From the Wikipedia page 
Storstrøms Amt (English Storstrøm County) is a former county (Danish: amt) on the islands of Zealand (Sjælland), Møn, Falster, Lolland and some minor islands in southeast Denmark. The county was formed on 1 April 1970, comprising the former counties of Maribo and Præstø. The county was abolished effective January 1, 2007, when it merged into Region Sjælland (i.e. Region Zealand).
The county was a somewhat artificial construction as the islands of Lolland and Falster did not traditionally share any institutions with Zealand, but rather with Funen. Locally, the adjectives nordenstrøms and søndenstrøms, i.e. "north" and "south of the Stream" were used to designate the Zealand-Møn and Lolland-Falster parts respectively, as they were separated by the strait Storstrømmen ("The Great Stream") from which the county derived its name.