Habitat indicators of stream health

9.2 Habitat indicators of stream health

HABITAT QUALITY
Water velocity

Water velocity

Water velocity depends mostly on the slope of the stream, though in streams in rolling to hilly catchments that have the pool–riffle–run structure (see pages 9.28–9.29 for an explanation) there will be variations in flow velocity over a short distance. Selecting a “run” area as a monitoring site helps to ensure that the velocity is easy to measure (no obstructions or very slow-moving areas).

The water velocity measured from the rate of movement of a floating object is only a rough indication because water moves at different speeds at different depths and distances from the stream edge. In a shallow stream the measurement is an approximation of maximum water velocity, because water speed is greatest where friction is least – at the surface near the centre of the channel. Near the bed, water movement is slower because of friction between the water and the substrate. Obstructions in the stream (e.g., logs, debris, silted-up areas) also slow down the flow.

Water velocity represents one of the most important environmental factors affecting the biota of streams. On one hand, water delivers food materials and oxygen and removes waste materials; on the other hand, flow is a direct force on organisms. Both invertebrates and plants have evolved ways to withstand high flows and to take advantage of the associated high oxygen content and nutrient supply. These include taking refuge in the slower moving areas, for example, beneath or behind large stones on the stream bed, or within the leaves and stems of water plants. This means that even faster flowing waters can support diverse and healthy communities of invertebrates and periphyton.

Very slow flows represent the most unfavourable conditions for stream life. Under these conditions the oxygen supply is restricted and there is potential for wide variations in both pH and temperature (see sections below) which can be harmful to aquatic invertebrates.

Water velocity is a local factor that generally cannot be altered, though very slow flows may be a result of an obstruction or damming downstream of the site. Seasonal growth of water plants may affect water velocity. If flow velocity differs markedly from one monitoring to the next, then some activity upstream or downstream could be causing the change.

Less than 0.1 m/sec
Rating: very poor
Score: 1

Less than 0.1 metres/second

The stream is very sluggish and invertebrates that require rapidly flowing, clean, waters such as mayflies and stoneflies will probably not be present. The bed is likely to be fairly silty and could well have profuse aquatic plant growth. Mats of long filamentous algae can grow from stable attachment points and snails, midges and worms probably dominate the invertebrate community.

This could be a natural condition for your stream, and in this case it is especially important that the stream is protected from other sources of degradation.

0.1 to 0.29 m/sec
Rating: good
Score: 8

0.1 to 0.29 metres/second

This is a low to moderate velocity and capable of supporting a range of both pollution-tolerant and pollution-sensitive invertebrates.

0.3 to 0.69 m/sec
Rating: very good
Score: 10

0.3 to 0.69 metres/second

This is the range of moderate stream velocities, which is ideal for healthy invertebrate and periphyton communities, though the exact components of the community will also depend on other factors, particularly stream substrate.

0.7 to 0.99 m/sec
Rating: very poor
Score: 5

0.7 to 0.99 metres/second

This range represents moderate to high stream velocities that are likely to preclude some invertebrates such as snails and some caddis larvae from living in the stream. In high-velocity streams in mountain areas (i.e., areas with little or no agricultural activity) mayflies and stoneflies should dominate and the bed should be composed of gravels and cobbles. However, when these velocities occur in low-altitude streams flowing across agricultural land, it generally means that they are flowing in erosion gullies where the stream bed is mixed material and sediment movement is high. Such an environment is not good for the development of diverse, healthy stream-bed communities of either periphyton or invertebrates.

1 m/sec or more
Rating: poor
Score: 3

1 metre/second or more

Stream velocities are high. In mountain streams, this will result in a bed dominated by cobbles, usually with only thin mats/films of periphyton and invertebrates dominated by mayflies/stoneflies and some caddis. Midges can also grow in the crevices between (and underneath) the stones. However, such streams are unlikely to be on agricultural land, and velocities of over 1 metre per second on farm properties usually indicate flow in severe erosion gullies with associated silt movement and unstable bed material.

Water pH

Water pH

pH is a measure of the concentration of hydrogen ions in the water, and hence the strength of acid present. The pH scale ranges from 1 to 14, with pH 7 indicating neutral conditions. Under 7 is acid and over 7 is alkaline. With increasingly acid waters, numbers of species and individuals of aquatic organisms decrease. For example, some species avoid egg-laying in low pH conditions.

