What makes a good indicator of river health?
A major part of monitoring river health is first establishing a full and accurate assessment of the waterway’s current ecological state. This will help to identify and predict causes of poor water health, and to assess the effectiveness of planned management activities. Such an assessment requires suitable indicators of river health to be selected, taking into account several general criteria as well as factors specific to the site and project. Dr Nick Bond explains.
There are a number of desirable features that good ecological indicators should meet in any context. They should:
- Reflect important ecological values and threats to these
- Provide outputs that are easy to interpret
- Respond predictably to damage caused by humans
- Relate to appropriate scales of time and space
- Be cost effective to measure
- Relate to management goals for the specific river system
- Be scientifically defensible.
The set of indicators chosen for any river health assessment program should also encompass as many cause-effect pathways as possible based a conceptual model of the ecosystem involved. This will ensure that all of the potential threats and assets of a given river system, as well as the causal linkages among these factors, are assessed.
Indicators can be classified differently depending on which aspect of river health they assess. For instance, the Pearl River pilot study in China used six broad indicator categories that captured a range of river health factors. These include water quality, hydrology, physical habitat, invertebrates, algae and fish.
Each of these categories contains a number of different indicators that respond differently to different forms of human impact. The final selection should therefore be connected to the characteristics outlined above, for the river system being monitored. For example, the factors outlined below were used to determine the assessment indicators for the Pearl River pilot study.
This assessment will not directly indicate the river condition. However, defining land use within a river catchment is fundamental for understanding and monitoring river disturbances, and the consequent effects on assessments of individual sites. Basic classifications of usage patterns are generally used, such as whether the surrounding land usage is forest, agriculture or urban.
Hydrological data are frequently incorporated into river health monitoring programs to better establish physical disturbances to the river system and their effects on flow. Changes in hydrology happen primarily through flow regulation and water extraction, or a combination of both. Dam construction is a common example of physical infrastructure that can impact heavily on these factors by disturbing the river connections and altering the volume and timing of flows.
The streamside or ‘riparian’ zone is an important interface between the river and catchment, and therefore is a good indicator of how much the river is naturally protected from surrounding activities. Specific indicators under this category include level of human disturbance, dominant vegetation and evidence of erosion.
Measuring physical factors can indicate how the river system has been physically altered by human activities. Examples of such physical alterations are channeling the river to improve navigation, removing large trees that have fallen into the river, and extracting gravel from the riverbed.
Water quality is already widely monitored in China, so existing datasets can be analysed to assess water quality indicators. This includes physico-chemical properties such as water temperature, pH and conductivity, nutrient concentrations, and the presence of toxins such as heavy metals.
Indicators of aquatic life are an integral part of river health monitoring. However, such assessments can be problematic. This is because the extensive natural variability and diversity in aquatic ecosystems will yield an equally diverse group of indicators to be measured. As well as macroinvertebrates, which are already widely used in river health monitoring programs, in the Pearl River pilot we are exploring indices based on the characteristics of benthic diatom (unicellular algae) and fish assemblages.
Ecosystem processes or functional indicators are used to estimate the rate at which important natural processes are occurring in the river system, such as oxygen production, oxygen consumption or the breakdown of organic material.