Swelinia - [svɛ'ɭiːɲa], also known as Swelina or (in German) Grenzfluß, is a name of a brook between Northern part of Sopot and Southern Gdynia (Poland). Original slavic name Swelinia occurred first time in documents from 13th century (Rivulum qui Swelina dicitur, "a creek known as Swelina"), but subsequently that name was not used in any official documents or maps until it was mentioned again in the Geographic Dictionary of Kingdom of Poland in 1888.
The brook was always a border between Sopot and other territories. The Treaty of Versailles specified it (without mentioning its name, however), in the Section XI. and article 100, as a border between Free City of Danzig and Poland.