Rainwater is normally slightly acidic because it contains dissolved CO2 from the air. A small proportion of the gas dissolved in pure water forms carbonic acid (H2CO3). Stream water usually contains bicarbonate and carbonates and the carbonic acid itself can dissolve carbonate rocks to form carbonates; all this tends to neutralise the acid so that natural waters do not usually experience wide fluctuations in pH.

Industrial activity in northern Europe has lead to the formation of strong acids (sulphuric and nitric acids) in the atmosphere and this has contributed to a lowering of pH in surface waters of the Northern Hemisphere. Such effects are not common in New Zealand. However, many streams have naturally low pH due to high concentrations of humic substances – the products of vegetation decay which are picked up as water percolates through the ground before reaching a stream. In brown-water streams draining swampy areas pH measures of 5 or less are quite common.

pH 5 or less
Rating: poor
Score: -5

pH: 5 or less

pH is very low. This could be due to the waters draining a swamp or some pollution discharge/seepage (e.g., from cleaning agents). This level of acidity can result in reduced numbers of species and individuals of invertebrates. There is evidence that naturally acid waters are less harmful than those acidified by atmospheric pollutants. One suggested reason is that non-natural acid waters take up metals and retain them in solution more readily and it is the metals that are detrimental to stream life (see Allan, 1995, p. 40–41, for examples).

pH 5.5 - 6
Rating: fair
Score: 5

pH: 5.5 to 6

pH in the 5.5 to 6 range is moderately low and would normally signify an influence of waters draining swampy areas. A pH in this range is not normally detrimental to stream life. However, in farm streams not connected to swampy areas, the acidity is probably a result of non-natural inputs and there is potential for deterioration if no action is taken to trace the source.

pH 6.5 - 7.5
Rating: excellent
Score: 10

pH: 6.5 to 7.5

The pH range around neutral is good for stream life.

pH: 8 - 9 Rating: fair
Score: 5

pH: 8 to 9

pH in the 8 to 9 range is moderately high and usually signifies intense photosynthetic activity by periphyton and macrophytes. This can be checked by taking another recording the following morning before 8 a.m. If the pH has dropped back to 7.5 to 8.0, then the high daytime reading is probably a result of aquatic plant growth.

pH 9.5 or more
Rating: poor
Score: -5

pH 9.5 or more

A pH of greater than 9 is high and usually signifies intense photosynthetic activity by periphyton and macrophytes. This can be checked by taking another recording the following morning before 8 a.m. If the pH has dropped back to 7.5 to 8.0, then the high daytime reading is a result of aquatic plant growth.

Water temperature

Water temperature

Stream temperature is important because every species has a preferred temperature range. The range varies considerably from species to species. Sometimes, a temperature change is important, for example, as a trigger for egg hatching in some mayflies. Many organisms are unable to survive in temperatures above about 30 °C (except for some adapted to life in hot springs). At the other end of the scale, temperatures below freezing point constitute a very harsh environment because of the effects of ice.

A single temperature measurement is not particularly informative, but a series over time will provide a rough picture of the temperature regime in a stream. The longer the series and the closer together the measurements, the more informative the series will be.

Temperature depends largely on time of year and weather conditions. Stream type also plays a part. For example, lowland streams tend to experience quite stable temperatures (i.e., closely following average air temperatures). Shading along streams reduces the occurrence of extremely high water temperatures.

Water temperature fluctuates on a daily basis and for this reason it is suggested that measurements are always conducted at the same time of day.

Less than 5
Rating: fair
Score: 5

Less than 5 oC

Values below 5 ºC are low and indicative of winter conditions in southern regions. Invertebrate and periphyton growth would be slow in such waters. Some species may be excluded.

5 to 9.9
Rating: good
Score: 8

5 to 9.9 oC

Values of 5 to 10°C are moderate to low and indicative of winter conditions. Most invertebrates and periphyton can survive well in these temperatures.

10 to 14.9
Rating: excellent
Score: 10

10 to 14.9 °C

Values of 10 to 15°C are very suitable for most invertebrates and periphyton.

15 to 19.9
Rating: good
Score: 5

15 to 19.9 oC

Temperatures of 15 to 20°C will start to be stressful for some invertebrates (e.g., stoneflies).

20 to 24.9
Rating: fair
Score: 5

20 to 24.9 °C

Temperatures of 20 to 25°C are moderately high. Some invertebrates, such as some mayflies, stoneflies, and some fish, such as trout, are unlikely to survive such conditions for prolonged periods (e.g., several weeks).

