The Beijing-Hangzhou Grand Canal, stretching over 1,700 kilometers, is the world's longest artificial waterway. Centuries before European engineers popularized double-gate pound locks, imperial Chinese canal builders faced a critical problem: navigating barges across sudden shifts in terrain elevation and water levels. Their solution was the Haoba, or double inclined stone slipway lock.
Constructed from finely dressed granite slabs coated with slippery clay or grease, a Haoba consisted of two opposing ramps meeting at a raised stone crest that acted as an embankment dam. To transfer a heavy grain barge from a lower canal section to a higher one, teams of oxen or dozens of laborers used massive winches, ropes, and windlasses attached to capstans to haul the vessel directly up the greased stone slope. Once over the ridge, the boat slithered safely down the opposing incline into the higher body of water.
This mechanism preserved precious canal water by eliminating the need to flush massive volumes downstream, a common drawback of early European flash locks. While pound locks eventually replaced many Haoba during the Song Dynasty under engineer Qiao Weiyue, these stone slope locks remained essential for centuries in drought-prone sectors where water conservation was vital to keeping imperial grain flowing to Beijing.