Forsån ( Swedish: "Rapids' Stream") is a

stream A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a strea ...

in southern

Stockholm Stockholm (; ) is the Capital city, capital and List of urban areas in Sweden by population, most populous city of Sweden, as well as the List of urban areas in the Nordic countries, largest urban area in the Nordic countries. Approximately ...

Sweden Sweden, formally the Kingdom of Sweden, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. It borders Norway to the west and north, and Finland to the east. At , Sweden is the largest Nordic count ...

. It is also known as Stortorpsån and Forsen. Leaving lake

Magelungen Magelungen () is one of the biggest lakes in Stockholm, Sweden, located between the municipalities of Stockholm and Huddinge. It is considered as of great recreational value and is popular for bathing, yachting, and fishing in summers, and tour s ...

calmly by a small sandy beach, Forsån's upper reach passes through a furrow blown in the 1860s to lower the level of the lake resulting in the present wide torrents passing through the bedrock. In contrast, the last 500 metres of the course are characterized by flat shores and stagnant, overgrown waters. The stream has a vigorous population of

signal crayfish The signal crayfish (''Pacifastacus leniusculus'') is a species of crayfish indigenous to North America. Introduced to Europe in the 1960s to supplement the North European ''Astacus astacus'' fisheries, which were being damaged by crayfish plague ...

and is the only winter habitat in Stockholm for

white-throated dipper The white-throated dipper (''Cinclus cinclus''), also known as the European dipper or just dipper, is an aquatic passerine bird found in Europe, Middle East, Central Asia and the Indian Subcontinent. The species is divided into several subspecies ...

.Forsån

Catchment area

Approximately half of the catchment area is occupied by contaminated ground and polluting operations in the Larsboda industrial area; with the remaining area composed of parks and green spaces. One of the minor allotment-garden's is located in an area which is crossed by two major traffic routes and a railway.Vattenprogram, p 30.2

Environmental influence

The inflow from Magelungen brings approximately 2 tons of

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

and 30 tons of

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

to Forsån, the contribution from the catchment area itself, 10 kg and 40 kg respectively, is insignificant in comparison. No estimates of the level of metals brought to the stream has been done, but low or moderate levels of common metals were reported in river sediments in 1997. Although very high levels of

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

next to the road Magelungsvägen were reported.

Fauna

Samples of bottom fauna taken in 1998 documented the presence of nationally rare species of

gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...

such as '' Hippeutis complanatus'' and bivalve swan mussel; and during the preceding decades two species of limnetic gastropods were documented. Larvae of white-legged damselfly and

blue-tailed damselfly The blue-tailed damselfly or common bluetail (''Ischnura elegans'') is a damselfly, belonging to the family Coenagrionidae. Subspecies and varieties Subspecies and varieties include: *''Ischnura elegans ebneri'' Schmidt, 1938 *''Ischnura elegan ...

were recorded in summer 2000.

Signal crayfish The signal crayfish (''Pacifastacus leniusculus'') is a species of crayfish indigenous to North America. Introduced to Europe in the 1960s to supplement the North European ''Astacus astacus'' fisheries, which were being damaged by crayfish plague ...

and

trout Trout (: trout) is a generic common name for numerous species of carnivorous freshwater ray-finned fishes belonging to the genera '' Oncorhynchus'', ''Salmo'' and ''Salvelinus'', all of which are members of the subfamily Salmoninae in the ...

have been introduced in the stream, of which the former succeeded in establishing while roach is believed to be unfavourable to the latter. The

white water Whitewater forms in the context of rapids, in particular, when a river's gradient changes enough to generate so much turbulence that air is trapped within the water. This forms an unstable current that froths, making the water appear opaque ...

rapids make a section of the stream ice-free during the winter time which attracts

s and

common kingfisher The common kingfisher (''Alcedo atthis''), also known as the Eurasian kingfisher and river kingfisher, is a small kingfisher with seven subspecies recognized within its wide distribution across Eurasia and North Africa. It is resident in much of ...

s. In 1996, traces of

beaver Beavers (genus ''Castor'') are large, semiaquatic rodents of the Northern Hemisphere. There are two existing species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers are the second-large ...

were reported.Vattenprogram, p 30.3

Notes

References

* * {{DEFAULTSORT:Forsan Geography of Stockholm Rivers of Stockholm County

Catchment area

Environmental influence

Fauna

See also

Notes

References