Ölfusá is Iceland's largest river by volume, born where two very different waters meet just north of Selfoss — the glacial Hvítá, milky with the rock-flour it carries off Langjökull, and the Sog, the country's greatest spring-fed river, gin-clear from Þingvallavatn. From that confluence it runs only twenty-five kilometres to the Atlantic, but it moves an average of more than four hundred cubic metres a second through a broad lowland channel, twenty-five metres wide and nine deep at the town, spreading to a five-kilometre estuary of lagoons and sandbars at its mouth. It drains a vast basin of some 5,760 square kilometres of southern lava plain. The character is big, powerful, semi-glacial water on an easy gradient — coloured and strong when the glacier feeds it, clearer when the spring water dominates. Wading is a question of volume and soft margins rather than slippery rock.

Wading: Sheer volume and soft estuarine margins