Low flows play a key role in maintaining the health of aquatic habitats during dry periods. Although small in volume, low flows provide a continuous or intermittent flow over the bottom of channels, providing connectivity between aquatic refuges, refreshing in-channel pools. Low flows help maintain water quality by flushing nutrients and pollutants that accumulate. They provide and maintain aquatic refuges during dry periods, and support the life cycle of water dependent plants and animals. Low flows help to maintain water critical to wildlife during drought, they also maintain riparian vegetation alongside river banks which protects the banks from erosion. Changes to flow patterns impact the whole river system including aquatic and terrestrial wildlife dependent on drought refugia, placing pressure on riparian vegetation and terminal wetlands and estuaries.
River flows are inherently variable, with the flow in a river at any moment in time determined by a range of natural factors including antecedent weather (e.g. precipitation, insolation, winds) and drainage basin (catchment) characteristics (e.g. size, topography, geology, soils, vegetation and stream network characteristics). In addition to natural variability, river flows are also influenced by anthropogenic activities including:
- Surface and groundwater abstraction (e.g. for domestic, agriculture and commercial uses);
- Land use (e.g. changes to surface permeability) and narrowing of channels;
- Waterway barriers and flow regulation (e.g. dams and weirs); and
- Water discharges (e.g. discharges from industry).
Most streams will display annual variation in flows due to seasonal changes in weather, with some streams ceasing to flow for parts of the year (i.e. intermittent rivers) or only flowing after precipitation events (i.e. ephemeral rivers). During dry periods river flows generally decrease, with much or all of the flow in a river derived from groundwater. During prolonged dry weather, the flow of water in a stream may drop to a level known as a low flow. This is different to a drought, which is a more general phenomena than low flow.
The amount of flow in a river has major impacts on the aquatic life within the river as well as on water quality, fluvial geomorphology and riparian vegetation.
Impacts to natural flows can cause:
- decline in dissolved oxygen levels and higher pollutant levels resulting in fish kill events;
- aquatic wildlife and other dependent terrestrial species that depend on them to become stressed and ultimately starve; and
- decline in water quality leading to greater outbreaks of blue-green algae (i.e. cyanobacteria).
While aquatic organisms are generally adapted to the hydrological regime of the waterway in which they reside, including low flows, anthropogenic changes to the river flow can severely impact on river aquatic life. While some anthropogenic impacts are relatively permanent (e.g. dams) other human activities are more changeable. Water abstraction is one of these. Groundwater abstraction will also impact on river flows, due to the connectivity between groundwater and surface water.
Permanent anthropogenic impacts such as dams are important because they provide security of water supply. But the impact of dam development impedes flow and parts of the catchments go without flowing water for longer periods of time. With dam interceptions effectively lengthening the time that catchments go without water, the no-flow period is getting longer and catchment health is declining. We know this because water quality is decreasing, watercourse pools are drier for longer, water dependent vegetation and fish are disappearing and plants and animals are struggling to reproduce and survive.
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The City’s Catchment Management Unit works towards healthy and resilient waterways and environments, and have undertaken a number of actions to assist with the management of low flows. This includes the establishment of the pilot City Flows Project to install continuous discharge and water quality gauges at key locations. Also, the Springbrook Groundwater Investigation is piloting a groundwater monitoring program that will contribute to our understanding of the City’s water balance.
Property owners and water users can help by:
- using recycled water in lieu of river and groundwater abstraction where appropriate;
- retaining native vegetation around waterways;
- ceasing, delaying or limiting activities including surface water and groundwater abstraction or diversion of water from a natural source; and
- preventing the discharge of pollutants to waterways.
The environmental benefits of securing low flows will require all water users to work together. Improving the health of waterways will achieve a range of short and long term responses including a longer flowing period and maintenance of permanent pools, improved diversity of macroinvertebrates (i.e. water bugs), recolonisation of water dependent plants, improved stability of watercourses and healthier native fish populations. Restoring low flows is a critical step in the progress towards improving catchment health.
For further information and advice contact the City’s Catchment Management Unit on 1300 GOLDCOAST (1300 465 326) or 07 5582 8211.