Lake Inari is the third largest lake in Finland as measured by surface area. The lake is situated at some distance from industrial centres. Its drainage basin is sparsely populated and is mainly used for reindeer herding. Most of the basin is still in a near-natural state. The main part of Lake Inari drainage basin is situated in the valley of the River Ivalonjoki. The River Lemmenjoki conservation area, which is the largest in Finland, is situated in the drainage basin. The forests are composed mainly of pines and are used as a source of raw material in forest industries. Forests of spruce are rare. In the northern part of the drainage basin there are some barren arctic mountains, with small clusters of arctic birch.

The lake is situated on the border of subarctic and temperate climatic zones and belongs to the sub-boreal vegetation zone and to the palaearctic zoogeographical region. The bedrock of the lake was formed during the Precambrian period and is composed of granite, gneiss and granulite. The dominant mineral soil type is moraine. The lake basin was formed as a result of tectonic movements and is surrounded by seven fault blocks. The lake was not in contact with the ancient Baltic Sea and therefore does not contain marine relics. The nearest marks of the ancient sea level have been found in the area of Saariselka, south of the lake. In ancient times the River Ivalonjoki probably drained into the Baltic Sea.

Lake Inari is the central lake in the drainage basin of the River Paatsjoki. The shoreline is very broken and there are many islands. The major part of the influent water enters the lake through the rivers Juutulanjoki and Ivalonjoki and from the drainage basins of Lake Paatari-Mutusjarvi and the Rivers Vaskojoki, Menesjoki, Lemmenjoki, Kaamasjoki, Yla-Ivalonjoki and Ala- Ivalonjoki. The water from the lake is discharged into Barents Sea through the River Paatsjoki.

The lake is oligotrophic and the quality of water is good. Even in winter under ice cover the oxygen saturation of the hypolimnion is above 80% and of the epilimnion above 90%. Effects of wastewater discharged to the lake from municipalities are observed locally. Typical features of the water quality of the lake are low concentrations of humic substances and nutrients, low values of hardness and colour, and pH close to neutral. Local and temporal differences in water quality are small.

The most important factor affecting the present state of the lake is the hydroelectric power plant constructed in the River Paatsjoki in USSR. This plant started operation in 1934 and it has regulated the water level of Lake Inari ever since. Because of the regulation, the water level of the lake is about 0.5 m higher than in its natural state. The regulation has detrimental effects on the benthic fauna and on fish. The growth and catches of some species of fish, e.g. lake trout and arctic char, have decreased since the beginning of regulation. Efforts have been made to improve the efficiency of fishing in the lake, e.g. by using new types of fishing tackle.

The drainage basin of the lake will probably always be sparsely populated and without major industry. The stress due to tourism will be limited to certain areas in order to protect the lake as a whole (Q).