Issue 1: Modification of stream flow

Stream flow

Two important characteristics of streams and rivers are the velocity and volume of flow. Flow rate controls many factors such as oxygen content, water temperature, composition of the streambed, amount of food available, and pollution levels. These in turn directly affect the types and numbers of plant and animal species that exist in the stream/river ecosystem. Changes in land use in the watershed may shorten the length of time it takes for rainfall to enter the stream, resulting in sudden changes in velocity and volume of flow.

Flow (or discharge is the volume of water moving past a point in a unit of time. Two components make up flow: the volume or amount of water in the stream, and the velocity or speed of the water moving past a given point.

Flow affects everything from the concentration of various substances in the water to the distribution of habitats and organism throughout the stream. Flow affects the amount of oxygen dissolved in the water. Higher volumes of faster moving water can increase the surface area and increase the transfer of oxygen into the water. The temperature of the water can also be affected by stream flow. Smaller volumes of slower moving water can heat up more dramatically hot conditions. The amount of sediment and debris a stream can carry also depends on its flow.

Stream flow interacts with the gradient and substrate of a stream to determine the nature of physical features, such as the types of habitats (pools, riffles etc) present, the shape of the channel, and the composition of the stream bottom. The volume of stream flow is determined by many factors. Precipitation is generally the key factor. There is usually a lag period between the time a storm reaches it highest intensity and the time the stream reaches it peak flow. This lag time can be changed by land use practices in the watershed. Vegetation can affect the lag time and volume of stream flow. Channelling a stream and removing woody debris and other large object can increase the velocity of flow and significantly affect the character of the stream. The volume of stream flow may go from high flow during a storm, to no flow during a dry period. It is very difficult for organisms living in the stream to deal with this much change in the conditions of their environment.

GIWA definition

Modification of stream flow refers to an increase/decrease in the discharge of streams and rivers as a result of human interventions on a local/regional scale. See Issue 19 for flow alterations resulting from global change.

Environmental impacts

1. Modification of riparian habitats

2. Depletion of fish stocks and species diversity

3. Water quality change

4. Decreased wetland areas

5. Capacity to transport sediments

6. Reduced groundwater recharge

7. Saltwater intrusion

8. Changes in biological diversity and food webs

9. Changes in sediment budgets

Indicators

The following are environmental impact indicators for modification of stream flow.

Select the environmental impact indicators that are relevant from the list below and input data into the appropriate indicator sheets.

Hydrological

Map and satellite derived

Biological

Parameters

Pollution