Kathmandu, Nov. 11 -- Under the October sun, workers rest on boulders by the banks of the Rolwaling Khola. They are building what they already know may not last: a temporary bridge between the villages of Dongang and Thanding directly downstream from the Tso Rolpa glacial lake.

Tso Rolpa used to be a debris-covered glacier with a few ponds 30 years ago. Since then it has steadily expanded into a lake 3.2km long and 100m deep dammed by a fragile moraine perched precariously above the Tama Kosi valley.

The bridge is just a few long logs lashed together, anchored to giant boulders on both sides of the roaring rapids. Until a more permanent trail bridge is built, the community will have to rely on these makeshift crossings that are swept a...