Tidewater Glaciers — What You're Watching When the Ice Falls

The star of every Alaska cruise is a river that forgot to melt: a tidewater glacier — a glacier whose face flows all the way down to the sea and ends in a wall of ice standing in saltwater, shedding pieces the size of appliances, houses and occasionally apartment blocks. Knowing what you’re watching upgrades the show considerably, so here is the five-minute glaciology every deck deserves.

The mechanics, minus the mathematics

A glacier is snow that outlived its winters: compressed over centuries into dense ice, deep enough that gravity puts the whole mass in motion — flowing downhill like an extremely patient river, at speeds from centimeters to (in Alaska’s fast cases) meters per day. A tidewater glacier is one whose valley reaches sea level before the ice runs out: the face meets the ocean, the ocean undercuts the face, and the glacier does what rivers do at their mouths — it lets go. Calving is that release: ice cracking off the terminus and dropping into the sea, from popcorn-sized “bergy seltzer” to full-face collapses that send swells across the fjord (which is why the boats hold their respectful distance — captains are reading the face constantly).

The two spectacles

• White thunder — the Tlingit name for the sound is the honest description: calving cracks like artillery and rumbles like weather, often before you see the fall. When the deck goes quiet and everyone stares at the face, they’re listening for it.
• Blue ice — glacier ice runs deep blue because centuries of compression squeezed the air out: dense ice absorbs the red end of the spectrum and hands you back the blue. The bluest bergs are the oldest, freshest-calved ice; overcast days — Alaska supplies them — paradoxically show the blue best. The otters and seals hauled out on the bergs are using the ice exactly as advertised: predator-free furniture.

Where the show runs

Alaska’s cruise fleet works the great tidewater faces daily in season: Kenai Fjords (Aialik and Holgate glaciers from Seward), Prince William Sound (the twenty-plus-glacier circuits from Whittier, including some of the state’s most active faces), and the Juneau-area day boats (compared here). The etiquette of the wait is universal: boats sit engines-low before the face for twenty-odd minutes, everyone watching; ice falls on its schedule, not yours; and roughly half the calving happens the moment you look down at your camera roll. Keep the lens up.

One sobering footnote the naturalists deliver honestly: many of Alaska’s tidewater glaciers are in historic retreat — the cruise is partly a document of a landscape in motion. It sharpens the watching; the show is not guaranteed to look like this forever.