This cache is located at the mini bridge near Norðurá. There's a tiny little bridge. You won't miss seeing it. Here's some info about Norðurá:
Norðurá in Borgarfjörður is considered by many anglers to be one of the best salmon fishing rivers in the country with an average annual catch of around 2000 salmon in the last 10 years. The river has its source in Holtavörðuvatn, about 62km from the sea, and the river's catchment area is considered to be about 518km2. There are about 170 marked fishing spots in the river, and their diversity is great from narrow to large and watery broads. The fishing areas therefore offer everything you can think of in one salmon fishing river. In Norðurá there are 3 main waterfalls, Laxfoss,Glanni and Króksfoss, each more beautiful than the other.