Kymijoki River’s watershed is 37 107 km2 which is about 11 % of the surface area of Finland, and its percentage in lakes is 19,7 %. The northern most parts are in Mid-Finland, Kinnula and Pihtipudas municipalities and in Northern Savo, Keitele and Pielavesi municipalities. The average precipitation in the north of the area is about 570 mm and in the south 610 mm. The highest parts of the area are more than 200 meters above the sea level, e.g. Laajavuori in Jyväskylä 228 m. The highest lake, Ylä-Kintaudenjärvi in Petäjävesi is 154 m above the sea level and the average height of the area is 120 m above the sea level.
Kymijoki proper starts in South-Eastern part of Päijänne lake, Kalkkinen village in Asikkala municipality. It has a 203 kilometers’ route to the sea, the Ahvenkoski bay, and drop height of 78,5 meters.
Hydropower
The first and the most important usage of the watercourse is hydropower. On Kymijoki River there are, side channels excluded, all in all 13 hydroelectric power plants, the earliest of which were built in the late 1800’s. The most notable free running rapids are Ahvionkoskis, 4,5 km above the branching spot, whose fall height is about 1,90 m aned Pernoonkoskis in the eastern main fork, fall height about 4,0 m.
Water regulation
Kymijoki River flow is regulated from Päijänne Lake. The purpose of the regulation is to increase the power plant’s economic benefits as well as to prevent flood damages as far as possible. The regulation aims at cutting out the biggest floods in the spring and at leveling the flow during the summer. Unlike many other big rivers there’s no short-term regulation at Kymijoki River, and that’s why there’s no daily fluctuation in the flow either.
Kymijoki cleanings
Between 1920 and 1932 cleanings were performed in the upper Kymijoki River to prevent floods. For example, Pyhäjärvi Lake’s level decreased about 1,5 m and considerably vast areas of land became arable. In 1926 farmers in the lower municipalities of the river made an initiative to clean Susikoski and the rapids below. The initiative resulted in a few plans that were never realized until professor P. Kokkonen’s mission from 1925 to evaluate the benefits of cleaning Kymijoki River lead to cleaning the river according to plan in 1929, first in Laajakoski and later in Susikoski, Ahvionkoski and Kultaankoski.
In Kymijoki’s western fork they cleaned Hirvikoski, Suomenkylän Koski, Strömfors’ fork, Paaskoski, Hattarinvirta and Klåsarönkoski. Later Hirvivuolle and the eastern forks’ Pernoonkoski were also cleaned. These cleanings resulted in 6 840 hectares of useful land.
The detriments caused by the cleanings are meant to be compensated in the following years by renovating some of the rapids into spawning areas for salmon.
Timber rafting in Kymijoki River
For over 200 years there’s been timber rafting on Kymijoki River. Korkeakoski sawmill operated in the eastern fork already before the Greater Wrath and it was renewed in the 1720’s. In the late 1700’s there were several sawmills on Kymijoki’s riverside and at that time timber rafting was particularly active. The government started to take a firmer approach towards timber rafting in the late 1800’s and in 1880 the first rafting ruling was confirmed and the state’s timber rafting equipment were surrendered to Kymi timber rafting company established in 1873. The company folded in 1907 and timber owners founded a new society called Kymijoen Uittoyhdistys. The society’s rules were ratified in 1945 and it is still running. On the river above Kuusankoski rafting ended in 2002 and between Kuusankoski and the sea already in 1966.
The water quality in Kymijoki River
The quality of the water in monitored in the lower Kymijoki. The quality has improved significantly in the last decades. The improvement is the result of the decrease in industrial strain. The strain biologically consuming oxygen has fallen to a percent from the 1980’s. The solids strain has fallen to a tenth and chemical oxygen consumption to a fifth. Kymijoki River water is slightly colored by humus, but the quality is good. According to the last usefulness rating Kymijoki River water fulfills the qualifications of good water.