and sometimes swim upriver to the waterfalls Faxi in River Tungufljot or Waterfall Gullfoss in River Hvita.  Some of the islet in the estuaries are breeding areas for some bird species and islet Laugadaelaholmi, 20 km upriver, has a small breeding colony of eider ducks, which is threatened by minks.