From the German Wikipedia

"Laui (Giswil)
Laui, also called Gross Laui in its upper reaches, is a nearly ten-kilometer-long torrent in Giswil in the canton of Obwalden in central Switzerland and left tributary of the Sarner Aa. It is not to be confused with the Lauibach in Lungern, which is a few kilometers away.

Headwaters

The headwaters of the Laui lie in the area of the mountain range between the Glaubenbielenpass pass in the south and Glaubenberg in the north. On the eastern slopes of Nünalpstock (1894 m a.s.l.) and Sattelstock (1769 m a.s.l.) the source streams have created deep erosion funnels (Lätzengraben, Rohrgraben, Unterwengengraben and Gipsgraben). The stream initially flows eastward and overcomes a difference in altitude of more than 1000 meters during its first six kilometers.

Torrent structures

Numerous torrent barriers tame the water of the Laui in this section. They are intended to prevent the debris from reaching the valley floor of Giswil. At the foot of the mountain range to the west of Giswil, a large forested alluvial fan has formed over time from the debris of the torrent. Here, too, it was necessary to dam the stream and build barriers.

Valley floor

At Giswil the Laui reaches the valley floor of the Obwaldner main valley and separates here the two districts of Grossteil and Kleinteil. It finally joins the Aa, the outflow of Lake Lungern and the Kleine Melchaa to form the Dreiwässerkanal, which flows into Lake Sarnen.

Floods and protective dams

During heavy thunderstorms and prolonged precipitation in its source region, the Laui is responsible for flooding the valley floor of Giswil. In 1629, for example, it washed away the old parish church and forced the people of Giswil to abandon an entire part of the village. This catastrophe coincided with the era of witch hunts. Someone had to be blamed for this flood and the destruction of the church. The parish priest recognized the culprits in his parish, and so the biggest witch hunt in the history of Switzerland took place in Giswil. Sixty-three people, besides the "witches" seven men and five children (four boys and one girl), were tortured, sentenced to death by the sword, executed, quartered and burned. According to tradition, the execution took place in the place where the Old Church stands today, built on the foundations of the tower of the destroyed church.
Thanks to the flood protection dams built at the turn of the millennium on the Laui River, major damage was prevented during the August 2005 floods. The protective dams were built from the excavated material from the Giswil bypass tunnel and the Zollhaus cut-and-cover tunnel.
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