Geology of the area


The western part of the area is characterised by a hilly landscape with peaks up to 300 metres above sea level. The landscape is full of many small lakes and large marshy areas. Those located under the highest coastline (HC) were created by post-glacial rebound as they became cut off from the coastal zone of the Baltic. Most of the hilltops are forested and are used for timber production today. Some farming is still pursued in the valleys.

Säterdalen Valley here is the result of geological processes at the end of the last ice age. Large amounts of sediment, mainly in the form of silt but also sand and clay, were deposited on the sea bottom at the edge of the retreating ice sheet. Because these soils are easily eroded, the Ljusterån River and the Hyttbäcken Creek dug ever deeper channels as they flowed towards the Dalälven. When saturated with water, silt becomes unstable and can easily form ravines. The ravine is still growing and changing as it has done for many thousands of years, and is gradually being absorbed into the surrounding agricultural landscape. The ravine system of Säterdalen Valley is Sweden’s prime example of a living ravine landscape.

Downstream of Avesta, the Dalälven flows out into fairly flat terrain, which covers most of the area. It passes through the sub-Cambrian peneplane, a flat, eroded area with only small topographical differences. In the western part, the average height is about 60 m, to then gradually descend towards where the river runs into the sea. 

Large parts of the river’s surroundings are wooded and are used for forestry. In the flat terrain, the river channel splits into many smaller channels, winding through the landscape and creating a characteristic, mosaic-like appearance to much of the Nedre Dalälven area. The many islands in the bays give the area a unique appearance. Many sections of the river are used for hydropower production, which means that the natural fluctuations in water level that previously defined the area surrounding the river have largely ceased to exist. 

Clearly visible delta landscapes are found where the river changes from a single channel to a fluvial lake, for example at the mouths of Bäsingen, Färnebofjärden bay, Hedesundafjärden bay, downstream of Untra before and in the Marmafjärden Bay and where the river runs into the sea. Other parts of the river’s landscape are also constantly changing due to erosion and sedimentation. This is clearly visible, for example, in Lakes Hovran and Bäsingen.
As the river approaches the coast at Marma, it passes through the ancient glacial river delta areas that characterise the landscape. Huge amounts of fine-grained material, such as sand, have collected here, giving the terrain a different shape with softer edges, while making the soil layer increasingly dry. The river runs out into the Gulf of Bothnia at Skutskär, where a small delta region is continually being reshaped.

Bedrock 
The bedrock in the Nedre Dalälven area is a part of the Baltic Shield and consists almost entirely of extremely old rock of about 1.9 billion years old. The western part of the area lies in the actual Bergslagen region, which is the ore belt of central Sweden. The bedrock here is varying, consisting largely of old volcanic rock such as meta-volcanic rock and granitic gneisses. The meta-volcanic rock contains the iron ore and sulphide ore (primarily copper, zinc and lead) that form the basis of the ore mining that has gone on here since prehistoric times. Sulphide ore is still mined in Garpenberg. Countless abandoned mine shafts in the woods speak of the small-scale mining operations of earlier eras, occasionally accompanied by mounds of waste rock and foundry slag. In some places primary limestone is found, favouring the growth of plants that require alkaline soil. In the eastern part of the area, the soil cover rests on bedrock that has eroded down to a flat, almost plane surface, called peneplane. The bedrock here consists primarily of granite, but has some elements of greenstone, primary limestone and sandstone.

The melting of the inland ice sheet and the highest coast line 
The latest inland ice sheet receded from the area 9,900–10,000 years ago. The landmass had been pressed down by the weight of the ice, so that most of it lay under the surface of the glacial lakes and bays that existed before the Baltic Sea. Thus, most of the area is under the Highest Coastline (HC), which is now about 190 m above sea level. Only in the west were there a few heights that were not covered by water. In many places we currently see clear sea walls made of stones and boulders – rubble fields – marking various locations where the shoreline once lay.

 

Soils 
Moraines 
Sandy till covers much of the area. For the most part the moraines are now covered with forest.

Sedimentary deposits 
Throughout the area are sedimentary deposits of various types and ages, such as esker ridges, glacial river deltas, distal deposits in the form of sand, silt and glacial varved clay, post-glacial fluvial sediments and levees, etc. The sedimentary soils in the area are concentrated to the river valley in the western part of the river, upstream of Avesta, where they consist mainly of silt and sandy soils, which are used for agriculture. The sediment close to the river channel was mainly deposited by the river and its tributaries. One striking example of such deposits is the formation of the ravines in Säterdalen and Solvarbo. At Avesta there is an outcropping of rock that has resisted erosion better than the mountains farther to the south and east. Because of this, there have been waterfalls and rapids in Avesta, while the river north of Avesta was dammed up and flows more slowly. Upstream of Avesta, much larger areas than now have been flooded by the river, and previous sediment deposits have shifted in later floods (lamination). Closest to the river channel, where the water moves faster, sand was deposited, while finer particles sank to the bottom farther from the strongest current. These river sediments are all rich in nutrients, and the sandy soil closest to the river is particularly well suited for growing potatoes. Of the once-larger flooded area around the river, all that now remains is the fluvial Lake Hovran and a number of shallow, lagoon-type lakes rich in nutrients, such as Trollbosjön, Flinesjön, Fatburen, Svinesjön and Amungen. The river’s lamination process continues on a regular basis as material is washed away from certain areas and deposited elsewhere to form reefs, levees and deposits on the islands, particularly visible in Lake Hovran.

