A canal needs a watertight bridge deck
A navigable canal cannot simply stop at a river valley. At a waterway aqueduct, the canal itself is carried across the gap in a long, watertight trough. Its sides hold the water in place, while the structure beneath supports the weight of both the water and passing vessels.
That load is substantial: water is heavy, and the trough must also resist the outward pressure of water against its walls. The bridge’s supports transfer these forces down into the ground, while the channel’s lining and joints keep water from leaking into the structure.
Keeping boats on one level
The key advantage is continuity. A boat stays on the canal’s existing water level as it crosses above the river. Without the elevated crossing, the canal might need locks—enclosed chambers that raise or lower boats between different water levels—on both sides of the valley. Locks take time and water, and they require boats to stop while the water level in each chamber changes.
An aqueduct does not eliminate every engineering challenge. It must connect smoothly to the canal banks, remain strong under changing loads, and be maintained so that leaks or damage do not interrupt navigation. But where the terrain allows it, carrying the canal across the obstacle preserves a steady route.
Two waterways, one crossing
The design separates the traffic of the river below from the navigation channel above. River water can continue along its course while boats travel overhead, avoiding a descent to the river and a climb back to canal height. The striking part is not just that a bridge holds water: it makes the canal behave as though the valley were never there.
