Kristian Thomsen and S. A. Angelo drew the plans in 1909. Nine years passed before anyone broke ground, because nothing could start until an agreement was reached with the Gudenå Commission. Work finally began in August 1918, and what followed was earthmoving on a scale Denmark had not attempted before. Five hundred men shifted roughly 300,000 cubic metres of soil, and they shifted it by hand. Before they could build upward they had to dig down, stripping between three and eleven metres of loose overburden to reach clay that would actually hold water back. The finished embankment runs 800 metres long and stands 12 metres high, ten metres across at the crest and fifty at the base — a wall of Jutland set against Jutland's longest river. From the reservoir behind it, water travels a channel 500 metres long and six metres deep to reach the turbine hall.

The station was commissioned on 20 December 1920 and formally inaugurated on 8 January 1921. Inside sat three double Francis turbine-generators, 1.1 megawatts apiece, and for a peninsula of farms and market towns they represented the future arriving in a shed beside a river. Annual production settled at around 11 gigawatt-hours. That figure has not collapsed; the country simply grew past it. What was once nearly a quarter of a region's supply is now a rounding error in a national one. The plant is still Denmark's largest hydroelectric station, which says less about Tangeværket than it does about Danish topography — a landscape so flat that 12 metres of head counts as a resource worth fighting over.

Behind the dam, the Gudenå backed up into its own valley and stayed there. Tange Sø spreads across roughly 540 hectares, three metres deep on average and ten at its deepest, holding some 20 million cubic metres — Denmark's largest artificial lake. Filling it required emptying it first. Five houses and twenty-two farms went under the rising water, and the families who worked that valley floor moved out so that the turbines could turn. A century on, the lake looks like it has always been there. Herons stalk the shallows, anglers work the margins, summer houses line the shore, and the people who use it have no memory of anything else. That is exactly what makes its future so hard to decide.

The dam has always had a fish ladder. Whether a ladder is a substitute for a river is the question that has never been resolved. In 1980 a new one was fitted, carrying 150 litres per second — a sevenfold improvement on the 20 litres per second of its predecessor, and still a trickle set beside the flow of the Gudenå itself. When the original 80-year concession approached expiry in 2001, every argument that had been deferred for decades arrived at once. Environmental organisations pressed for the lake to be drained and the river released to run free for salmon again. A preservation society formed to defend the lake that three generations had grown up beside. Both sides were describing the same body of water and neither was wrong about what it was.

Danish politics handled this the way politics handles anything with no painless answer: in increments. The concession was extended in 2000, then 2002, then 2007, in stretches of two to six years. In 2013 the Folketing scrapped the 1918 concession law altogether and folded the plant into ordinary permitting. Viborg Municipality granted an open-ended permission in February 2015, and on 21 February 2019 the Environmental and Food Appeals Board struck it down, ruling that a proper habitat assessment was required first. The turbines kept turning under transitional provisions. In 2020 the environment minister framed the whole question in terms of the EU Water Framework Directive, which leaves room for "good ecological potential" rather than full restoration. Then in 2025 the representative board of Norlys, the utility that owns the plant, voted down a proposal to dismantle it and drain the lake. Nothing is settled. The water still falls twelve metres, and the fish still take the ladder.