This river is in its natural state. It runs from the headwaters into the sea, adapting to the landscape that it flows through. Along its course there are bursting rapids, fast flowing streams and slow flowing pools. In the rapids the fall height is most intense, making water splash from rock to rock, oxygenising the water and keeping the river bed free of silt. The aquatic life in pool sections resembles that of small ponds and lakes. The main channel of the river widens as more tributaries join its flow.
Streams and floods shape the channel and the riverbed
The river makes its way through the landscape based on altitude, flow intensity and the geological character of landscape and soil. The flow of the water constantly erodes material from the river channel, transporting it along the channel before depositing it to a new place. The river channel is sometimes straight but often twists and splits, forming smaller river branches and islands. Each river has its natural balance of erosion and sedimentation.
The river often floods in spring when the snow melts. Heavy autumn rainfall also contributes to the flooding. Due to climate change, flooding has become more frequent in winter and summertime too.
During flooding, the amount of water in the river and the flow rate of this water are high, causing an increase in erosion and transportation of sediments. The water flow removes soil from the river bank, forming new waterways and thereby determining its own direction and shape of the channel. During the flooding, water rises and spills over the banks creating flood plains.
During flooding, rough bottom material gets loose due to the power of the water and gets carried down the riverbed. It is deposited according to size, forming either narrow passages or sandbanks. Clay and other fine sediments are mixed with the water making it muddy. The fine matter is removed from the fast flowing sections and is deposited in the pool areas and floodplains. The flood acts as an annual spring clean-up for the river.
In the stream rapid sections, the riverbed is coarse in structure, forming pores, compartments and labyrinths that act as home for stream-inhabiting species.
Life below the surface is adjusted to the river flow and its changes by attaching itself to solid bottom material or searching for shelter behind stones and other current-free pockets. The biodiversity of the riverbed is rich and diverse. Algae attaches to the stones, whereas fish and other river inhabitants find resting places in the changing forms of the riverbed.
Migratory fish move along the free waterways
Fish swim freely through the rivers and lakes, looking for suitable feeding and spawning areas. Salmonids dig spawning beds in the gravel where the roe can develop safely protected by the gravel and oxygenated by the flowing water.
The salmon and trout fry grow up to become migrating smolt, which swim downstream to the sea or to the lake. As adults they return to their home river for breeding. The freshwater pearl mussel can also thrive if the river is inhabited by salmonids. Freshwater pearl mussels spend their adolescent lives in the gills of salmonids.
Riparian vegetation adapts to the floods
The riverside vegetation is adapted to the proximity of water and frequent flooding. The plants that favour the flood and endure it the most grow the closest to the river. Floodplains are covered with deciduous tree forests and flood meadows rich in biodiversity.
The riparian area with its vegetation acts as a filter and keeps the water clean. The trees give shade from the heat of the sun and keep water in the river cool and oxygenated during the summer period, even when the water level is low. Debris from the trees in the water slow down and diversify the flow.
Fast flowing water has a rich benthic fauna
In the headwaters, most of the nourishment for the river organisms originates from trees and other riparian vegetation. From there the nutrients gradually enter the river. Leaves, branches and other detritus are processed by bacteria, fungi and other benthic organisms, especially the benthic insects. They spend most of their lives as aquatic larvae. After metamorphosis into their adult form, they only live for a few days or weeks.
Benthic insects include caddisflies, crane flies, stoneflies, mayflies, black flies, beetles, damselflies and dragonflies. Other benthic organisms are mussels, snails and crayfish. They are a standard food source for many fish, birds and mammals, such as otters.
River Tornionjoki as seen from the hills of Aavasaksa. It is situated on the Swedish-Finnish border and it is, along with the rivers Teno and Simo, one of Finland’s few big rivers that have been completely unaffected by the construction of hydropower plants. Picture: Virpi Sahi 2017