In English the Gudenå is routinely called Denmark's longest river, and at 149 kilometres it is comfortably the longest thing of its kind in the country. In Danish the phrasing is more careful: Danmarks længste vandløb, Denmark's longest watercourse. The distinction is not pedantry. Danish separates a flod, a proper river, from an å, a stream or small river, and the Gudenå is an å - the name says so. Danish hardly ever applies flod to anything at home. In a country whose highest point is under 200 metres and whose rivers move slowly through soft glacial material, the Gudenå is the largest freshwater system there is, draining a catchment of 2,638 square kilometres, and it is still, by the standards of most of Europe, a modest stream. Both things are true, and holding them at once is the only accurate way to describe it.

The valley is roughly 15,000 years old, cut at the end of the last glaciation when meltwater and glacial streams carved their way out of the retreating ice. That origin explains why the Gudenå behaves less like a river than like a chain. Instead of running through a single trench it threads a whole string of lakes through the high, hummocky country of Søhøjlandet - Mossø, Gudensø, Rye Mølle Sø, Birksø, Julsø, Borre Sø, Brassø, Silkeborg Langsø, Sminge Sø, and others besides. Between them the water widens, slows and turns glassy, then narrows into channel again. Himmelbjerget rises above Julsø, one of Denmark's best-known hills, and the reason it feels like a mountain to Danish eyes is that the ice piled the land up here and then cut it apart with water.

People have been putting the Gudenå to work and to pleasure for a long time. Silkeborg grew up on its banks around a paper mill and a harbour, and from that harbour the paddle steamer Hjejlen still runs across Julsø to Himmelbjerget, carrying passengers on a route that has been a Danish summer ritual for generations. In canoe season the upper reaches fill with hire boats working downstream between the lakes. In December 1920 the river was dammed at Tange to feed Tangeværket, the country's largest hydroelectric plant, and the reservoir behind it became Tange Sø. Whether that dam should stay, go, or be bypassed to let migrating fish through has been argued in Denmark for decades and is not settled yet - a genuinely difficult question, because the lake is now a century old and has its own constituency.

The Gudenå once ran salmon, and the loss of them is older than the dam. Major dredging of the channel had already stripped out many of the good spawning gravels before the twentieth century's engineering arrived, so the story of what the river lost is a story of accumulated small decisions rather than one dramatic act. What remains is still substantial. The system is fed by a dense web of streams and wetlands along its whole length, it shelters a wide range of animals, and stretches of its course fall under Natura 2000 protection, the European framework for conserving habitats. From Tinnet Krat the water goes north through the lakes, past Silkeborg, on through the flat eastern farmland, and finally into Randers Fjord at Randers - a long, narrow inlet of the Kattegat that gives the Gudenå its exit to the sea. One hundred and forty-nine kilometres, and it never once hurries.