A significant main watershed runs through the territories of Hollola, Lahti and Nastola: lakes and ponds north of the First Salpausselkä Ridge belong to the Kymi River system, while the surface waters on the south side end up in the Porvoo River. On the terminal moraine of Salpausselkä there are small kettle ponds, such as Likolampi, Mytäjäinen, Häränsilmä, Kintterönlampi, Sietikka and Valkealampi in Lahti. The kettle ponds receive their water directly from the groundwater table or from precipitation. They were formed when giant blocks of ice buried in the soil during the ice age melted, leaving behind large potholes.
In addition to Lake Vesijärvi, there are several smaller lakes in the region. These have high recreational value, as they are located close to residential areas, many of them right in the middle of the city. However, the recreational value of most of the small lakes has been under threat in recent decades due to increased eutrophication. In addition, the state of Lake Vesijärvi is still no better than satisfactory.
The lakes are affected by many things around them, e.g., housing, economic activity, forests and farmland. Nutrients that cause eutrophication also enter the lakes from the atmosphere: rain water nutrient levels may occasionally be much higher than those of lake water.
Lahti is a densely populated and industrialized city and has a lot of activity that puts a strain on the waters. The eutrophication, which has been going on for a long time, has, in certain places, resulted in the depletion of oxygen and severe internal loading. The recreational use in summer is particularly affected by intermittent mass occurrences of planktonic algae, or algal blooms, some of which have been caused by toxic blue-green algae. Lakes in the Lahti region are managed jointly by the municipalities and the Vesijärvi Foundation. In addition to the management of the lakes, measures are also needed in the catchment areas so that the external loading caused by farming and surface runoff will be reduced.
Most of the lakes in the Lahti region are included in the national quality classification scheme. In compliance with the Water Framework Directive, the objective is to have the lakes in good or excellent condition by 2021 and, regarding Lake Kymijärvi, by 2027. At the moment, Lake Kymijärvi’s quality is rated “passable”. The lakes Iso-Kukkanen, Kärkjärvi, Alasenjärvi, Oksjärvi and Arkionmaanjärvi are rated “good”.
In 2015, the Environmental Services carried out several measures for improving the quality of surface water. A comprehensive report prepared by Lahti Region Environmental Services on the results of the ditch load monitoring study 2008–2015 was completed. It was found that the external load on Lake Vesijärvi had decreased, but further decrease is required for the lake to reach “good” condition. To reduce the load, the Purailanviepä water conservation wetland was reconditioned. Lahti Region Environmental Services’ long term monitoring programme had revealed that it was in need of improvement. A two-hectare water conservation wetland was created on Sammalsillansuo bog south of Lake Sylvöjärvi. The purpose of the wetland is to reduce the nutrient load into the lake and to improve the ecological state of the lake, which is now in satisfactory condition. The finishing work on the wetland will be carried out later once vegetation has grown in the excavation areas.
In addition, the Environmental Services organized an improvement project for the Hammonjoki River and was involved in the reconditioning of the Virojoki River. Both rivers flow into Lake Vesijärvi. The aim is to improve the trout breeding areas in the lake.
The contribution of surface runoff to the load on Vesijärvi’s Enonselkä was studied and the report was published in 2015. Surface runoff from the city is a major contributor to the loading of Enonselkä now that other loading has been reduced.
A year-round measurement station was installed in the Lankiluoto Deep on Lake Vesijärvi in 2015. The station comprises a buoy and an underwater sensor. The station measures the quality of the water every hour along the entire height of the water column and the results are available online.
|Vesijärvi, Lankiluoto in Enonselkä basin||5,8||6,8||4,8||25||22||11||13||11||12||76||4,5||14||13||16||8,7||10|
|Hollola, Lake Arkiomaanjärvi||3,7||2,9||7,3||4,4||6,7||5,7||4,6||3,2|
|Nastola, Lake Salajärvi||8,1||9||17||31||7,4||13||12||11||10||15||14||9,5||8,1|
|Vesijärvi, Enonselkä basin||2,8||1,9||2,0||2,9||2,3||2,2||1,8||2,1||1,8||0,9||2,2||2,3||1,8||1,8||1,7||2|
|Hollola, Lake Arkiomaanjärvi||2,0||2,0||2,0||2,4||2,0||1,8||2,0|
|Nastola, Lake Salajärvi||1,5||2,0||2,5||3,0||2,0||2,0||1,8|
|Tons of fish||5||86||94||200||159||184||111||54||137||216||201||206||218||159||140||113|
|In good condition||11||11|
Future challenges related to recreational and natural values of lakes:
• Reducing the external load into Lake Vesijärvi
• Achieving the targets of the Water Framework Directive for all lakes