DRIFT is an interactive, holistic approach for advising on environmental flows for rivers. The DRIFT methodology, together with multicriteria analysis (MCA), can be used to provide flow scenarios and descriptive summaries of their consequences in terms of the condition of the river ecosystem, for examination and comparison by decision-makers. The essential features of DRIFT, the output of workshops where it is applied, and the development of the DRIFT database are described. Modules within the database include DRIFTSOLVER and DRIFT CATEGORY. DRIFTSOLVER contains an integer linear programming MCA method, which generates optimally distributed flow scenarios for different total annual volumes of water. DRIFT CATEGORY facilitates evaluation of these in terms of river condition. These two modules are explained in detail and illustrated with examples.

The DRIFT (downstream response to imposed flow transformation) methodology was applied as part of a comprehensive Reserve determination study on the Olifants-Doring River, Western Cape, South Africa. DRIFT was used to provide flow scenarios, and descriptive summaries of their consequences in terms of the condition of the river ecosystem, for examination and comparison by decision makers, managers and users. The methods used and results obtained in that study are presented and discussed in the light of the study constraints of time and money, the lack of historical data, and the urgent need to provide clear, easily-understandable information on the consequences for the river ecosystem of various tradeoffs characteristic of water resource developments. The results indicated that abstracting during high flows in winter and river restoration work can have positive ecological and agricultural returns.

The Pongola Floodplain in the Makhathini Flats is an area of low topographic relief between the 1973-commissioned Jozini Dam, and the Usuthu River which borders Mozambique. The floodplain system is characterised by a complex mosaic of meandering river channels, levees, and floodplains interspersed with pans (or depressions) and wetlands. The landmark 1982 study of the floodplain, Man and the Pongolo Floodplain, suggested a pattern of flows to ‘maintain the floodplain’ based on socio-ecological criteria. Since 1998, however, annual releases have been primarily targeted at the needs of recession agriculture and inundation of the floodplain in the Ndumu Reserve. No releases have been specifcally aimed at maintaining the floodplain ecosystem and the services it delivers to support the livelihoods of local communities. In 2013, the Department of Water and Sanitation commissioned an Ecological Reserve study of the Usuthu/Mhlatuze Water Management Area, which incorporates the Pongola Floodplain. This paper describes two-dimensional hydrodynamic modelling using RMA2 to inform this flow assessment. Four computational Pongola Floodplain models have been developed since 1979, including cell-based, one- and two-dimensional approaches. The RMA2 model is based on existing topographical, hydrological and hydraulic information, and was calibrated and verifed for the period 2008 to 2010 using water-level data from the local hydrometric monitoring network. Generally, good replications have been achieved in terms of peaks, rising and recession limbs, recession of ponded pan water-levels, and low-flow river stages. The RMA2 modelling represents an advancement of previous hydrodynamic studies of the floodplain and contributes to an improved understanding of its hydraulic behaviour. Model application was for the 15-year period 1990 to 2004, and simulations included naturalised, present management (2014), and 7 potential dam operational scenarios. The results were post-processed for analyses in the DRIFT DSS, described in the companion paper.

This paper attempts to address some of the problems that are faced in the endeavour to minimise the time and financial costs of environmental monitoring. At the same time, it aims to assess the usefulness to freshwater studies of some of the advances being made in the marine sciences. The paper examines the results obtained and the conclusions drawn using several recognised techniques for measuring aquatic macroinvertebrate communities. The physical, chemical and macroinvertebrate data used were collected upstream and downstream of a land-based trout farm situated on a tributary of the Elands River in the Molenaars River catchment, south-western Cape, South Africa. Five measures of community stress were examined to determine the extent to which they summarised community changes in response to the trout farm effluent. They ranged from those requiring quantitative sampling and species-level identification to those requiring only qualitative sampling and an ability to distinguish between taxa. The results obtained suggest that: the incorporation of an abundance measurement can greatly enhance the sensitivity of an index; combining the results from different microhabitats can artificially inflate the scores obtained at a site; change in community structure following a perturbation may be more marked at higher taxonomic levels, because the community response is more evident above the natural environmental noise; identification to resolution lower than family (e.g. order) may result in important information being missed, and increase the chances of the effects of a perturbation going unnoticed. It is concluded that quantitative or semi-quantitative family-level data probably represent the best compromise between the costs of collecting and processing the samples, and the information content of the resultant data, but that the use of several different techniques would enhance the reliability of the conclusions drawn. In addition, it is recommended that some of the advances being made in the marine sciences, such as the 'marine' ranked Abundance Biomass Curves (ABCs) should be developed and implemented, since they can provide a much needed functional underpinning for results obtained.

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