M.Sc. / The Mvoti River is referred to as a ‘working river’ in that it is highly utilized and developed. It is also, however, in a severely degraded state according to recent ecological assessments that were carried out on the Lower Mvoti River in 2000 and 2005. Previously, this was mostly attributed to the specific activities related to a pulp and paper mill. However, findings of the abovementioned studies indicated that there are multiple stressors present in the system and to derive the combined effect of stressors in an environment affected by multiple activities, a characterization of the different contributing activities is required. Thus, this assessment was undertaken to determine the impact of one of the contributing stressors, i.e. sugar milling activities on the ecological integrity of the Mvoti River. The survey assessments, incorporating local and international accredited methods and techniques, were carried out over a high and low flow period during 2006. A BACI (before-after controlled impact)-research design was applied in this study. Four sites were selected on the Mvoti River, up and downstream of the Glendale Distillery (GDR and GDS) and the Ushukela Milling Company (USR and USS). The former sites were selected to assess the impact of a sugar mill alone and the latter to determine the combined effects of sugar milling and pulp and paper activities. Additionally, 2 sites were selected on the Amatikulu River, up and downstream of the Amatikulu Mill (AR and AS), to assess the effect of the sugar milling activities alone. This study consists of two components, namely the abiotic driver component and the biotic response component. The abiotic driver component involved the analysis of water quality, sediment and habitat quality in which physico-chemical variables of water and sediment were carried out during high and low flow periods and the habitat indices, Habitat Quality Index (HQI) and Integrated Habitat Assessment System (IHAS) were implemented. The biotic response component involved the assessment of the macroinvertebrate and fish community structures, as well as the determination of acute and chronic toxicity and mutagenicity resulting from exposure to mill effluent. The macroinvertebrate communities were assessed using the South African Scoring System Version 5 (SASS5) index and the fish communities were assessed using the Fish Assemblage Integrity Index (FAII). The toxicological assessments were carried out via the use of the Direct Estimation of Ecological Effect Potential, or DEEEP, methodology, and biomarkers.