25 to 29.9
Rating: poor
Score: 0

25 to 29.9 °C

Temperatures between 25 and 30°C are likely to be stressful to fish, stoneflies, mayflies and some caddis flies. Such high temperatures may be a result of lack of shading and very sluggish flows.

30°C or more
Rating: poor
Score: -5

30 °C or more

Temperatures over 30°C are likely to be very stressful to most stream life and result in their death. Again, such high temperatures may be a result of lack of shading and very sluggish flows. However, stream temperatures will rarely get to these levels.

Water conductivity

Water conductivity

Conductivity is a measure of the total ionic strength of the water and is widely used in water quality studies as a quick field indication of the level of enrichment (i.e., nutrient content) of the water. It is measured in microSiemens per centimetre (mS/cm). The method has limitations. In particular, areas that receive geothermal inputs or are subject to tidal influence will show higher readings because of the presence of non-nutrient ions (for example, the sodium ions in seawater). For most farm streams a conductivity reading should give a reasonable indication of nutrient levels.

All stream waters contain some nutrients as a result of natural conditions and processes. The underlying rock type determines the “base” level of nutrients in streams (see pages 9.25–9.27). Runoff and seepage are natural processes which add extra nutrients. In agricultural areas these inputs may increase because of both non-point-sources, such as gradual runoff from cultivated land, and direct inputs from stock faeces and urine.

Nutrient level is important in stream health because, under suitable conditions, excessive amounts encourage proliferations of algal growth which can in turn lead to wide daily fluctuations in both pH and dissolved oxygen levels which are known to be harmful to fish. Thick growths also exclude certain invertebrate species (e.g., mayfly larvae), and there is an overall degradation of waterways in both biology and appearance. In larger streams and rivers, periodic high flows can flush out algal growth. However, in small farm streams, the degradation may be less easy to reverse.

Note that after prolonged or heavy rain, the conductivity of stream water may be elevated due to increased runoff.

Under 50
Rating: excellent
Score: 20

Less than 50 mS/cm

Very low concentrations of dissolved ions. Nutrient enrichment is highly unlikely unless there is a specific waste-water discharge. Usually expect only thin films of periphyton.

50 to 149
Rating: good
Score: 16

50 to 149 mS/cm

Low concentrations of dissolved ions. Nutrient enrichment is unlikely unless there is a specific wastewater discharge. Usually expect only thin films or mats of periphyton.

150 to 249
Rating: fair
Score: 10

150 to 249 mS/cm

Slightly enriched waters. Thick mats of slime and some green filamentous periphyton growths may occur in summer if the stream bed is cobbly and water velocities are 0.31 to 0.7 m/s during summer low flows.

250 to 399
Rating: poor
Score: 6

250 to 399 mS/cm

Moderately enriched waters. Thick mats of slime and green filamentous periphyton growths may occur on any stable objects in the stream during summer low flows.

400 or more
Rating: very poor
Score: 1

400 mS/cm or more

Enriched waters. Extensive mats of green filamentous periphyton should be expected in low velocity areas ( under 0.3 m/s), particularly in summer. Your stream may be draining a catchment with a siltstone/sandstone geology. In some areas, conductivity can be very much higher than this (e.g., up to 700 or 800 mS/cm).

Water clarity

Water clarity

The clarity of stream water has a powerful influence on people’s perception of stream health: clear water is generally assumed to be clean.

Clarity is also an important feature of the habitat for stream life because it affects the amount of light that gets through to the stream bottom. Plants (in this case, periphyton – algae) need light for photosynthesis, and hence growth. If algal growth is strongly inhibited by low water clarity, and if the problem persists for long enough, there may be flow-on effects. For example, invertebrates that consume algae may die out.

Water clarity generally reflects the amount of fine suspended sediment in the water. The size of the sediment load depends on underlying rock type and the amount of erosion or stream-bed disturbance going on upstream. On farms, a range of activities may contribute: cultivation too close to the stream edge; stock in the stream; bank collapse. Recent rainfall may cause sediment input to increase. In slow-flowing streams, sediment may settle out onto the stream bed, with the potential effect of smothering invertebrate and periphyton habitat.

The apparatus provided with this kit (“clarity tube”) is based on a scientific method of measuring water clarity called the black disk method (Davies-Colley 1988). The clarity tube has been designed for use on farm streams that are fairly turbid (see Kilroy & Biggs 2002). The following is a commentary on the range of readings that may be obtained.

