The largest river in Rokua Geopark is the Oulujoki River. It was first formedapproximately9,500 years ago when the land areas in Vaalankurkku rose above the surface of Ancylus Lake, what is now the Baltic Sea, and dammed Lake Oulujärvi, turning it into a separate lake. Initially the Oulujoki River only linked Lake Oulujärvi to the sea by means of a short waterfall but around 4,000 years later the river delta reached as far as Muhos. From there the river fed into the sea via Pyhäkoski, Finland’s biggest waterfall. After being separated from the sea, the tributaries of the Oulujoki River also started to develop and carve deep river canyons into the sand of the sea bed.
The Oulujoki river traverses a range of heights
At Muhos the riverbed is higher than the fracture line of the bedrock, which causes a drop of almost 50 metres within a short distance. The powerful current arising from this height difference has been harnessed for electricity production since the 1940’s at Pälli, Pyhäkoski and Montta power stations. The courses of tributaries of the Oulujoki River also run through the Oulujoki Valley, sometimes in canyons or ravines tens of metres deep, some of which are crossed by suspension bridges. In flat areas the tributaries are sometimes extremely winding. The bends are called meanders. The Muhosjoki River is a particularly good example. Water also flows underground in the Rokua Geopark, as on the sandy soil, much of the rainwater filters through to become groundwater.