From the sluice of Geesthacht (at kilometre 586) on downstream the Elbe is subject to the tides, the tidal Elbe section is called the Low Elbe (Unterelbe). Soon the Elbe reaches Hamburg. Within the city-state the Unterelbe has a number of branch streams, such as Dove Elbe, Gose Elbe, Köhlbrand, Northern Elbe (Norderelbe), Reiherstieg, Southern Elbe (Süderelbe). Some of which have been disconnected for vessels from the main stream by dikes. In 1390 the Gose Elbe (literally in ) was separated from the main stream by a dike connecting the two then-islands of Kirchwerder and Neuengamme. The Dove Elbe (literally in ) was diked off in 1437/38 at Gammer Ort. These hydraulic engineering works were carried out to protect marshlands from inundation, and to improve the water supply of the Port of Hamburg. After the heavy inundation by the North Sea flood of 1962 the western section of the Southern Elbe was separated, becoming the Old Southern Elbe, while the waters of the eastern Southern Elbe now merge into the Köhlbrand, which is bridged by the Köhlbrandbrücke, the last bridge over the Elbe before the North Sea.