What are River Habitats?

A river can be defined as a flowing body of water supplied primarily from lakes, ponds or streams which runs into a larger river, a lake, or the sea. Rivers are important habitats for large numbers of aquatic organisms including many common fish which use the rivers to migrate to and from their spawning and nursery areas.

Rivers affect the land beyond their banks. These areas, known as flood plains, have special plant communities. Rivers tend to be exposed to direct sunlight; they are warmer than their feeder streams, and consequently, support a great quantity and variety of animal and plant life.

How are River Habitats Disrupted?

Throughout the year, the volume and level of water passing through a river changes substantially. This variation is most noticeable at the peak of spring floods and during the late summer when water levels are low. During periods of high runoff the water current quickens, sweeping debris, such as logs and rocks, downriver. The debris scours the river banks and bed especially where the river bends. The result can be the loss of aquatic plants and protective riverside vegetation, such as trees and shrubs, which otherwise helps to reduce bank erosion.

DDT, lead and cadmium can accumulate throughout the food chain.

The long-term discharge of nutrients, in the form of leached fertilizers or sewage, will enrich the river environment. The effects of these nutrients, such as algal blooms, are most noticeable during the summer when biological activity is at its highest. They will continue to affect the river even after the discharge has stopped. However, as nutrients and the breakdown products of the enriched growth move downriver, plant and algae production will slow down and bacterial levels, from decomposing materials, will decrease. The river system will gradually return to its pre-enrichment state.

The development of urban or rural areas within the watershed can also have longterm effects on river habitats. Even with the use of control measures, such as sewage and toxic waste treatment and disposal, the effects of development may be felt. The river may be altered by the regular runoff of salt and oils from the highways and roads, and leaching of fertilizers from lawns and pastures. Usually the river will be able to minimize the adverse effects of runoff materials if their concentrations remain relatively low. However, even low levels of discharge over a long period of time can cause changes in plant and animal communities.

How Severe are the Disruptive Effects and How Long Will They Last?

Single events, such as the release of a concentrated amount (slug) of silt or a toxic chemical will displace or destroy sensitive species. However, these species will return, or be replaced by fish from unaffected areas within the watershed, after the slug has passed.

Long-term events, such as a continuous discharge of a toxic chemical will affect the river as long as the discharge continues. Persistent chemicals, such as DDT, or heavy metals, such as lead or cadmium, can accumulate throughout the food chain. Once the discharge is stopped, the material will work its way downstream or be covered by newly deposited sediments.

When the water level is high, many insects and fish normally found in the main channel move into protected areas such as quiet backwater holes, crevices or lakes. As the level drops, fish may become trapped in shallow pools and side channels where they are more exposed to predation from birds and fish-eating mammals and face stronger competition for the limited food supply.

The condition of the river will reflect the presence of materials introduced upstream. These may include fertilizer or toxic chemical runoffs, sewage, or silt from eroding river banks. It will also reflect the presence of dams or other obstructions. During high water levels, insects and fish may move into protected backwater holes. During low water levels, fish may become trapped in shallow pools.

Toxic Chemicals

Toxic chemicals can destroy individual or complete groups of fish, insects and plants. Persistent chemicals can build up in fish and plant tissues (bioaccumulation) and can result in the contamination of all portions of the food chain. Herbicides, used to control vegetation, can leach out of the soil and enter the river, where they may have a toxic effect on plants and animals.


Irrigation will increase the amount of material that is flushed into the river. This may result in bioaccumulation of pesticides or increased salinity, which may render the water unsuitable for many freshwater species. Fertilizer runoff may cause excessive plant and plankton (algal blooms) growth in the river and in downstream lakes.

Excessive amounts of silt or mud in the water can destroy river habitats by smothering invertebrates and fish eggs. Suspended silt will block the amount of sunlight entering the water and result in a reduction of plant and plankton (algae) growth.


Permanent changes in water levels, such as those that occur with the installation of a dam, have very dramatic effects on river habitats. As conditions are altered from a fast flowing state (riverine) to a slower, lake-like state, the aquatic plant and animal communities also change.

A reduction of flowing water will decrease food and oxygen in the river, causing the original animals and plants to be replaced gradually with lake species. Without a source of suitable replacements, the result can be a large, natural-looking body of water with relatively little aquatic life. Physical or chemical barriers, such as beaver dams or toxic spills, that block a river can hinder the migration of fish. If fish cannot reach their spawning grounds or feeding areas, their populations may be seriously reduced. An impassable barrier on a river can have a profound effect throughout the entire watershed, both above and below the obstruction.

Sensitive Habitats: Rivers

Land clearing operations can increase debris in a river causing scouring, erosion and debris dams. Land clearance and development decreases the capacity of the land to retain water, intensifying the effects of flooding and droughts. The breakdown of cleared vegetation and leaching of soil-bound chemicals may enrich the water and cause algal blooms.

The disruptive effects of a permanent barrier, such as a dam, will last longer than the lifetime of the structure itself. If fish are prevented from passing upstream and downstream, certain migrating populations may be permanently lost.

How Do We Prevent Disruptions?

Many of the effects caused by the construction of dams are difficult to remedy. Fish ladders, designed for specific species, may help with the migration of these fish, but ladders cannot ensure the survival of other species. The flooded area behind the dam (headpond), will destroy river habitats in that area. Soil conservation techniques in agriculture, forestry and land development will help reduce the problems caused by erosion. However, the overall effects of removing vegetation from large areas of a watershed will still increase flooding and, possibly erosion, downriver.

Although the treatment of domestic sewage, industrial effluent and toxic wastes is relatively expensive, the cost of repairing the long-term effects of pollution on the river may be much higher. Jurisdiction over freshwater habitats and water use, as well as the types of activities outlined here, is shared among a number of federal, provincial and municipal/county departments.

For further information on these sensitive habitats, and how to minimize disruptions to them, contact.

Aquaculture and Inland Fisheries
P.O. Box 700
Pictou, Nova Scotia BOK 1HO

Top of Page AOC Fish Habitat Home Page up one level