Prior to the latest ice age the Dalälven went from modern-day Avesta south to what is now Lake Mälaren. But just south of Avesta, the old channel itself was filled with rocks and soil by the inland ice sheet, so the river’s water had to carve a new path northeast over a flat area where no river channel existed. This created the foundation for the river landscape we have today in the eastern part of the area, downstream from Avesta, featuring fluvial lakes, bays, deltas and brief stretches of rapids. During the brief period, geologically speaking, that the river has followed this new course it has not had time to form a noticeable valley. Instead, the river is characterised by the creation of deltas and deltaic lobes. This distinguishes the Dalälven from other large rivers in Sweden where erosion is the dominant process. 

To some degree the lower reaches of the Dalälven also experience repeated lamination. When water levels are low, the river erodes the deposited sediment. The majority of the material – primarily the 33heavier particles – is deposited on the shores closest to the river channel, forming deltaic lobes. Of the finer particles, only small amounts are deposited in the flooded lands farther from the river channel. The deposits created are therefore highest closest to the river channel and slope away from it. For this reason, it is common that the edges of the river are drier than the surrounding lands, which can be very waterlogged. In addition to new deposits by the river, there are also existing landforms that are built up by sediment. Several of the moraine islands in the river have expanded due to deposition.

Esker ridges 
Several esker ridges cross through the area in a north-south or northwest-southeast direction, a characteristic element of the landscape. The largest is Badelundaåsen, which can be followed from Nyköping under Lake Mälaren, through the province of Västmanland and into our area southeast of Avesta. The Dalälven breaks through the ridge at Brunnbäck south of Avesta and in Grådö south of Hedemora.

In Västerby north of Hedemora, the Svärdsjöåsen Ridge joins up with the Badelundaåsen Ridge. All along the ridge are several small lakes, called kettle holes, which were created by glacial retreat. Kettle holes are created when blocks of ice calve from the front of the receding glacier and are buried by the glacial outwash of sand and stone. When these ice blocks melt they become holes in the ridge, which are now often lakes or peat bogs. A defining trait for these kettle hole lakes is that they have no tributaries or distributaries.

Another mighty ridge that crosses the Nedre Dalälven is the Enköpingsåsen, which branches out into the Ockelboåsen Ridge from Tärnsjö through Färnebo(ärden Bay, where it forms narrow islands and the long, narrow isthmus of Strångnäs. Others are Gäveleåsen and Uppsalaåsen. In several places the ridges create peninsulas and islands when they cross the lower reaches of the river, such as at Hedesunda. The northernmost part of the Uppsalaåsen Ridge forms a 3 km long peninsula – Billudden – into the sea just east of where the Dalälven runs out into the Gulf of Bothnia.

Billudden is rich with fossil-bearing limestone, carried in by the inland ice sheet from a large limestone deposit in the Gulf of Bothnia. The high lime content also contributes to rich vegetation on Billudden, as in all of North Uppland, including many orchids. The ridges are exceptional groundwater supplies and are widely used to provide the built-up areas in the region with high-quality drinking water. The cities of Avesta, Hedemora and Säter are all in the running for the distinction of Sweden’s best drinking water. Humans have long used these esker ridges as travel routes. The ancient roads from the Lake Mälaren Valley north followed the Badelundaåsen and 
Enköpingsåsen for long stretches. 

In Hedemora, the Bådelundaåsen meets up with the Dalälven. The ridge and the river were historically the two most important travel routes to Northern Dalarna. Several national highways still run along the ridges. The eskers have also been used since ancient times for settlements and also to a great degree as sand and gravel quarries for construction and roads. To preserve the other important functions of the ridges – not least as groundwater supplies – Sweden now aims to replace its use of natural gravel like this with crushed material from rock quarries.

Peat bogs 
Peat is a common soil type in the area. It occurs in marshlands, in the form of raised bogs and fens. The most common type of marshland in the area occurs when lakes become overgrown with vegetation, forming a fen. Mud is deposited on the lakebed, while sphagnum mosses grow towards the shores. Once the lake is completely overgrown, a raised bog may form on the top. Raised bogs consist of sphagnum mosses, which require oligotrophic rainwater to grow. Peat can also develop via waterlogging of forest land.

Over the years humans have used peat bogs in many ways. Prior to the Second World War, they were mainly drained to create farmland. Draining forest land to achieve greater timber production continued from that period until the 1970s. In addition, peat was harvested in small and larger scale operations to use as litter for stables, fertilizer, soil improver and fuel. One example of large-scale peat harvesting operations still in use is Karinmossen northeast of Österfärnebo. Some 20,000 m3 of peat is harvested there annually, which corresponds to about 25% of the estimated growth of all peatlands in the area.