In reply to UKC Articles:
Nice overview / trip report - I walked down to the Mer de Glace from Montenvers in the mid 90s. I hadn't been there for nearly 20 years.
Almost reflexively, I took the path to the right which was the old route down, resulting in confusion and consternation.
I retreated, puzzled, and then noticed the sign "to the refuges" which was the "new" route down. Much longer than the old route.
It's instructive to use Google Earth's historical image data to see where the snouts of glaciers used to be. The retreat, even in the brief 10 years that the imagery covers is sobering. As is the prominence of the glacial river on the surface of the Mer de Glace these days.