Streams always reflect the surrounding topography and type of land cover. These broad-scale factors determine many of the instream habitat conditions that shape aquatic communities. The key aquatic habitat factors include current speed, water depth, type of streambed, type and amount of organic matter entering the stream, amount of instream cover and the essential water quality characteristics such as dissolved oxygen levels, temperature and contaminant levels.
Mountain and hill country streams
In mountainous areas the steep terrain creates fast-flowing streams with rocky beds, and the limited human interference in such areas means the water is likely to be of high quality. Mountain and steep hill country streams are often covered by native forest, and this is where you’re most likely to find pristine habitats with the most spectacular stream life. However, some harsh mountain environments can create stressful conditions for stream life, for example high altitude streams may be very flood-prone, or unstable because of rapid erosion, or quite sterile, and these streams may not support a wide variety of stream life.
Lowland, low-gradient streams
Lowland streams, which often run through farmland, tend to be slower flowing, less shaded, warmer and with muddy/weedy beds. Such habitats may be teeming with invertebrate life, but these invertebrates are likely to be smaller and less spectacular than those of forest streams. The quality of lowland streams is often affected by:
- the amount of sediment or nutrients entering the water,
- the quality of riparian (river margin) vegetation, and
- the amount of shade along the stream length.
Landuse practices can have drastic effects on stream habitats, for example stock access to streambanks accelerates erosion and sediment inputs, and exotic forests may provide 30 years of protection of stream habitats followed by sudden disruption during logging.
Urban streams
Urban streams are often highly modified, with flashy flows, straightened channels, piped reaches and contaminated water, and not surprisingly their biota can be quite limited. Many Auckland streams have been piped and these underground reaches provide almost no habitat value to fish or invertebrates. Other urban streams have been turned into concrete-lined drains and these habitats offer no cover for our secretive native fish, and no shelter from flood flows for fish or invertebrates. Urban streams are often heavily contaminated by heavy metals from road runoff, toxic hydrocarbons from roads and industrial wastes, sediment from earthworks, and sewage from leaking sewer systems. Not surprisingly, urban streams tend to support fewer fish and invertebrate species than rural streams, and those species living in urban streams tend to be very tolerant of a wide range of water quality conditions.
Riffles, runs and pools
Another way of looking at stream habitats is to look at the physical habitat type at a particular site. Stream biologists often refer to three basic habitat types. Riffles are shallow, fast flowing, usually stony bedded sites, and these habitats usually support the greatest diversity of stream life (the best places to look if you’re studying the state of stream invertebrate communities). Runs are sections of stream with an obvious flow, but the surface of the water is not broken (as in riffles) and the water is usually deeper than in riffles. Runs can support a high variety of aquatic life if there is a mixture of stony, woody or weedy bed material, and if dissolved oxygen levels are high and water temperatures are low. Pools are slow-flowing or stagnant habitats and they tend to support fewer invertebrate species than riffles or runs. Pools may be the best places for large native fish including eels and kokopu, especially if there is abundant cover in the form of undercut banks, large woody debris or boulders.
The perfect stream habitat
So what’s a “perfect stream environment”? Generally the greater variety of stream habitats the better. Streams with complex beds, providing lots of cover from floods and predators tend to support most species. A mixture of riffles, runs and pools also caters for a wide range of species. Cool temperatures, consistently high dissolved oxygen levels, and low levels of nutrients, suspended sediment and other contaminants, are all positive factors. Natural riparian vegetation providing shade, bank stability, woody debris and edible plant and animal matter, is also part of the “perfect stream environment”. Natural flow regimes are also important. Rapidly-draining urban areas cause unnatural flash floods in streams, and for this reason alone urban streams are unlikely to measure up to forest covered streams, or even most rural streams.
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