The National State of the Environment report for South Africa uses the Driving Force-Pressure-State-Impact-Response model to explain what is causing environmental change, how good or bad the conditions are, and what we can and are doing about it. Each of these categories is described using environmental indicators to give specific measures, benchmarked against international practice, legal requirements, and desired conditions. The National Water Act makes provision for monitoring a river’s current health (Present Ecological State) in relation to a desired condition (Desired Ecological State).

Aligned to both these processes, the RHP uses indices, or composite indicators, to report on the current state, the causes of change, the desired state, and makes recommendations for achieving this.

River health indices

A multitude of factors determine the health of a river ecosystem: its geomorphological characteristics, hydrological and hydraulic regimes, chemical and physical water quality, and the nature of in-stream and riparian habitats. It is impractical to monitor each of these factors in detail. Therefore, the RHP focuses on selected ecological indices that are representative of the larger ecosystem and are practical to measure. These indices are also mechanisms through which complex ecological data can be summarised and output can be provided in simple numeric format.

While biological indices are the main focus of the RHP, the development and inclusion of indices of physical and chemical parameters (e.g. habitat conditions, geomorphological characteristics, hydrology, water quality) are encouraged to increase the information value of the programme.

Indices in this report:

River Health Classification

In order to allow objective assessment of, and comparable reporting on, river health, a "scale of river health" has been established. The scale used by the RHP describes five different states of health (natural to unacceptable).

The attributes of a fish or invertebrate community that relates to a natural state for one river will not necessarily be the same for the next river. In fact, the biotic and abiotic characteristics that describe "natural" are likely to be different for different rivers. Therefore, a scale of river health needs to be calibrated for each river or even for distinctly different river reaches. This is done by reconstructing the conditions that represent a "natural state" for a particular river. These conditions are referred to as reference conditions. The rest of the classes then translate as a percentage of the reference conditions or "natural" state.

Ecoregions

For the purposes of this report, reference conditions were determined for each of the ecoregions within the study area. These are reaches that have been demarcated based on ecological similarity. In other words, monitoring sites within an ecoregion are ecologically more similar to each other than to sites in other ecoregions. Because of their similarity, these ecoregions also provide convenient boundaries within which to do ecological assessments and set ecological quality objectives.

Present ecological state

The results of applying the biological and habitat indices during a river survey provide the context for determining the degree of ecological modification at the monitoring site. The degree of modification observed at a particular site translates into the present ecological state. In this report, the present ecological state obtained at sites within the same ecoregion has, where possible, been summarised into one overall state.

Desired Ecological State

In order to put the present ecological state in context, it is important to have an idea of what is desirable and what not. Understanding of what the ecological state of a river should be could provide resource managers with directions for making decisions and implementing management actions.

Many factors could be considered in determining the desired ecological state for a particular river reach, including the strategic importance of the river for economic development as well as its ecological importance (in maintaining ecological diversity and functioning at local and wider scales) and sensitivity (ability to tolerate disturbances). The ecological importance and sensitivity of the river reach considers biodiversity, rarity, uniqueness, and fragility, from habitat, species and community perspectives.

For this report we have only considered the following in order to suggest a desired ecological state that is practical and achievable as well as protective in terms of ecological processes and functioning: