Reyðarvatn belongs to District Borgarfjordur.  It is located almost at the beginning of valley Lundareykjadalur.  During earlier geological and geographical stages, it was shortened by subglacial eruptions creating i.e. Mt Thverfell at the present end, furthest east.  At the same time the depression behind the mountain, where the lake is now started collecting water.

The lake area is 8,3 square kilometres and is lies 325 metres above sea level.  The stream Reydarlaekur spill into it from lake Stora Brunnavatn and stream Fossa.  From the northeast it is fed by the streams Fossa and Leira.  Between those streams are the ruins of a farm.  The discharge is called Grimsa.  The lake abounds with lake char.  Sometimes 12 pounders ar caught, but the most common size is 1-2 pounds.  There are quite a few good fishing spots around the lake, especially where river Grimsa discharges it.

The distance from the capital is about 75 kilometres, and the lake is accessible by car.