After the retreat of the ice sheet that covered the region after the last ice age, the river found itself rerouted by an esker onto a plain uniquely ribbed by the ridges of other eskers, where it formed a succession of rapids and wide bays (called fjärdar), which the river inundates during the spring floods.

This particular hydrography, along with the park's proximity to the ecological frontier between the north and the south, has favored the development of a fauna and flora of great biodiversity.

If the presence of humans was relatively discreet since the Stone Age, concentrated principally atop the eskers, the development of mining activity (in particular of iron) has profoundly affected the region.

[3] Most of the bodies of water called fjords in Norwegian were steep glacial valleys into which the sea had penetrated, which was the more restrictive definition which passed into French[4] and English.

[S 2] In addition, the ridge Enköpingsåsen cuts across the park, with a maximum height of 18 m (59 ft),[S 2] crisscrossing the region over 300 km (190 mi) between Trosa to the south and Bollnäs to the north.

[13] The river has largely flooded this flat and irregular zone, creating a mosaic of water, dry earth, and wetlands.

[14] The snow cover lasts a little over 3 months[15] and the river is frozen during a similar period, with the exception of the rapids, which remain ice-free for most of the winter.

[20] Färnebofjärden is the first big floodplain of the Lower Dalälven, situated between the rapids of Tyttbo to the west and Gysinge and Sevedskvarn to the east.

[S 3] While the flow of the river Österdalälven is controlled (at Trängslet and the lake Siljan), the course of the Västerdalälven is free, and thus has the largest seasonal variations.

[S 3] Subsequently, the area underwent a long period of erosion, so that by leading 600 million years ago the so-called Sub-Cambrian peneplain had formed.

[S 2] When the glacier withdrew, the soil had been so compressed by its mass that the entire zone situated to the east of Avesta found itself at sea level.

[N 1] During this maritime period, sediments deposited themselves on the bedrock, which explains why that region even today has superior fertility to that of the rest of the country.

[N 1] In effect, before the ice age, the river continued in the direction of the Mälaren, in which it emptied itself and it so had flooded a valley along this entire section.

[N 1] It thus had to adapt itself to the topology of the terrain, forming large fjärds with many isles in the flattest sections and by contrast a course narrower and faster at the different eskers encountered along the way.

[N 2] The park of Färnebofjärden is according to the classification of the WWF, situated in the terrestrial ecoregion of Sarmatic mixed forests,[25] not far from the frontier of the Scandinavian and Russian taiga.

[S 6] The forests of conifers, principally of Norway spruce, are in particular present in the south of the park,[26] around Tinäset, on the isle of Torrön and the peninsula of Öbyhalvön.

[S 10] These old trees as well as the presence of dead wood give this forest a very ancient appearance, and contribute to a great richness of lichens and fungi.

[S 10] The soil is often covered in moss, the characteristic species being Hylocomium splendens, but Anastrophyllum hellerianum and Nowellia curvifolia are also common.

[S 8] Thus, the Eurasian lynx can be observed regularly around Öbyhalvön, Tinäset and Gärdsvekarna, alongside brown bears and gray wolves, although these are less frequent.

[S 16] The waters abound also with northern pike, with specimens weighing sometimes 20 kg (44 lb), with European perch, with zander, and with grayling — which species is classified as endangered in Sweden, much like the brown trout.

[33] Between 2002 and 2008, the insecticide Bacillus thuringiensis israelensis was used to try to regulate these populations, which needed a waiver, its use being forbidden in Sweden and contrary to the principles of the national park.

[34] However, the project was not renewed, this method not being considered a durable solution and some scientists arguing that the mosquitos make up part of the region's ecosystem.

[34] 6000 years ago, the Lower Dalälven was a bay of the Littorina Sea, but as the land rose, the crests (eskers) became the best sites for the first permanent settlements.

[S 19] When sedentary agriculture developed in the region during the Bronze and Iron Ages, the slopes of the crests were again the favorite settlement sites, being easier to cultivate and above the floodplain.

[S 21] The forge's location permitted a good access to metal, extracted at the mine of Dannemora,[N 5] to wood and to the energy of the Gysinge rapids.

[38] In response the ornithologist Stig Holmstedt made contact with the Swedish nature conservation society and formed a group to work towards the protection of the zone,[38] which led to the formation of a proposition for a national park in 1975.

[40] The motive for creating the park was "to preserve a unique fluvial landscape as well as rich forests and surrounding wetlands in a relatively intact state".

[45] Naturvårdsverket is charged with the proposal of new national parks, through consultation with administrative councils of the counties and municipalities; a vote of the Riksdag endorses their creation.

[S 24] Elk, deer, and American mink hunting is allowed for population regulation purposes, but is forbidden between 1 January and 15 August.

[46] At Skekarsbo, an observation tower 20 metres (66 ft) in height was constructed in 1995 and offers a panoramic view over the major part of the park.