Shade from overhanging riparian (streambank) trees creates a moist micro-climate, which favours a diversity of plants such as orchids, ferns, mosses and lilies. A wide diversity of animals is also present including frogs, lizards and birds (e.g. owls and hawks) and other predators, which help control rats and other pests in adjacent crops.
Overhanging trees provide a variety of food sources to native aquatic animals. Leaf litter acts as a natural slow-release fertiliser, as well as providing shelter and food (through organic matter) for invertebrates such as crayfish, prawns and some aquatic insects. Fallen insects provide food for specialist insect-eating native fishes such as jungle perch (Kuhlia spp.) and archer fish (Toxotes spp.). Native fruits such as figs (Ficus spp.) and lillypillies (Syzygium spp.) provide a valuable seasonal food source for omnivorous fish such as black bream (Hephaestus fuliginosus).
Rock crevices, submerged tangled tree roots, leaf litter, fallen logs (snags) and undercut banks provide aquatic shelter needed by native aquatic organisms. Catfish often use undercut banks and rock crevices for daytime refuges. Submerged logs provide spawning sites for some fish such as gudgeons and shelter for territorial predatory fishes such as barramundi (Lates calcarifer) and mangrove jack (Lutjanus argentimaculatus).
The high diversity of native plants stabilises stream banks to greatly reduce erosion and maintain high water quality. Such a healthy environment supports a wide range of native animals.
A sustainable stream environment results from appropriate farming, urban and industrial management practices.

Appropriate farm management may include:
- keeping native riparian vegetation intact
- restricting stock access to stream banks
- reducing erosion by trash blanket harvesting of adjacent cane lands.

Good urban management practices may include:
- locating dumps away from flood-prone areas
- tertiary treatment of sewage sediment and use of gross pollutant traps (filters)
- promoting more efficient use of water (Waterwise).

Desirable industrial practices may include:
- appropriate treatment of effluent before disposal
- removal of toxic chemicals
- cooling of superheated water
- increasing oxygen levels before discharge.

6. Adequate stream flows, especially during the dry season, and flushing/flood events are essential for providing a sustainable stream environment.

A poorly managed stream

Removal of native trees enables more light to penetrate which favours the growth of introduced grasses, shrubs, trees and vines. Degraded areas are colonised by a narrow diversity of dominating pest plants such as:
- grasses - para grass (Brachiaria mutica) and guinea grass (Panicum maximum)
- shrubs - lantana (Lantana spp.), privot (Ligustrum spp.) and wild raspberry (Rubus spp.)
- trees - pond apple (Annona glabra) and African tulip tree (Spathodea campanulata)
- vines - thumbergia (Thumbergia spp.).
The lower plant diversity of degraded stream banks provides a smaller variety of food sources. Degraded streams favour introduced exotic pest fish species such as tilapia (Tilapia spp.), guppies (Poecilia reticulata) and platies (Xiphophorus maculatus).
In degraded streams:
- rock crevices may become covered by silt
- log ‘snags’ can disappear in silt or become fouled by algae
- undercut banks are absent due to the lack of riparian trees with complex root systems
- a poorer range of aquatic shelters leads to fewer native fish and lower fish diversity, often favouring introduced fishes such as tilapia.
Erosion of degraded stream banks silts the stream bed, fills water holes and increases turbidity. Shallow watercourses favour invasion of introduced grasses (para grass), which choke the stream, reduce water flow and further accelerate siltation. Slower flowing waters of degraded streams are characterised by conditions that favour introduced fishes:
- lower oxygen levels
- increased turbidity
- more extreme seasonal temperatures and algal blooms (green and blue-green algae).
Lack of appropriate management practices in farmland, urban and industrial areas leads to:
- erosion
- siltation
- fish kills (due to pollutants)
- water unsuitable for domestic use (high bacteria loads and excess nutrients, which can promote blue-green algal scums). Decaying vegetation in silt beds may release toxic leachates and gases (such as sulfides and methane) into the water.
Excessive water demands on streams result in extreme conditions, which may lead to a breakdown in the natural relationships between native plants and aquatic animals.

Comparative cause and effect table

Well managed stream

Cause/feature	Effect
1. Retaining native streambank (riparian) vegetation for shade	c. cooler water temperatures and fewer weeds due to shading from vegetation
2. Maintaining rich diversity of native plants	e. provides food and shelter for a wide variety of animals
3. Keeping features such as rocks, tree roots and spawning sites logs on banks and in streams	b. suitable habitats, shelter and for many animals
4. Preserving healthy riparian forest structure, high water including trees, shrubs and ground cover	d. reduces erosion and maintains quality for native animals
5. Managing urban and rural land appropriately	f. reduces pollution and erosion, producing healthy, sustainable streams
6. Ensuring natural patterns of high and low flow in streams	a. promotes a diverse stream environment

Poorly managed stream

Cause/feature	Effect
1. Removal of shady, native riparian vegetation	c. increases light and promotes growth of exotic pest plants
2. Loss of native riparian and instream plants	f. reduces food sources for native animals
3. Removal of rocks, tree roots and logs and build-up of silt on streambeds	b. removes habitat of native aquatic species
4. Unprotected and eroding stream banks	e. greatly decreases water quality by increasing sediment in streams
5. Poor land use management in catchments	d. leads to pollution, salination and siltation
6. Over-extraction of water from streams	a. native life in streams leaves or dies, exotic pest fish may survive

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Teacher resource d.4 Developmental

Death by degradation

A well managed stream

A poorly managed stream

Comparative cause and effect table

Well managed stream

Poorly managed stream

Teacher resource d.4
Developmental