The River Detective's

Ecological Training Tips

Factors that effect the Ecology of Rivers

When investigating a stream, river, or lake, here are the key ecological factors to consider. A competent river detective will have to think about each of these and how they relate to the condition of the river and the relationships between all the creatures that depend on the river.

River Characteristics
Rivers are flows of rainwater or snowmelt that runs or seeps off the land carrying with it eroded particles of rock or soil, dissolved minerals and gasses, organic chemicals and solid matter originating from land plants and animals.
Rivers have their own chemical signatures / fingerprints created from the particular cocktails of imputs into their waters.
Their flow patterns relate to the steepness of the terrain through which they flow and to the volume of run-off which is affected by local climate.

Dissolved Substances
In all water there are dissolved gases from the air and salts (or minerals) which are:

- washed from eroding rock and soil

- released from decomposed plants and animals

- released from animal wastes

- flushed as fertiliser from horticultural and agricultural land.
The concentrations in freshwater are normally very low

- The concentrations in seawater are quite high - about 35ä (± 32 to35 gram per litre).

- In estuaries the concentrations fluctuate between those for freshwater and seawater depending on the tide level and state, and the river flow.

Bush Clearance

Land covered with mature bush which has dense undergrowth and thick forest floor litter acts like a sponge to absorb heavy rainfall and then release it steadily. Removing bush from the land allows very rapid surface run-off of rainfall This can cause rapid erosion of soils and also precipitate landslips.
Bush that over hangs waterways provides shade that helps create conditions favoured by many freshwater animals. It also stabilises river banks. Removing riverbank bush results in bank erosion and reduction of freshwater life along the river edges.


Particles washed off the land form sediment in freshwater. They come from eroded rock and soil, and from decomposing plants and animals.
Large particles (boulders, cobbles, gravel, grit and sand) settle out quickly in fairly quiet water but silt and clay take at least a day to settle in calm, still water. Moving water keeps sediment stirred in suspension and the faster that the water moves the larger the particles it is able to pick up and carry along.

Recreational Activities

People like to use rivers and streams for a variety of recreational activities such as swimming, fishing, water skiing & scenic pleasure. These are all affected by the health of the waterway and of its banks and catchment. Some recreational activities affect the health or character of rivers.


Unnatural compounds or natural compounds in unnatural quantities are pollutants in rives and streams. E.g. Small amounts of sediment are natural and can be tolerated by stream life but large amounts may smother animals or choke their gills.
Higher levels of light, or pure but warm water can also be pollutants. Natural stream life like phytoplankton can also be pollutants if they occur in too high a concentration (called "blooms").
Waste outputs / effluents from industry and town sewerage are often the most obvious forms of pollution but high levels of fertiliser or pesticides washed off the land can be equally serious. These chemicals often change the acidity which is often critical to many freshwater animals.

Introduced Species

A number of fish, birds and plants have been deliberately introduced to New Zealand rivers and streams either for recreation or just to recreate surroundings similar to those that new settlers had been familiar with in their homelands. Other introductions have been accidental or negligent from, for example aquarium specimens being released into waterways.
Introduced species often compete with native species for food or space, or they may predate on them. They may also modify the environment in ways that harm native species. If there are no natural controls introduced species can become troublesome.

Native Species

New Zealand has a fairly small but diverse set of freshwater animals. These range from creatures that graze algal films from hard surfaces, filter freshwater plankton, predate on other animals, scavenge for carrion, and process organic deposits such as fallen leaves and animal dung.

Food from Rivers

Longfinned and shortfinned eels have been an important traditional food source since human setltement here and have been fished commercially for about 100 years. Whitebait, the fingerlings of native fish returning from the sea, have been a valuable seasonal luxury food and freshwater mussels, koura and adult native fish were once minor resources. Introduced fish, mainly salmon and trout have been important as game fish and flourished in New Zealand lakes and rivers.

Water Balance

The blood of freshwater animals is much more concentrated than freshwater so by the process of osmosis water constantly floods into their bodies. Consequently all have to work continuously to pump out excess water as fast as it flows in in order to keep the concentration of their blod and other body fluids at the correct levels.


Many freshwater animals depend on others to survive often by serving as links in the food chain but also by assisting with dispersal or controlling competitors. Stable and balanced community structures are important indicators of a river's health and these are only verified by careful and regular monitoring.


Freshwater animals and plants show many adaptations to surviving in an environment that naturally has varying flow rates and sediment levels. Their adaptations for catching particular foods, resisting being washed away by torrential flows, avoiding predators, mating and dispersal are important to survival in the stream and river environment.

Water Catchment Land Use

The use of land which drains into a stream or river greatly affects water quality and the lives of it animals and plants. Water flow fluctuates least in catchments covered in native bush and most where the land is cleared for farming, industry or residential settlement. These uses also affect what flows off the land into the waterways.

Biological Indicators

The presence or absence or the abundance of certain species of animals and plants can indicate whether a waterway is healthy or under stress. A knowledge of the population structures of different species is often more useful than some physical and chemical measurements because the organisms respond to abnormal events - such as a toxic spillage - and to general long term deterioration of water quality. Population changes can frequently be used to signal other changes which might be able to be corrected before they cause serious long term damage.

Weather and Climate

The natural characteristics of rivers and streams are greatly affected by the local climate and periodically affected by unusual weather events. These will affect the normal range of flow rates and water temperature and limit the colonisation of particular waterways to certain creatures.

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