To make her threat believeable she went barefoot on a protest march from Gullfoss to Reykjavik. In those days the roads weren't paved and when she arrived after 120 kilometers her feet were bleeding and she was in very bad shape.
The people believed her and listened and the powerplant at Gullfoss was never built.
Today one can see the memorial site of Sigríður that decipts her profile at the top of the falls.