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Rogsån below the first dam, Falu municipality, Sweden

Sponsored by

Sweden, Uvnäs, Falun

Rogsån is a small creek in central Sweden, that despite a long history of logging, mining, and being cut-off at both ends by dammed lakes, host a relatively rich ecosystem. This area is situated right below the dam separating the stream from lake Rog above, meaning a high flow year-round from the lake outlet, but with fluctuating water levels depending on the demand from the hydropower plant downstream.

Despite a long history of man-made changes to this section fish densities can be relatively high, especially in summer, when resident trout (Salmo trutta) and miller’s thumb (Cottus gobio) are joined by perch (Perca fluviatilis), pike (Esox lucius), and occasionally roach (Rutilus rutilus), that migrate upstream to take advantage of the cool, oxygen rich, water.

Submitted by
Emil Nordström
GPS
60.7057915, 15.5964003
Geographical region
Northern Europe
Drainage Basin
Mälaren Lake
River catchment
Dalälven River
Water body type
Creek
Water body name
Rogsån
Water body part
Open water
Water body course
Middle course
Water body: tributary of
Tributary name

Water Chemistry

Water information

Water type
Fresh water
Water color
Clear water
Water transparency
Medium
Concentration of sediments
Low
Water temperature
4-22 °C
Water flow/curent
Strong

Chemical parameters

pH
6.9
Conductivity
56
GH
KH
Dissolved Oxygen

Substrate in nature

Sand
Reddish
Pebble/Gravel
Mixed
Stone
Mixed
Stone form
Irregular
Silt/Mud
Grey
Leaves
Few
Driftwood
Many
Submerged terrestrial vegetation
no

Aquatic Biotope

Date of collecting
2022
Collecting area
River bank
Water depth
1,0m
Air temperature
Sunlight
Partial shade

Environment

Environment
Affected by human activity
Affected by human activity
Deforestation
Deforestation
Initial
Surrounding area

Right above this section is lake Rog, an oligotrophic lake that supplies the stream with clear and cool water year round, although in winter the warmer lake water prevents this area from freezing over. The other sides are covered by pine forest, with occasional birch or alder trees mixed in. The trees, along with the steep sides of the lake outlet channel, causes patches of strong light that changes position depending on time of day and year.

Underwater landscape

This section consists of the lake outlet channel and a small pool right below it, with fish moving out into the faster water to feed or resting in the calmer parts. The edges of the outlet section have been built of large rocks with with cut edges, while around the pool the shoreline consists of moraine, like in the surrounding forest. The substrate is a mix, with fine grey silt at the calmest spots, to large irregular boulders and human-added smooth cobbles where the flow is strongest. There are a few sedges around the edges, but no true aquatic plants, and the substrate is instead covered by algae or other biofilm (depending on time of year and amount of light), as well as numerous nest made by free-living Trichoptera larvae. Driftwood includes old planks from the dam and on one shoreline a few trees (mainly pines) that have fallen in.

Fishes:

  • Salmo trutta (Salmonidae)
  • Cottus gobio (Cottidae)
  • Perca fluviatilis (Percidae)
  • Esox lucius (Esocidae)
  • Rutilus rutilus (Cyprinidae)

Crustaceans:

  • Pacifastacus leniusculus (Astacidae)
Threats to ecology

Main threats include flow regulation for hydropower, causing low or unnatural flows, and the dam itself limiting fish migration and gene flow, potentially leading to inbreeding. This is a concern especially for the trout with its more limited population compared to the other fish, but is monitored by the local sport-fishing club. Otherwise this section is quite safe at the moment, lake Rog above is used as a drinking water reservoir meaning that pollution has to be kept low, and tree felling (a problem downsteam) is unpractical due to steep slopes and partly boggy terrain.

Riparian zone

Trees near the aquatic habitat
Many - Pinus sylvestris (Pinaceae)
Trees near the aquatic habitat
Many - Picea abies (Pinaceae)
Trees near the aquatic habitat
Many - Alnus sp. (Betulaceae)