How canal builders lifted boats over hills without flooding the valleys
Water always runs downhill, yet canals must stay almost perfectly level. Engineers solved the problem with chambers, lifts, bridges and tunnels, and China worked out the double-gated lock by the 10th century, some five hundred years before Europe. Its Grand Canal, 1,794 kilometres from Beijing to Hangzhou, still carries heavy traffic.
Strictly speaking, not every canal is a canal. A navigation follows a river valley, borrowing the calm stretches of the river and smoothing out the rest with weirs, dams and locks. A true canal crosses a drainage divide to join two separate river basins, which means it needs a water supply above its highest point. The Panama Canal is the best-known example, and such routes are harder to build, often needing aqueducts and viaducts along the way.
The pound lock does most of the climbing: a chamber with gates at both ends, filled or drained to raise or lower a boat between levels. Before it, boats used flash locks with a single gate, which only worked where water was plentiful, or were dragged up ramps, sometimes over rollers. Since every trip through a lock spends a lot of water, dry routes use boat lifts instead. Scotland's Falkirk Wheel carries vessels floating in a water-filled caisson from one level to the other.
Other obstacles call for bolder answers. The Pontcysyllte Aqueduct carries boats across the valley of the River Dee in Wales and is now a UNESCO World Heritage Site. The Harecastle Tunnel bores straight through a hill on the Trent and Mersey Canal, a trick that suits only small canals. Early contour canals took the lazy route, winding along ground of constant height to avoid any change of level. Where the soil leaks, builders line the channel with clay, a process called puddling.
Canals carried the coal and ore that fed the Industrial Revolution, then lost out to railways from the 1840s in Britain. By the early 1880s many had been abandoned. Astronomers once used the word for straight lines they thought they saw on Mars, which modern images show do not exist.
Source: Canal
