The waters flow east and west
There are large altitude differences at Lake Isojärvi
The latest Ice Age in Finland ended about 10 000 years ago. It left behind erratic boulders, smoothed the granite bedrock of the area, and moved the earth material. Accumulation of the boulders and earth material was affected by not only the melting waters but also the rift valleys, which is why long eskers did not form in Lake Isojärvi area. The bedrock fractured 200 million years ago when the earth's crust was moving. The fault lines run through the area from northwest to southeast. The most common soil type in the National Park is moraine, which was formed under the ice sheet, when stones which had come off the rock got mixed and ground up with the old soil.
Isojärvi National Park is, in average, 100 - 150 metres above the sea level, and it is part of a watershed region. The waters north of Lortikanvuori Hill flow into Lake Päijänne, and the waters south of it flow into Lake Längelmävesi.
Clear water lakes
Most of the lakes in this region are deep, barren, and have clear water. For example Lake Isojärvi is narrow a lake formed inside a fracture in the earth's crust. The shores of this lake are rocky. The Black-throated Diver (Gavia arctica) is one of the bird species typically nesting on the barren lakes of the area.
Shores of the small forest lakes grow sphagnum moss. These lakes are becoming overgrown and swampy on the edges. On the remote forest lakes live birds such as the Red-throated Diver (Gavia stellata).
Ancient Latokuusikko forest
The National Park is covered mostly by dry and moist forests. There is only a little bit over 0,01 sq.km. of herb-rich forest. The core of the forests is Lortikka area, located in the eastern part of the park, from Lake Isojärvi to the southeastern side of Lake Hevosjärvi. It has not been logged for a long time. There you find different types of forests, from small, luxuriant herb-rich forests to rugged pine forests on top of the rocks. In the other forests of the National Park, loggings continued almost until the National Park was established. Since the beginning of 1990s, the Canadian Beaver (Castor canadensis) has been doing its own forest works in the forests on the lake shores and in the hollows by brooks.
One of the most valuable areas is the old-growth forest of Latokuusikko, which represents the spruce-dominated herb-rich forest very typical for Central Finland. The trees in Latokuusikko are large, and the spruces are almost 30 metres long. In the middle of the spruce forest grow giant Aspens (Populus tremula) and a few smaller Littleleaf Lindens (Tilia cordata). There is abundance of decaying wood, and many shelf fungus (Polyporaceae) species of old-growth forests live on the trunks of the trees.
In addition to Latokuusikko, there are other areas where vegetation typical for herb-rich forests can be found, such as under steep cliffs, and as narrow ribbons along brooks. The species of herb-rich forest include the Littleleaf Linden (Tilia cordata), the Bird Cherry (Prunus padus), the European Meadowsweet (Filipendula ulmaria), the Ostrich Fern (Matteuccia struthiopteris), the Subarctic Ladyfern (Athyrium filix-femina), the Common Twayblade (Listera ovata), the Baneberry (Actaea spicata), the Lesser Butterfly-orchid (Platanthera bifolia) and the Bracken Fern (Pteridium aquilinum).
The North American beaver at Lake Isojärvi
The original beaver species in Finland was the European Beaver (Castor fiber), but they became extinct in the mid 1800s because of hunting. The fur of the beaver was a valued commodity, and magic medicine was made of castoreum (Finnish: "hauste") which the beavers anal glands excreted. In 1930s, European Beavers were transported to Finland from Norway, and Canadian Beavers from Canada.
All the beavers living in Isojärvi National Park and its surroundings are Canadian Beavers. They were first seen in the area in the end of 1980s, when they colonised Lake Salmijärvi on the eastern edge of the park. From Lake Salmijärvi, the beavers spread further, and in the 1990s they have built their nests in many places, for example at the brook hollow in Kaatkorpi, at Lakes Kurkilammi, Hevosjärvi, Mutkalammi , and at other small lakes. In almost all the forests around lakes, marks of the beavers have been found. Since 2000, the beaver population has decreased in the National Park area.
Beavers colonised Lake Salmijärvi again in 2003. It is typical for beavers to move from one area to another. When they can no more find food around the lake, they move on along the water system to the next lake, until a future generation maybe comes back to the starting point if there is a lot of food growing again. Beavers eat bark of deciduous trees, leaves, aquatic plants and their roots. By building dams in waterways, they extend the area where they can find food. In water the beavers feel safe, as there are not many predators. The door of their nest is under the water, so they dive to enter the nest.
Because the terrain is hilly and its features are intricate in Isojärvi National Park, not very much forest has died in the floods caused by beavers' dams. Many species benefit from the landscape shaped by beavers. For example the hole nesters have nesting trees there, broods of water birds find shelter in the tangled thicket, insects become more abundant and the fish come to eat larvae. In some places, the trails of the National Park have had to be changed because of the beavers' dams, and in other places the beaver has used parts of the duckboards for its own buildings.
Beavers mostly move during the evening and the night. The beaver is a cautious animal, but the population living in the National Park has got used to visitors. If you wait quietly, you can get to see the beavers going about their business; building, finding food, and wrestling. The beaver is peaceful and avoids humans but, if threatened or irritated greatly, it will defend its nest and the young.