If you stand on the Spreuer Bridge and look upriver, you cannot fail to see the Needle Dam (also known as the Reuss Weir). Built between 1859 and 1861, the weir is a historical monument and marvel of 19th century engineering.
The water level of Lake Lucerne is still regulated manually by the removal or insertion of timber «needles». This regulation – to prevent flooding, for instance – allows the level of the lake to be kept at a height to suit the seasons, regardless of the weather.
The weir was renovated and extended between 2009 and 2011. The work to extend the installation allows the outflow of the lake (e.g. at high water levels) to be increased by a third to around 430 cubic metres a second, thus mitigating the bottleneck effect at the mouth of the river.