Clear to bottom
Rating: excellent
Score: 10

Clear to bottom

For a farm stream this represents nice, clear water! Note that the New Zealand clarity standard for recreational use of fresh water (e.g., swimming) (MfE 1994) is a conventional black disk measurement of 1.6 metres. This is equivalent to a clarity tube reading of about 85 cm (see the SHMAK information sheet “Measuring Water Clarity: Tube, Disk and Turbidity” in Unit 15. Educational and training material).

70 to 99 cm
Rating: good
Score: 8

70 to 99 cm

Slightly turbid. This may inhibit plant growth and the suspended solids could settle on the stream bed.

55 to 69 cm
Rating: fair
Score: 5

55 to 69 cm

Moderately turbid water. It will be difficult to see the bottom in pools and at this level is probably starting to affect stream life, both through light restriction for photosynthesis and through settlement of sediment on the stream bottom. A review of what is happening upstream is needed.

35 to 54 cm
Rating; poor
Score: 5

35 to 54 cm

This very turbid water is likely to silt up the stream bed and be detrimental to most stream life. Again a review of what is happening upstream is needed. This clarity will almost certainly be caused by some fairly obvious disturbance.

Less than 35 cm
Rating: very poor
Score: 1

Less than 35 cm

This extremely turbid water will result in a silty stream bed and will be detrimental to most stream life. An immediate review of what is happening upstream is needed. This clarity will almost certainly be caused by an obvious disturbance.

Composition of the stream bed

Composition of the stream bed

A range of materials can make up the stream bed, from solid bedrock through to fine silt.

Bed material type has a major influence on the type of organisms found in a stream and this is reflected in the scores assigned to the different materials. To obtain an overall score for a given stream site, the monitoring system requires an estimate of the coverage of each different category.

Bed composition may not change greatly between monitoring occasions. Indeed, in healthy streams any changes will be minor. However, because of the potential for siltation, or clearing out of silt, on farm streams, it is useful to record the bed composition after any changes in the stream environment due to major natural events (e.g., floods) or changes in land-use practices.

Bedrock
Rating: poor
Score: -10

Bedrock

Bedrock – a solid rock surface – may occur in streams on upland farms. The rock tends to be smoothed from the action of water flowing over it. There are few crevices within which insects and native fish can live and/or hide, so bedrock is not a very good habitat for stream communities. Snails can often be seen crawling over the surface of bedrock areas grazing the attached periphyton. Aquatic mosses can grow well on bedrock and if these are extensive then they can provide some areas of good habitat for insects.

Boulders
Rating: good
Score: 10

Boulders

Boulders are defined as rocks that are more than 25 cm across (i.e., breadth rather than length – they will not fit through a square aperture with 25 cm sides.) Because of their size they tend to be very stable and have moderate amounts of space around their bases within which insects and native fish can live.

Large cobbles
Rating: excellent
Score: 20

Large cobbles

Large cobbles are 12–25 cm across (i.e., they fit through a square with 25 cm sides, but not through a 12 cm square). They are fairly stable and have extensive crevices between adjacent stones within which insects and native fish can live. Stream beds composed mainly of large cobbles can form excellent habitat for stream life as long as the gaps between the stones are not filled with silt.

Small cobbles
Rating: good
Score: 10

Small cobbles

Small cobbles (6–12 cm across) tend to be reasonably stable and have extensive crevices between adjacent stones within which insects and native fish can live. Stream beds composed mainly of large cobbles tend to be good habitat for stream life providing the gaps between the stones are not filled with silt.

Gravels
Rating: fair
Score: 0

Gravels

Gravels (up to 6 cm across) tend to be stable during average flows. Because of their size, they pack together with very small spaces in between. These spaces could form reasonable habitat for invertebrates. However, again because of their small size, gravels move quite easily if water flows increase. This movement tends to be detrimental to the stream life, by scouring off any periphyton growth and attached invertebrates and exposing invertebrate refugia (or hiding places).

Sand
Rating: poor
Score: -10

Sand

Sand is usually quite mobile and thus represents a poor habitat for most stream life. However, some worms and insect larvae borrow into sands and live below its surface. In stable spring-water streams, periphyton may grow on the sand surface.

Mud/silt
Rating: very poor
Score: -20

Mud/silt

Mud and silts tend to be very mobile and thus represents a poor habitat for most stream life. However, some worms and insect larvae borrow into sands and live below its surface. Mud and silt can also become stagnant and the resulting low levels of oxygen near the surface of the mud and liberation of sulphides can be very detrimental to stream life.

