Binnenalster (pronounced [ˈbɪnənˌʔalstɐ]) or Inner Alster Lake is one of two artificial lakes within the city limits of Hamburg, Germany, which are formed by the river Alster (the other being the Außenalster). The main annual festival is the Alstervergnügen. The lake has an area of 0.2 square kilometres (0.077 sq mi).