Historical Ostrobothnia was separated from southern Finland by Suomenselkä, a drainage divide which directs the flow of water north to the Merenkurkku or Perämeri and south to the Gulf of Finland or Selkämeri. To the east, it was bordered by Russian Karelia on the Maanselkä watershed, which divides the estuaries of the Oulujoki and Iijoki from the estuaries of rivers flowing to the White Sea. The northern borders are poorly defined, partly because permanently fixed habitations were a relatively new phenomenon at the time of the introduction of the county system, which replaced the older provincial divisions. On the coast, the border ran somewhere between the Torniojoki and Iijoki, without any formal definition inland, though the watershed between the Kemijoki, Iijoki and Olhavajoki estuaries serves as an approximate definition.