Man-made
Rating: very poor
Score: -20

Man-made

Most man-made bed materials, such as concrete, tend to be poor habitat for stream life, except in situations where other stable habitat may be missing (e.g., shopping trolleys in urban streams can be good habitat for fish and invertebrates).

Woody debris and water plants
Rating: fair
Score: 0

Woody debris

Woody debris is often utilised by stream insects as habitat, and is particularly important in soft-bottomed streams, where it may be the only ‘stable’ habitat feature available. Removal of woody debris can be detrimental to stream communities in such streams.

Score summary for stream bed composition

Water plants (rooted in the stream bed)

Water plants usually grow in streams with silt or sand beds. They provide an extra layer of habitat above the bed. Many aquatic plants such as water cress (emergent plant) and oxygen weed (submerged plant) are used by snails, worms and caddisfly larvae as habitat. However, some aquatic plants can reduce stream health by choking waterways and inducing harsh instream conditions.

Understanding your stream bed scores: summary

Score: -20 to -10
Much of the stream bed is silt, sand or artificial surfaces. The former two are unstable and all three are poor habitat for most invertebrates and periphyton.

Score: -10 to 0
Bedrock does not have many crevices for invertebrates to hide in so is not very good habitat. Snails can occupy the surfaces. Sand tends to be very mobile and poor habitat also. Gravels, woody debris and water plants can provide limited habitat for invertebrates, but are generally poorly colonised by periphyton.

Score: 0 to 10
Moderately high proportions of cobbles provide good invertebrate and periphyton habitat.

Score: 10 to 20
Large proportions of the bed composed of cobbles provides really good habitat for invertebrates and periphyton.

Deposits

Deposits

Deposits (fine, loose, usually pale brown “flocculent” [almost floating]) material covering or partly covering the stream substrate) can form a temporary covering on the stream and generally indicate that some recent event has caused excess suspended sediment to settle out. This can happen when extra silt has entered the stream, or when the water flow has slowed down to below “normal”. Streams with sand or silt beds may also have additional deposits and these are identified from material settling out on leaves of water plants near the banks. Deposits are not always easy to identify, especially on streams that already have silt or sand-covered beds.

Little/none Rating: excellent
Score: 10

Little or none noticed

Clean gravel/cobble/boulder/bedrock surfaces are good for stream life.

Fine cover
Rating: good
Score: 5

Fine cover (less than 1 mm thick) of silt mainly in edge areas

Limited amounts of fine deposits in edge areas should not inhibit invertebrates greatly. If composed of organic detritus, these deposits may provide food for some invertebrate groups.

Moderate cover
Rating: fair
Score: 0

Moderate cover (up to 3 mm thick) in edge areas and elsewhere

A moderate amount of streambed siltation. This can inhibit the development of healthy invertebrate communities.

Moderate to thick
Rating: poor
Score: -5

Moderate to thick layer (3 mm thick or more), patchy, most of bed

A moderate to high degree of streambed siltation. This is likely to reduce the quality of the habitat for streambed life.

Thick
Rating: very poor
Score: -10

Thick (over about 5 mm thick) on most horizontal surfaces

A severe amount of streambed siltation. This will eliminate some invertebrate species (e.g., some caddis larvae) that are sensitive to silt and reduce the diversity of periphyton communities.

Bank vegetation

Bank vegetation

The vegetation growing on the banks of a stream forms an important part of the stream habitat. Taller plants provide shade and important habitat for adult insects. Shade helps to reduce temperature extremes, especially in slow-flowing streams. On small streams, even short vegetation gives valuable shade. All substantial vegetation (i.e., not crops or close-cropped grass) protects the stream from exposure to sunshine and winds. Root systems help to stabilise the stream bank and prevent erosion, especially if vegetation grows right down to the water’s edge. Inputs such as leaf fall add organic matter to the system and are an important food for some invertebrates. Vegetation growing immediately next to the water provides shelter for fish. Bank vegetation as a whole acts as a filter for runoff from the surrounding land, removing nutrients and thereby partly “purifying” the water before it enters the stream. A diverse, native vegetation is most desirable.

