Björkfjärden, Fjard in eastern Mälaren, Sweden

Björkfjärden, properly Södra och Norra Björkfjärden, is Lake Mälaren's largest fjard, covering 349 square kilometres with a maximum depth of 54 metres. Its northern part holds the islands Kurön, Adelsön and Björkö, the last being the site of the Viking town Birka; the southern part includes the islet Midsommar and Slandön. The boundary between the two halves is where Uppsala, Stockholm and Södermanland counties meet.

Björkö, in the southern Björkfjärden, was the site of the Viking town of Birka. The three counties of Uppsala, Stockholm and Södermanland meet at the boundary between the northern and southern parts of the fjard.

The name Björkfjärden comes from the Swedish word for birch, reflecting the birch forests that once bordered these waters. Today, locals use the fjard for fishing and boating, with islands serving as popular destinations for walking and exploring nature.

The fjard is best explored by boat from various access points along the surrounding shore. Visitors should prepare for changing weather conditions and be aware of the scattered islands when navigating the waters.

The border between Södra and Norra Björkfjärden is where Uppsala, Stockholm and Södermanland counties meet, a tripoint in open water. The northern part also holds Norra Björkfjärdens nature reserve, created in 1968 and enlarged in 1995, and Svalgarn, a reserve of Upplands-Bro since 1993.