Caribou cross open lakes as a calculated shortcut during migration, using frozen surfaces to travel faster and more directly. But these crossings come with real danger, especially as climate change makes ice conditions less predictable. What happens on the lake depends on the season: in winter, caribou glide across packed snow and ice with remarkable efficiency; in spring, they risk breaking through thinning ice and drowning; in summer, they crowd along lake shores and snowfields to escape relentless insect swarms.
Why Caribou Choose the Lake
Frozen lakes are essentially highways for migrating caribou. The surfaces are flat, wind-swept, and covered in packed snow, which makes them ideal for moving quickly during long-distance travel. Visibility is better on a lake than in dense boreal forest or broken terrain, so caribou can spot predators from farther away. For a herd covering hundreds of kilometers between seasonal ranges, cutting straight across a frozen lake saves significant time and energy compared to walking around it.
Caribou can and do swim across open water, but swimming is far less efficient and more energetically costly than walking. So when a herd encounters a large water body, they face a real tradeoff: cross it to save energy and time, go around it to access better foraging, or risk the water itself. The decision depends largely on ice conditions and season.
How Caribou Stay Upright on Ice
Caribou hooves are built for frozen surfaces. The outer shell of the hoof provides a hard edge that digs into ice for traction, while fur on the underside of the foot acts like a natural anti-slip pad. This plantar fur also enlarges the contact area with the ground, spreading the animal’s weight more evenly and reducing the chance of punching through thin ice or snow crust. These adaptations let caribou move confidently across surfaces that would send a moose or wolf scrambling.
The Danger of Spring Crossings
The most dangerous moment for caribou on a lake comes during spring migration, when warming temperatures weaken ice from below. Caribou that crossed the same lake safely in November may find it barely holding together in April or May. When ice is solid enough, herds cross rapidly, sometimes at a near-trot. But when conditions deteriorate, caribou change their behavior entirely. They’ll detour around open water, sometimes adding several extra kilometers to their route rather than risk a direct crossing.
Those extra kilometers aren’t trivial. Spring migration coincides with calving season for many herds, and pregnant females are already burning through energy reserves. Longer detours mean more time walking, less time feeding, and later arrival at calving grounds. For caribou that misjudge the ice, the consequences are worse. Drowning and death from exposure after breaking through ice have been documented across multiple Arctic herds, including the Peary caribou of Canada’s northern islands.
Climate change is compressing the window of safe ice travel. Springs are arriving earlier across the Arctic, which means caribou encounter thinner, more fragile ice during the same migration timing their ancestors followed. Some herds have begun adjusting their routes, choosing more complicated overland paths to avoid unreliable ice. But caribou migration routes are deeply ingrained, passed from mothers to calves over generations, and the animals don’t always reroute in time.
What Happens in Summer
Lakes play a completely different role for caribou in summer. Once the ice is gone, caribou aren’t crossing lakes so much as gathering near them. The reason is insects. Arctic summers bring clouds of mosquitoes and parasitic flies that can drive caribou to near-constant movement and serious blood loss. A single caribou can lose enough blood to mosquitoes in a day to measurably affect its health.
To cope, caribou crowd together in large aggregations around bodies of water, snowfields, and wind-exposed ridges. These open habitats have less vegetation, which means fewer insects. The water itself provides some relief, as mosquitoes and flies are weaker fliers over open water and windswept shorelines. Caribou will also seek out river gravel bars and lingering snow patches for the same reason. During peak insect season, a herd’s entire daily pattern revolves around finding these refuges rather than feeding, which can slow weight gain during the critical summer months before winter.
When Herds Lose Their Ice Routes
For island-dwelling caribou populations like the Peary caribou, frozen lakes and sea ice aren’t just convenient shortcuts. They’re the only connection between islands. As sea ice declines, some island populations are losing the ability to reach neighboring islands entirely. This fragments what was once a connected population into smaller, isolated groups with less genetic diversity and fewer options when food runs short on any single island.
Mainland herds face a less dramatic but still meaningful version of the same problem. Lakes and rivers that once froze reliably enough to cross every year are now unpredictable. A herd that historically migrated in a straight line across a chain of frozen lakes may now need to navigate a patchwork of solid ice, open water, and slush. Each obstacle forces a decision, and each detour costs energy that pregnant females, calves, and older animals can’t easily spare.
The cumulative effect is that open lakes, once among the easiest parts of a caribou migration, are becoming one of the most consequential. Whether a lake is frozen solid, barely frozen, or fully open determines not just which route a herd takes but how much energy they burn getting where they need to go.