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Riihimäki is located on a low plain between the Salpausselki ridges, partly in the watershed of both Kokemäenjoki and Vantaanjoki. The central area of the city is cut by the Vantaanjoki, which flows down from Erkylänjärvi as a stream-wide bed. Upstream of the river there is a recreation area at Käräjäkoski, where there is also a nature trail. To the south of the city, the river Herajoki branches off. Arolammi is the river's first and only pond-like widening of the Vantaanjoki in the southern part of the city. Arolammi is a lush, wetland surrounded by fields. The area is an important nesting and feeding place for water and shorebirds.
The Riihimäki city centre-Peltosaari area was previously designated as a significant flood risk area and it is still an area where flooding is possible. The Vantaanjoki River has a large flow variability and flooding in the watershed can occur in all seasons.
All Riihimäki's lakes are in the southern part of the city. Hirvijärvi and Suolijärvi are located on the border of neighboring municipalities. Hirvijärvi has a swimming beach with campfire places built for city dwellers to refresh themselves, and there is also a nature trail. Paalijärvi also has a city beach. The cleanliness, hygiene and safety of swimming beaches are regularly monitored by health inspectors.
Vantaanjoki originates from Hausjärvi's Erkylänjärvi and flows through Riihimäki towards the south. Upstream of the river is the recreation area Käräjäkoski. There is also a nature trail on Käräjäkoski. At the beginning of the 2000s, the upper parts of Vantaanjoki underwent fishery rehabilitation. The primary purpose of the work was to rehabilitate the currents to make them suitable for salmon fry production. The work restored rapids close to the nature reserve and also built the best possible habitats for chicks of different ages. The measures also included the construction of spawning grounds and the removal of obstacles to the migration of fish and other organisms. In addition to the main channel of Vantaa, renovation work was also carried out in Epranoja and Uhkolanoja. The river that flows through the city with low water passes a significant distance in the area of Versowood Oy's industrial facilities. The Vantaanjoki is also burdened by the waste water treatment plant of the city of Riihimäki. To the south of the city, the river Herajoki branches off.
Arolammi is a verdant expansion of the Vantaanjoki River, the restoration of which was carried out simultaneously with the restoration of the fishery. The goal of the renovation was to prevent the overgrowth of the pond by increasing the water volume. During the renovation, a bottom dam was built at the bottom of the pond to raise the water level during low tide. In addition, nesting islands for birds were built in the pond from dredged materials.
Regular water quality monitoring of Riihimäki's surface waters started in 2006. You can familiarize yourself with the surface water monitoring program from this link. The last monitoring was carried out in 2021, when the water quality of the lakes was studied.
Hirvijärvi and Suolijärvi are valuable nature and recreation destinations. The lakes are also part of the backup raw water system in the capital region. The monitoring results of the lakes show that the water quality of the lakes is excellent, and there is no evidence of a change in a more lush direction.
Paalijärvi and Vähäjärvi are shallow and lush lakes. The physicochemical condition of Paalijärvi is satisfactory. Local water protection work has been active in the Paalijärvi area. In recent years, the usability of the lush lake has been good, as the abundance of blue-green algae has hardly occurred and the hygienic quality of the water has been good.
The ponds included in the monitoring, Kalaton, Myllylampi and Hatlampi, are small humus ponds where the water was acidic and heavily browned. In addition, in 2024, the water quality of Juppalanlammi was studied.
The latest monitoring reports are from 2021 and 2024.
Water quality monitoring results are also added To Järviwiki, where they can be found under each lake and pond's own site over one hectare in size.
The water quality of the Vantaanjoki, which flows through the center of the city, is monitored annually at four observation points on Riihimäki, and in addition, the Herajoki and in 2018 also the Paalijoki. The results of river waters are reported annually in the Vantaanjoki joint monitoring report, which is available to read From the website of the Vantaanjoki and Helsinki Region Water Protection Association.
The most important groundwater area for Riihimäki's water supply, the Herajoki groundwater area, is a ridge section that extends from Riuta under the motorway to Herajoki. The quality of drinking water is regularly monitored by the health protection authority through laboratory analyses. The boundaries of the groundwater area can be seen, for example, on the Riihimäki map service.
The water quality of the Herajoki groundwater area is regularly monitored by the water supply company from groundwater pipes.
The Vantaajoki and Helsinki Region Water Protection Association has updated the Riihimäki groundwater protection plan, coordinated by the city's environmental protection department, in 2024. The Riihimäki City Council has approved the update of the protection plan in March 2025. The protection plan is used as a tool to take groundwater protection into account, mainly in official work.
The protection plan covers the Herajoki groundwater area and part of the Hirvenoja groundwater area. The Herajoki groundwater area is classified as a Class I groundwater area important for water supply.
The protection plan describes the current situation of the groundwater formations and the risks to the groundwater. An important part of the protection plan are recommendations for measures to eliminate or reduce risks. Groundwater protection must be taken into account when planning land use and construction. When developing areas, the location of water intake areas and the effects of construction on groundwater conditions must be taken into account. Stormwater management must be planned taking groundwater conditions into account.
It is necessary to continue the oil tank information aimed at property owners in all groundwater areas of Riihimäki. The information ensures that property owners are aware of the leakage risks of old oil tanks and the obligation to check the oil tank periodically. If the tank is found to have caused pollution, the property owner is responsible for the costs caused by soil (and groundwater) pollution.
The status of the Herajoki groundwater area has been assessed as good, but the area is subject to construction pressure and there are already existing risk activities. Business activities in the small industrial area of Mattila involve the processing and storage of substances harmful to groundwater. There are also more than ten properties in the Herajoki groundwater area where the soil is contaminated or potentially contaminated. There are approximately 130 heating oil tanks. Other possible risks are old landfills, road traffic and anti-slippage, sewer network leaks, waste water treatment from scattered settlements, animal farms and geothermal wells.
The most urgent measures to protect groundwater in the Herajoki groundwater area are the completion of soil and groundwater remediation of areas found to be contaminated, assessment of the contamination and the need for cleaning of suspected contaminated land, and the reduction of groundwater risks caused by road traffic and road maintenance.
In 2020, Riihimäki's small water survey was carried out, in which a survey of small water sites according to the Water Act and other valuable small water bodies was prepared.
In the work, already existing information on the selected 44 small water bodies was collected, new information was acquired through terrain surveys and the catchment areas of the selected bodies were determined in the area of the city of Riihimäki. The aim of the work was also to present restoration measures that might be necessary for the selected small water bodies, as well as issues to be taken into account in stormwater management and land use planning.
The small water survey can be used, for example, in the preparation of a nature conservation program, land use planning, water protection, recreation area planning and stormwater management solutions.
The Riihimäki city center area has been removed from the list of nationally significant flood risk areas at the end of 2024. However, flooding is still a risk that must be prepared for. Flood warnings from the Häme regional website You can find more information about floods and, among other things, about floods that have occurred in Riihimäki in the past.
It is possible and advisable to prepare for flood risks in advance if the property is located in an area where flooding is possible. More information about flood preparedness can be found on the vesi.fi website. from the flood preparedness section and from the Riihimäki City website at Safety and preparedness.
Environmental Director (deputized by Jenni Lehtonen)
Vitality
Management of the area of responsibility for environmental protection, development projects, environmental status monitoring, new environmental permits, water protection and nature conservation matters
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