Native trees
Rating: excellent
Score: 10

Native trees

Native trees and shrubs (and also non-coniferous introduced evergreens) form excellent stream bank vegetation, providing large shaded areas which reduce the potential for increased water temperatures and the problems this may cause (e.g., blooms of algae). They are good filterers of pollutants in runoff. Leaves fall into the stream all the year round and are a steady source of nutrients, and food for invertebrates

Wetland vegetation
Rating: excellent
Score: 10

Wetland vegetation

Marginal wetlands are extremely good for filtering out pollution from the land such as silt and nutrients, particularly when stock are denied access to these sensitive areas. Wetland vegetation is often recommended as suitable for riparian planting (see Collier et al. 1995) and the restoration and protection of natural wetlands is encouraged. Waters flowing from wetlands are often “tea-stained” from the tannins released from decomposing vegetation. There is usually very little pollution from nutrients and silt in such streams.

Tall tussock grassland
Rating: good
Score: 8

Tall tussock grassland (not improved)

Large tussock bushes overhanging streams provide excellent shade, some bank stability, and help to filter out pollutants in runoff.

Introduced trees (willow, poplar)
Rating: good
Score: 8

Introduced trees (willow, poplar)

Poplars and, particularly, willows provide excellent shading for streams, helping to maintain lower stream water temperatures during summer and the reduction in light helps prevent periphyton blooms. Willows have often been used very effectively to help stabilise stream banks, but are aggressive colonisers and may outcompete native wetland and forest vegetation (see Howard-Williams and Pickmere 1994). Introduced trees are often deciduous, releasing large amounts of leaf fall into the stream over a short period in autumn. Submerged willow roots provide excellent habitat for eels and other native fish, but may fill-up streambed interstices and reduce the available habitat for invertebrates

Other introduced trees
Rating: fair
Score: 5

Other introduced trees (conifers)

Conifers provide the same shade, stabilisation and nutrient filtering benefits as the native and deciduous introduced trees. However, their leaves contain resins and generally decompose less quickly within the stream.

Scrub
Rating: fair
Score: 5

Scrub

“Scrub” refers to weedy species (e.g., gorse, broom) and regrowth. This can provide reasonable shade for very small streams, thereby helping prevent high water temperatures and periphyton blooms. On wider waterways scrub is less effective as shade; also scrub tends to be in areas that may be experiencing increased erosion as the stream channels go through the transition from one vegetation phase to another (e.g. pasture reverting to native forest).

Short tussock
Rating: poor
Score: 3

Short tussock grassland, improved

Short tussock mixed with pasture grasses provides little shade and reduced filtering capacity compared to tall tussock.

Rock, gravels
Score: 5

Rock, gravels

Rock and gravelly banks are of little value to stream ecosystems, except where this is a natural feature of the stream type (e.g. braided rivers). They do not have negative impacts, but neither do they benefit the stream by providing shade or filtering runoff from surrounding lands. Such banks may be more easily eroded during floods than tree-lined banks. Overall their influence is “neutral”.

Pasture grasses and weeds
Rating: poor
Score: -10

Pasture grasses and weeds

Short vegetation does not benefit the stream either by providing shade or by filtering runoff from the surrounding area. Such banks are also more easily eroded during floods than banks lined with trees or mixed tall vegetation.

Bare ground, roads, buildings
Rating: poor
Score: -10

Bare ground, roads, buildings

Bare ground or built-over areas near or adjacent to a stream edge help to increase direct runoff into a stream. Unsealed roads or open areas may be a source of silt that could find its way into the stream. Sealed roads or other areas with vehicles are liable to contain pollutants.

Score summary for bank vegetation

Understanding your bank vegetation scores: summary
Score: 8 to 10

Trees provide shade for the stream bed which helps reduce daily fluctuations in water temperature and reduces light penetration to the stream bed if the channel is narrow enough. This can reduce the potential for blooms of filamentous algae and maintain better water temperatures for invertebrates and fish. Tall tussock plants have a similar benefit in narrow streams and, together with wetland vegetation, can also act as a good filter for land runoff.

Score: 5 to 8

Trees and scrub provide shade for the stream bed which helps reduce daily fluctuations in water temperature and reduces light penetration to the stream bed if the channel is narrow enough (5–6 metres). This can reduce the potential for blooms of filamentous algae and maintain better water temperatures for invertebrates and fish. Tall tussock plants have a similar benefit in narrower streams and, together with wetland vegetation, can also act as a good filter for land runoff. Trees and tussock are better for this (and more permanent) than scrub.

Score: 0 to 5

The lack of tall vegetation on the stream banks is potentially a limiting factor for healthy stream communities.

Score: -10 to 0

Pasture, bare ground, roads and buildings right to the stream edge at the site will reduce the quality of the habitat for instream life. Filamentous algal blooms could occur periodically and invertebrate communities may not be very diverse.