By ship movements, no artificial seaway in the world carries more traffic. Around 26,000 to 30,000 vessels pass through in a typical year — 26,659 in 2023 — moving on the order of 70 million tonnes of cargo. Panama and Suez handle bigger ships and far greater tonnage; the Kiel Canal handles sheer numbers, a near-continuous procession of coasters, feeder container ships, tankers and yachts, roughly seventy a day. It is fresh water throughout, and it picks up one more connection along the way: at Oldenbüttel the short Gieselau Canal links it to the navigable River Eider. From the towpath the effect is faintly surreal — a 200-metre ship sliding through farmland, above the level of the fields, apparently moving overland.

There had been a link before, dug when this coast answered to Copenhagen. The Eider Canal, completed in 1784 during the reign of Christian VII of Denmark, used stretches of the Eider itself: 43 kilometres of cut forming part of a 175-kilometre waterway from Kiel to the river's mouth at Tönning. It was 29 metres wide and three metres deep, which capped the vessels that could use it at 300 tonnes. That was adequate for eighteenth-century coastal trade and useless for anything the nineteenth century produced. By the time Prussia held Schleswig-Holstein, the Eider Canal was less a shortcut than a reminder that a proper one did not exist.

Construction began at Holtenau in June 1887 and took eight years. Up to 8,900 workers shifted roughly 80 million cubic metres of earth, and the German Reich paid the bill of 156 million marks. On 20 June 1895, Kaiser Wilhelm II opened the waterway by transiting it from Brunsbüttel to Holtenau; the following day, at a ceremony in Holtenau attended by dignitaries including Prince George, Duke of York, he laid the final stone and named the canal after his grandfather, the Kaiser Wilhelm Kanal. Regular traffic began on 1 July. The British film pioneer Birt Acres filmed the opening, and the surviving footage is preserved by the Science Museum in London — among the earliest moving pictures of a civil engineering project anywhere. Ships had already been through: an aviso made the passage in late April to confirm the canal was usable, and a tender followed in May.

The canal was barely a decade old when it became too small. The Imperial German Navy was building Nassau-class battleships, and a strategic shortcut that a dreadnought could not fit through was no shortcut at all. Between 1907 and 1914 the channel was widened to 102 metres and deepened to 11, at a cost of 242 million marks — more than the original construction. New locks went in at both ends. The 1895 chambers measured 125 by 22 metres; the pairs completed in 1914 measured 310 by 42, which gives a fair sense of what had changed in nineteen years. After the war, the Treaty of Versailles required the canal to stay open to the commercial and naval vessels of any nation at peace with Germany, while leaving administration in German hands — a formula the United States opposed, wary of the precedent it might set for Panama. Hitler's government repudiated the canal's international status in 1936. It reopened to all traffic after 1945.

Passage is tightly governed. Every vessel is sorted into one of six traffic groups by its dimensions; larger ships must take a pilot and a specialised canal helmsman, sometimes a tug as well, and may be ordered to tie up at bollards along the bank to let oncoming traffic through. The hard limits are 235.50 metres in length, 32.50 metres in beam and seven metres of draught, though a ship under 160 metres may draw as much as 9.50. Every fixed bridge, without exception, clears at 42 metres. There are eleven of them, the oldest still in service being the Levensau High Bridge of 1894, due to be replaced as the current expansion advances. The Rendsburg High Bridge carries the railway and, slung beneath it, a transporter ferry that was destroyed in a collision with a ship in 2016 and returned to service in 2022. Fourteen ferry lines cross the canal for local traffic, all run by the canal authority, all free. The widening never really stopped: the western half was enlarged between 1965 and 2001, and work on the eastern half began in 2020, with the first four-kilometre section opening on 10 November 2025 and five more to follow.