The glacier is named for a nunatak — a peak that stands clear of the ice — which from a distance reads blue-black against all that white, and which older eyes saw as a figure walking with a pack on its back. Its apparent size changed through the season as the snow rose and fell around it, and people in the valleys below used to read the change as a forecast for the coming grain harvest. Blåmannen, the Blue Man, gave his name to the whole icecap: Blåmannsisen, the Blue Man's ice. In Sami the glacier is Ålmåjalosjiegŋa. Both names point at the same thing — a dark human shape in an otherwise featureless white field, the one landmark on a surface that offers almost none.

Blåmannsisen is the fifth-largest glacier on the Norwegian mainland, straddling the boundary between Fauske and Sørfold municipalities just two kilometres west of the Swedish border. Its high point, the Blue Man himself, stands at 1,560 metres; the lowest ice reaches down to about 810. The area is the part that keeps moving. In 1961 the icecap covered 87.8 square kilometres. By 2011 it was down to roughly 80. Along the southern margin the ice has pulled back far enough to expose a forefield — bare ground uncovered since the Little Ice Age maximum — with a well-formed end moraine still marking where the front used to stand. The equilibrium line, the altitude above which more snow falls than melts, sits around 1,100 metres on the eastern side. Satellite imagery shows a great deal of old firn exposed above it, which is what a glacier looks like when it is spending more than it earns.

Three outlet glaciers run off the icecap, and each behaves differently. To the east, the largest of them descends toward the lake Leirvatnet and calves into it, breaking off ice at the snout. To the west, another drops steeply and is torn open by the gradient. To the north, a small outlet spills across a buried ridge and dams the lake that periodically empties itself. The glacier also calves into Blåmannsisvatnet, and the ice above that lake is shattered accordingly. Crevassing at every margin makes a traverse genuinely dangerous — most of all in late spring and early summer, when a smooth cover of snow is still lying over gaps it is no longer strong enough to bridge. The middle of Blåmannsisen is a plateau. The edges are not.

Almost none of this meltwater is allowed to find its own way to the sea. Tunnels take roughly a quarter of Blåmannsisen's runoff into Sisovatnet to feed the Siso power station; the rest drains into the Norwegian networks operated by Elkem and Saltens Kraftsamband, or eastward across the border into the Lule River system in Sweden. That arrangement is why the outburst floods are measured so carefully — a sudden 40 million cubic metres is a hydrological event, but it is also a reservoir management problem. There is a particular northern logic to it: an icecap that has been shrinking for six decades is still, for now, one of the region's most reliable sources of power, and the more it melts the more it delivers. That bargain has an end date. Everyone involved knows roughly what it is.