A causeway is a raised road or pathway built across low, wet, or submerged ground, allowing people and vehicles to cross water, marshland, or other terrain that would otherwise be impassable. Unlike a bridge, which spans open water from above, a causeway typically sits on or near the surface, supported by packed earth, stone, or concrete fill. Some causeways are permanently above water, while others are designed to be submerged during high tide or heavy rainfall.
How Causeways Differ From Bridges
The key distinction is how they interact with the landscape beneath them. A bridge lifts traffic high above water or land, leaving open space below. A causeway pushes through the environment at ground level, often built by filling in shallow water or soft soil with compacted earth, gravel, or stone and then paving over it. This makes causeways cheaper and faster to build than bridges, especially over long distances of shallow water or swampland.
That said, the line between the two blurs in practice. Many modern causeways include bridge sections to allow boats to pass or water to flow beneath them. The 25-kilometer King Fahd Causeway connecting Saudi Arabia to Bahrain, for example, combines long stretches of filled roadway with elevated bridge segments over deeper channels.
What Causeways Are Made Of
Materials depend on the era and purpose. Ancient causeways were built from packed earth, timber, and stone. Modern causeways use reinforced concrete slabs laid over compacted embankments, with stone revetment (heavy rocks layered along the sides) protecting against erosion from flowing water. U.S. Army Corps of Engineers specifications call for selecting stone size based on the speed of water approaching the causeway: faster currents require heavier rock to prevent washout.
Where causeways cross streams, irrigation canals, or areas with continuous groundwater flow, engineers install small bypass pipes beneath the road surface to let water pass through without overtopping the structure. These pipes, typically about 100 millimeters in diameter, connect the upstream and downstream sides so water pressure doesn’t build up and damage the roadway.
Ancient Causeways and the Egyptian Pyramids
Causeways are among the oldest engineered structures in human history. In ancient Egypt, every major pyramid complex included a long, sloping causeway connecting two temples: a valley temple at the water’s edge and a mortuary temple beside the pyramid itself. These weren’t just practical walkways. They were ceremonial roads, integral to the religious rituals surrounding the pharaoh’s burial.
A 2024 study published in Nature found that many of these causeways run perpendicular to a now-dry branch of the Nile River, terminating directly at what was once its bank. The valley temples at the ends of the causeways served as river harbors, where visitors and funeral processions arrived by boat before walking the raised path up to the pyramid. When the river branch eventually dried up thousands of years ago, the causeways lost their connection to the water, but the structures themselves survived.
Natural Formations Called Causeways
Not every causeway is built by humans. The Giant’s Causeway in Northern Ireland is a geological formation made up of roughly 40,000 interlocking basalt columns, created about 62 to 63 million years ago when volcanic lava reached the surface, cooled, and solidified into rock. The process happened as tectonic forces were pushing North America and Europe apart, forming what is now the North Atlantic Ocean. The columns, many with distinctive hexagonal tops, look strikingly like a paved road descending into the sea, which is how the formation got its name.
Natural land bridges connecting islands to the mainland at low tide are also commonly called causeways. Lindisfarne in northern England, for instance, is accessible by a tidal causeway that disappears under several feet of water twice a day.
Famous Modern Causeways
Some of the world’s longest road connections are causeways. The Lake Pontchartrain Causeway in Louisiana stretches nearly 39 kilometers across open water, making it one of the longest overwater highway structures on Earth. The King Fahd Causeway between Saudi Arabia and Bahrain, completed in the 1980s at a cost of $800 million, runs 25 kilometers and carries tens of thousands of vehicles daily.
Shorter causeways are far more common. Coastal communities around the world rely on modest causeways connecting barrier islands to the mainland, crossing marshland, or providing access to otherwise isolated peninsulas. In many tropical and low-lying regions, causeways remain the most practical way to build roads across wetlands where traditional foundations would sink.
Environmental Trade-Offs
Because causeways sit directly in the landscape rather than above it, they can significantly alter the ecosystems around them. The most documented problem is water flow disruption. A causeway built across a lagoon, bay, or tidal flat acts like a dam, splitting what was once a single body of water into two separate environments that behave differently.
Research on North Bull Island in Dublin, Ireland, showed exactly this effect. A causeway built to provide access to the island split a back-barrier lagoon in two, and the consequences were measurable. Tidal water on the far side of the causeway rose 10 to 25 minutes later and drained on a different schedule than the near side. Over time, these altered tidal patterns changed how sediment accumulated on the salt marshes flanking each side of the causeway, meaning the two halves of what was once a single ecosystem began evolving in different directions. With sea levels rising, that divergence will only accelerate.
Sedimentation is another major issue. When a causeway blocks natural water movement, sand and silt build up on one side, gradually filling in channels that once flushed the area clean. This is precisely what happened at Mont Saint-Michel in France, one of Europe’s most iconic landmarks. A concrete causeway built in the 19th century trapped so much sediment that the island, famous for being dramatically surrounded by the sea, was increasingly surrounded by dry sand instead.
The Mont Saint-Michel Fix
France’s solution at Mont Saint-Michel illustrates how engineers now think differently about causeways. Starting in the 2000s, the old concrete causeway was demolished entirely and replaced with a bridge on stilts, allowing water to travel freely beneath it. A new eight-gated dam was built on the Couesnon River to regulate flow between the river and the sea, using the current to actively scour away the accumulated sediment at the island’s base.
The results were tangible. Ground levels around the island dropped, the sea moved faster through the area, and the river began flowing on both sides of the mount again. The project had a dual purpose: restoring the island’s historic character as a place surrounded by tides, and better managing the millions of tourists who visit each year. It’s a case study in why modern infrastructure projects increasingly favor bridges or causeway-bridge hybrids over solid-fill causeways in ecologically sensitive areas.