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 |
| High quality site characteristics |
Impacted site characteristics |
| Forested headwater stream reaches are generally heavily shaded by overhanging
riparian vegetation. Shading acts to reduce primary production (i.e.,
autochthonous inputs) within the stream, thus the primary food sources
for aquatic invertebrates are generated from outside the stream (i.e.,
allochthonous inputs) and consist of leaves, twigs, and other larger wood
debris. Headwater stream reaches act as accumulators, processors,
and transporters of terrestrial derived material to downstream reaches.
The water for these reaches originates from groundwater sources.
Fine sediment loads are generally low. Cascades and step-pools are
common instream habitat features. |
In comparison to unlogged stream reaches, logging impacted streams
can have either an overabundance or lack of instream woody debris. Construction
of near-stream roads and removal of watershed vegetation increases erosion
and can lead to filling of pools with fine sediment and embedding riffle
gravels and cobbles in finer sediment, which reduces crevice habitat for
aquatic invertebrates and young fish. |
| Potential impacts |
|
| Logging, near-stream road construction, especially stream
crossings, heavy livestock grazing, and mining activities |
| Hydrology |
|
| Discharge regime - small upstream watersheds and typically a constant
ground water inflow reduce high and low flow extremes.
Temperature - little thermal variance due to constant ground water inputs
and an overstream vegetation canopy that shades the stream. |
Discharge regime - variable, with rain and snowmelt events having a
greater impact on instream conditions due to faster and greater runoff
volumes.
Temperature - variable, but temperatures are generally warmer in summer
and lower in winter than unlogged stream reaches. Logging within riparian
areas will increase solar input to the stream. |
| Instream habitat |
|
| Coarse substrate with large amounts of coarse organic matter, such
as leaves, shrub roots, and tree limbs. Organic debris dams are common
and form stair-step pools and riffles. Coarse organic debris accumulates
in pools and provides food and habitat. |
Increased fine sediment amounts in pools and riffles. In pools, this
reduces habitat availability for fish. In riffles, increased fine sediment
levels reduces crevice space among gravel and cobble particles which eliminates
habitat for invertebrates and small fish. The removal of wood from the
instream or riparian areas can cause changes in the shape and stability
of the channel. Too much wood in the stream channel can reduce oxygen levels
in the stream. |
Aquatic invertebate sampling
equipment
kick nets or Surber
nets in small streams, Hess nets
in larger streams |
|
| Dominant functional feeding groups |
|
| Shredders and predators |
Typically and increase in scrapers when the canopy is opened, more
light allows more algae to grow. There may also be an increase in
collector-filterers and gatherers and decreases in shredders. |
| Invertebrate assemblage members |
|
Coleoptera: Elmidae
Diptera: Blephaceridae, Chironomidae, Tipulidae
Ephemeroptera: Ephemerellidae, Heptageniidae
Megaloptera:
Plecoptera: Perlidae, Pteronarcidae
Trichoptera: Rhyacophilade, Limnephilidae |
Typically an increase Chironomidae and Simuliidae and a decrease in
taxa that need abundant crevice space (e.g., Megaloptera and Plecoptera)
or live on the tops of rocks (e.g., Ephemerellidae, Heptageniidae) |
