Western opening
From the early Middle Ages until 1825, the Limfjord was only open to the sea at its east end. In that year, the North Sea broke through from the west and created a second opening, turning the northern part of Jutland Vendsyssel-Thy into an island.
An isthmus of shifting sand had separated the Limfjord from the North Sea during historic times. The present inlet, Thyborøn Channel, has existed only since 1862. A passage also existed during the Viking Age. Based on place names and the geography, it is thought to have been to the south of the present one, between Ferring Sø (locally still called 'the Fjord') and Hygum Nor. Canute the Great sailed into it in 1027 on his way back from England. According to Saxo Grammaticus it closed at some time around 1200. There are records of several floods piercing the isthmus during the 16th to early 19th century.
On 3 February 1825 a flood pierced a permanent opening, the so-called Agger Channel, in the north of the 13 km long and less than 1 km wide isthmus, the Agger Tange, which had until then linked Thy with the rest of Jutland. The western part of North Jutland lost its road connection with mainland Jutland, but the towns and harbours in the western part of the Limfjord could benefit from becoming directly accessible from the North Sea, to the dismay of Aalborg. From the 1840s the western route became increasingly important, as Britain had opened for import of grain, and ships could return with British coal. However, the instability of Agger Channel made the towns of the western Limfjord look for a second option.
In 1862, a flood pierced another opening, the Thyborøn Channel, through the remainder of Agger Tange (see satellite image[1]). Agger Channel was continuously filling with sand and eventually closed in 1877. Since then, the remaining Thyborøn Channel has been kept open and navigable by dredging. The harbour of Thyborøn was built in 1914–1918 and a town was founded. The two isthmuses have shifted eastwards since the 1800s. They have only been preserved by groynes, persistent sandpumping and two road dams along their inward (eastern) side.
Plans for re-closing the western inlet
A 1946 act provided for re-closing the Channel with dams and sluices at Thyborøn, but this was never carried out. This idea re-emerged in 2005 and is now[when?] officially being investigated. It is thought that the isthmuses would be easier to preserve, and that the water level of the Limfjord would be more controllable. In periods of persistent western winds, flooding occurs on low-lying land and harbour areas in the towns of the western Limfjord, since the water can't escape through the narrow, eastern part of the Limfjord.[2]
Historically, a North Sea to Limfjord canal has also been proposed between Hanstholm harbour and Vejlerne (or Klitmøller and Thisted), e.g. by Jørgen Fibiger in 1933,[3] the engineer behind the project of Hanstholm harbour.