It starts at kilometre 586, at the sluice of Geesthacht, where the Elbe forms the border between Lower Saxony and Schleswig-Holstein.

In Hamburg the Unterelbe forms two anabranches, Norderelbe and Süderelbe, the latter now partially a cut-off meander.

Where Norderelbe and Süderelbe used to reunite, at kilometre 634 there is a bay called the Mühlenberger Loch [de; nds].

It continues further into the Wadden Sea as the Außenelbe, where it passes the Hamburg islands Scharhörn and Nigehörn.

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