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Rivers and streams

Urban streams

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Example of an urban stream

Puhinui Stream is an example of a soft-bottomed urban stream in the Auckland region. The ARC has been monitoring its water quality since 1994.

What is an urban stream?

An urban stream flows through a catchment area with a high amount of urban development for most, or all, of its length. Urban development covers just over one tenth (about 47,000 hectares) of all the land in the Auckland region.
The health of an urban stream and the types of life (ecology) in and around it are influenced by the following factors.

Riparian vegetation

Trees and shrubs are often cleared from urban stream banks so there is no shade. The lack of shade exposes the stream to direct sunlight, causing a permanent rise in the water temperature. It also encourages algae and other nuisance plants to grow. These decrease the levels of dissolved oxygen in the water and make the stream unsuitable for native aquatic organisms.

When there is no riparian vegetation the rainwater can run straight into the stream because there is no buffer zone of vegetation to absorb the water and release it gradually. Consequently, floods are more likely.

Land use in the surrounding catchment

The land in an urban catchment area has been transformed from its original native bush by human activities. Land clearance and excavation, the construction of roads, industrial and domestic dwellings, and the creation of open spaces and urban parks means that all, or almost all, of the land has been disturbed in some way.

The natural contours and shape of the land may be changed by excavation or infilling. The path of the stream may be changed and natural meanders and bends straightened out. This can increase the speed of the water flow and make erosion of the stream banks more likely.

The urban catchment means that the water in an urban stream is often contaminated with faecal coliform bacteria and other pollutants. There may also be high levels of suspended sediment that make the water more turbid. Rainfall that drains into the stream is often warm because it has run off urban land cover, such as roads and roofs, that have absorbed heat from the sun.

Rock type and stream bottom

Urban streams in the Auckland region may be lined with timber or concrete in places, or channelled through culverts or pipes. This removes the variety of natural habitats that are necessary for many stream invertebrates.

Some types of culvert, particularly wide, flat-bottomed culverts and perched culverts, can act as barriers that restrict the movement of fish along the stream.
When the stream is lined, groundwater cannot seep into it. This means the water level may fall to very low levels during long periods of dry weather but when it rains, the water level may rise very quickly.

All of these factors mean urban streams have the lowest water quality and poorest physical habitats of all the stream types in the Auckland region.

Typical water quality of an urban stream

Generally, water quality decreases and water temperatures rises as the amount of urban land cover increases, hence urban streams usually have the worst results of all types of stream found in the Auckland region.

In particular, urban streams have:

  • the highest water temperatures
  • the highest levels of suspended solids and turbidity
  • the highest levels of faecal coliforms
  • low levels of dissolved oxygen.

Typical invertebrate ecology of an urban stream

Urban streams that are not lined provide soft-bottomed stream habitats that are more suitable for invertebrates that prefer slow-moving water, but these type of streams have the lowest numbers of taxa in the Auckland region. Urban streams with particularly poor water quality and / or poor habitats may have only 5 to 9 different taxa. The regional average is 18.

Pollution-tolerant, non-insect taxa such as crustaceans, molluscs, and worm-like organisms tend to dominate urban streams.

What is being monitored?

The Freshwater Ecology Programme is monitoring the biological health and the physical condition of some streams in the Auckland Region.

The Rivers & Streams Water Quality Programme is monitoring the water quality.

When the results from both programmes are combined, they give a good indication of the overall health of a stream and its physical, chemical, and biological condition.

Technical Publications

The ARC has a number of technical publications relating to this subject. Follow this link to see the full list of ARC technical publications.

Continue to Rural streams