The River Detective's
Ecological Training Tips
Factors that effect the Ecology of Rivers
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
- 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
- - washed from eroding
rock and soil
- - released from
decomposed plants and animals
- - released from
- - flushed as fertiliser
from horticultural and agricultural land.
in freshwater are normally very low
- - The concentrations
in seawater are quite high - about 35ä (± 32 to35 gram
- - 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.
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
- 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
- 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.