The Alster is a right tributary of the Elbe river in Northern Germany. It has its source near Henstedt-Ulzburg, Schleswig-Holstein, flows somewhat southwards through much of the Free and Hanseatic City of Hamburg and joins the Elbe in central Hamburg.The Alster is Hamburg's second most important river. While the Elbe river is a tidal navigation of international significance and prone to flooding, Alster is a non-tidal, slow-flowing and in some places, seemingly untouched idyll of nature, in other places tamed and landscaped urban space. In the city center, the river forms two lakes, both prominent features in Hamburg's cityscape.