Northern arm of the Baltic Sea .
Extending between Sweden and Finland, it covers about 45,200 sq mi (117,000 sq km). It is 450 mi (725 km) long and 50–150 mi (80–240 km) wide; its average depth is 965 ft (295 m). Because many rivers drain into it, its salinity is very low; ice cover consequently lasts up to five months of the year.