The history of the Karelian Isthmus is closely intertwined with post-glacial changes and the opening and drying up of water routes created by land uplift. The rise in sea levels caused by the meltwaters of the continental ice sheets and land uplift have dramatically shaped the conditions of the Baltic Sea basin and altered the discharge routes of the waters of lakes Saimaa and Ladoga. The rivers of these large water systems have served as travel routes between east and west, and north and south, and have determined the directions of settlement expansion and trade.

After the Ice Age, settlement spread across the area of present-day Finland as the continental ice sheet retreated from the region. People migrated to the area of Finland from the south across the sea, by land via the Karelian Isthmus, and from the east from the region between Lake Ladoga and the White Sea.

Evidence of migration is left, for example, by the Stone Age find from Antrea, which a local resident, Antti Virolainen, made in 1913 while deepening a drainage ditch in the marsh meadow of his home farm. In archaeological excavations, the remains of a fishing net, among other things, were found in the mud, which have been dated to the year 8540 BC, that is, over ten thousand years ago. A settlement site of the same age has also been investigated in Ristola, Lahti.

Lake Ladoga is becoming a separate lake

The Earth's crust pressed down by the ice sheet is still rising, although the uplift was considerably faster after the Ice Age. Land uplift is fastest in the Kvarken region and slower elsewhere. In Karelia, this means that the land surface is tilting towards the southeast.

Around 13,000 years ago, the Baltic Ice Lake covered the present-day Baltic Sea, the Gulf of Finland and Lake Ladoga, and at its widest extent, the waves of the Baltic Ice Lake lashed the slopes of the Salpausselkä ridges. Under the influence of the retreat of the continental ice sheet and land uplift, the water levels in the Baltic basin and the oceans fluctuated in such a way that the Baltic basin was at times filled with brackish sea water, and at other times with fresh lake water.

Lake Ladoga finally separated into an independent lake 9,800 to 9,700 years ago, when the water level of the Baltic Sea basin, or the Ancylus Lake of that time, dropped rapidly. The water level of Lake Ladoga remained about twenty metres higher than the surface of the Ancylus Lake, and the lake's waters discharged into the Ancylus Lake at Kexholm and Vyborg via the Heinjoki river. The waters of Lake Saimaa, in turn, drained into the Gulf of Finland via Kivijärvi and the Kymijoki river.

Land uplift gives birth to the Vuoksi and the Neva

The situation changed about 5,700 years ago (3700 BC), when a new outlet route, the Vuoksi, was formed for Lake Saimaa. The Vuoksi, which begins in Imatra, merged with the discharge route of Lake Ladoga's waters in the Antrea region. The torrential current coming from the vast Lake Saimaa also raised the water level of Lake Ladoga by a metre or two, and according to Matti Saarnisto, who has studied the geology of the Karelian Isthmus, the water submerged Stone Age dwelling sites located on the shores of Lake Ladoga.

Rapid and uneven land uplift weakened the flow of the Heinjoki and raised the water level in the southern parts of Lake Ladoga. Around 3400 years ago (1350 BC), the waters of Lake Ladoga broke through a new route from the southern part of the lake into the Gulf of Finland. Thus the River Neva was born, which later developed into a central route for traders of the East and the West. The Neva was utilised by the Vikings and later by traders of the Baltic Sea region, including during the Hanseatic League.  

Through the Neva, a massive volume of water discharged into the Baltic Sea, and the water level of Lake Ladoga dropped by twelve metres in a short time. The difference in water level between Lake Ladoga and the Gulf of Finland decreased from 20 metres to eight metres. Lake Ladoga's previous discharge route from Kãkisalmi to Vyborg Bay dried up, and the waters of the Vuoksi began to flow towards Lake Ladoga.

The birth of the Neva and the lowering of the water level of Lake Ladoga also altered the Vuoksi river system. Before the water level dropped, there was a waterway connection from Lake Ladoga at the Taipaleenjoki river to Lake Suvanto and further via Noisniemi to Kexholm. Following the breakthrough of the Neva, the level of Suvanto dropped by about 15 metres, and an isthmus less than a kilometre wide formed at the village of Taipale, separating the lake from Lake Ladoga. The waters of Suvanto drained via a small stream across the Kiviniemi threshold into the Vuoksi.

Traders move along the waterways

The water systems of the Karelian Isthmus formed an important fur-trade network, the control of which interested the Eastern Slavs and the Western Swedes. In the Viking Age, the Neva became the most significant trade route between East and West, along which both goods and slaves were transported to the eastern markets and from there to the West.

From the 8th century onwards, Scandinavian Vikings increasingly began to utilise Russia's dense river network – the Great Eastern Route – along which connections were established via the Dnieper to the Black Sea and Constantinople, as well as along the Volga to the Caspian Sea and all the way to the Arab world. The Neva was the most important route from the Baltic Sea to the Great Eastern Route. Trading centres developed around Lake Ladoga at important trading locations, the most significant of which grew to be Novgorod along the Volkhov River, which flows into Lake Ladoga. The Karelian Isthmus remained somewhat to the side of the main Neva River thoroughfare, but connections certainly functioned.

From Kexholm and Vyborg to administrative centres

 The name Karelia in various spellings appears already in the Viking sagas and in the First Novgorod Chronicle under the year 1143. There it is in the form Korela, which refers to the Old Fortress of Kexholm. Following the Treaty of Nöteborg concluded in 1323, the part of Karelia belonging to Novgorod was administered from Korela, and from the late 13th century onwards it developed into the centre of the Eastern Isthmus and Ladoga Karelia.

The western part of the Karelian Isthmus was within Sweden’s sphere of influence, and to consolidate its position, the King of Sweden had a castle built on the Gulf of Vyborg in 1293. In the Treaty of Stolbovo in 1617, Sweden gained control of the Karelian Isthmus and with it Kexholm, but lost the isthmus and the shores of Lake Ladoga to Russia in the Treaty of Nystad, concluded in 1721. At the same time, Sweden's era as a great power came to an end. In 1812, Karelia was incorporated into the Grand Duchy of Finland.