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, the 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. Geography In total, the Alster is long and has an incline from 31 m to 4 m above sea level. Its drainage basin is about . Left tributaries to the Alster are: Rönne, Alte Alster, Sielbek, Ammersbek, Drosselbek, Bredenbek, Rodenbek, Lohbek, Saselbek, Osterbek, and Wandse (Eilbek); right tributaries to the Alster are: Mühlenau (or Mühlenbach), Diekbek, Mellingbek, Susebek, Tarpenbek, and Isebek. Upper course The source of the Alster is a small bog pool in the Timhagen Brook near Henstedt-Ulzburg, approximately north of Hamburg. The small brook makes a couple of curves before entering Hamburg territory at Duvenstedt and Wohldorf-Ohlstedt. At that point, the Alster has passed a couple of smaller locks and has been passed by 15 bridges. From here on, Hamburgers use the river for water sports, especially for canoe and kayak trips on the river, or hikes along its shores. At Wellingsbüttel, the Alster runs past Wellingsbüttel Manor. At Ohlsdorf the Alster reaches a weir, approximately at the height of the former Fuhlsbüttel Lock, the last water level regulation before reaching the inner city. From Fuhlsbüttel downstream, the Alster's course, has been straightened on several locations, with old and new river beds occasionally forming parallel canal beds. Between city limits and the inner city's first lake − the Außenalster − the river is crossed by 42 bridges. Lower course For Hamburg's inner city, the river is a major geographical feature and greatly defines its cityscape. Through ponding of the river in the 13th century, two artificial lakes, the Außenalster (Outer Alster) and the smaller Binnenalster (Inner Alster) were created in central Hamburg. These two lakes and the surrounding parks serve as important recreational areas in the heart of the city. Binnenalster and the subsequent Kleine Alster form important urban spaces. The Alster's final section between Kleine