The Sog drains Þingvallavatn, Iceland's largest lake, and runs some twenty-two kilometres of the clearest, steadiest water in the country down to its meeting with the glacial Hvítá, where the two form the Ölfusá. It is the greatest spring-fed river in Iceland: fed from the lake rather than from rain or ice, it holds a stable flow of around a hundred and ten cubic metres a second and never blows out in spate nor drops away in drought. In places it spreads to a hundred and fifty metres over an irregular rocky bed, gin-clear above the stones, threaded through farmland and birch with three mid-century hydro dams on its upper reaches. This is sight-fishing water for salmon and Arctic char — clear, broad and even-tempered. Wading is over uneven rock and ledge; the clarity that makes it beautiful also makes the fish wary.

Wading: Uneven rocky bed under very clear water