The River Ełk is a right tributary of the Biebrza River. It is 113,6 kilometres long. It starts at the Szerska Góra, near Gołdap. Its initial section, from the estuary to the Łaśmiady Lake is called Czarna Struga and Łaźna Struga. It becomes „the River Ełk” after passing through the Lake Ełckie into the south. It flows into the Biebrza River, in the vicinity of Osowiec, situated on Biebrza Marches. The River Ełk is the part of many surrounding canoeing tracks.