Skiensvassdraget

The Skien watershed is the third largest watershed of Norway (after the Glomma and the Drammenselva watersheds) and includes rivers which feed the Nordsjø above Skien;
* Vinje-Tokke watershed, which includes lakes Totak, Bandak, Kviteseidvatn and Flåvatn.
* Bø watershed, which includes lakes Sundsbarmvatn and Seljordsvatn.
* Tinn watershed, which includes lakes Møsvatn, Kalhovdfjord, Tinnsjø and Heddalsvatn.

The Skien river (Skienselva) begins in Skien, at the Telemark Canal's first lock, and runs through Porsgrunn to the mouth of the river at Frierfjord at Norsk Hydro's factory complex.

Through Porsgrunn municipality the river is called the Porsgrunn river (Porsgrunnselva).