We’ve all heard the story: at the end of the last Ice Age, the immense glaciers that covered Canada melted, releasing a colossal weight from the Earth’s crust. Like a memory foam mattress slowly regaining its shape, the land began to rise—a process known as post-glacial or isostatic rebound. Today, in the heart of this ancient ice sheet’s footprint around Hudson Bay, the land is still rising, in some places by over a centimetre a year. But travel to the southern and southeastern coastlines of the bay, particularly in Ontario and Quebec, and you’ll witness a stunning geological paradox: the land is sinking.

This counter-intuitive phenomenon is turning a vast, remote landscape into one of North America’s most vulnerable coastlines, creating a slow-motion drama with profound consequences for its geography and the First Nations communities who have lived there for millennia.

The Glacial Seesaw: A Rebound with a Twist

To understand why a coastline is sinking next to a rising landmass, we need to think of the Earth’s crust not as rigid, but as a flexible surface floating on the semi-molten mantle below. The Laurentide Ice Sheet, which was up to three kilometres thick at its centre over Hudson Bay, created a massive depression in this surface.

As the ice melted and the centre began to spring back up, the areas on the periphery of the former ice sheet—which had bulged upward slightly to compensate for the central depression—started to collapse. This is often called a “peripheral forebulge collapse.”

Imagine pushing your fist into a soft cushion. The area directly under your fist is depressed, but the area around your fist puffs up. When you remove your fist, the centre rises, and the puffed-up ring around it slowly sinks back down. The southern coast of Hudson Bay and James Bay sits right on this collapsing “ring.”

This process of subsidence, or sinking, is compounded by a global factor: climate-change-driven sea-level rise. While the land itself is sinking by a few millimetres per year, the world’s oceans are simultaneously rising. For the communities along this coast, it’s a double whammy. The combination of sinking land and rising seas—what geologists call “relative sea-level rise”—is happening here at a rate faster than almost anywhere in Canada.

A Dissolving Landscape: The Geography of Submergence

The Hudson Bay Lowlands are one of the world’s largest wetlands, a vast, flat expanse of peat bogs (muskeg), marshes, and meandering rivers. The coastline isn’t a neat line of sandy beaches or rocky cliffs; it’s a gradual, muddy, and ambiguous transition from land to sea. This unique geography makes it exquisitely sensitive to changes in sea level.

The effects of the sinking coast are stark and visible:

• Increased Coastal Flooding: Storm surges, which are natural parts of the coastal system, now push seawater much farther inland than ever before. What might have been a minor flood a few decades ago can now become a major, destructive event, inundating entire sections of coastal communities.
• Accelerated Erosion: The soft, unconsolidated silt and clay that make up the shoreline are easily washed away. The sea isn’t just rising; it’s actively nibbling away at the land, causing riverbanks to collapse and coastlines to retreat at an alarming rate.
• Saltwater Intrusion: As the sea pushes inland, saltwater contaminates the freshwater ecosystems. It seeps into coastal ponds, alters the chemistry of the soil, and pushes its way up rivers. This disrupts the delicate balance of the sensitive peatland ecosystem and threatens the freshwater sources that both wildlife and people depend on.

The result is a landscape in flux, where the boundaries between land, river, and sea are constantly being redrawn by a combination of geology and climate.

A Human Story on a Sinking Shore

This is not just an abstract geological phenomenon; it is the lived reality for the Cree First Nations whose ancestors have called this coastline home for thousands of years. Communities like the Kashechewan First Nation, the Fort Severn First Nation, and the Attawapiskat First Nation in Ontario are on the front lines.

The impacts on their lives, culture, and infrastructure are profound and escalating:

Threats to Homes and Infrastructure

The most immediate threat is to the physical existence of the communities themselves. Many are built on low-lying river deltas just metres above sea level. Critical infrastructure—homes, schools, power plants, and the vital airstrips that connect these remote communities to the rest of the country—is increasingly at risk from flooding and erosion. The Kashechewan First Nation, for instance, has endured repeated, devastating floods, leading to emergency evacuations and an ongoing, decades-long discussion about relocating the entire community to higher, more stable ground.

Impact on Traditional Livelihoods

The cultural and economic fabric of these communities is woven from the land and water. The sinking coast directly threatens the traditional practices of hunting, trapping, and fishing that provide both sustenance and cultural identity.

• Saltwater intrusion changes the fish species found in the coastal rivers.
• Erosion and changing flood patterns alter the migration routes of caribou and moose.
• Coastal marshes, which are critical habitats for migratory birds like geese and ducks—a key food source—are being drowned and altered.

Loss of Cultural Heritage

Beyond the immediate threats, there is the heartbreaking loss of heritage. Ancestral burial grounds, historic campsites, and culturally significant places located along the coast and riverbanks are being washed away by the encroaching sea. Each metre of eroded coastline represents an irreversible loss of history and connection to the past.

Living on the Edge: Adaptation and an Uncertain Future

Responding to this challenge is immensely complex. Short-term adaptation strategies include building and reinforcing protective dikes, but as relative sea levels continue to rise, these are often just temporary solutions. For some communities, planned relocation is becoming the only viable long-term option, a process fraught with emotional, logistical, and financial challenges.

The sinking of the Hudson Bay coast is a powerful, localized example of how intricate Earth systems and global climate change can converge. It’s a story written in the mud and marshes of the Lowlands, a reminder that while parts of our planet are still rebounding from the last Ice Age, others are facing a very modern threat. For the people of Canada’s sinking coast, the rising tide is not a future problem—it’s a present-day reality.