The preceding review of the quality of watercourses in Hong Kong and the trends in pollutant levels indicates that whilst many of the rivers are still not compliant with each individual WQO, significant improvements in water quality have been achieved in recent years, and the overall compliance level has steadily increased. Much of this improvement can be attributed to extension of the sewerage network to rural areas previously unserved by sewers or treatment plant, improvements in agricultural and chemical waste management, as well as a decline in agricultural intensity generally. Implementation of further work programmed in the SMPs should continue to bring about benefits through reduced organic pollutant loadings.

Nevertheless, a number of issues threaten the long term quality and assimilative capacity of freshwater systems in Hong Kong. Pollution of watercourses through illegal dumping of waste materials (see Paragraph 5.3.3.1) and contravention of discharge consents poses a threat to enhancement of assimilative capacity. Measures to address these problems lie either in encouraging better practice and environmental protection through education programmes or through penalising offenders through stiffer fines and greater enforcement of regulations. Abstraction of freshwater from rivers (also see Paragraph 4.2.1.27) for use in irrigation, homes and industry also affects water quality, particularly during dry weather conditions when river flows are already low. Further abstraction of water results in reduced dilution capacity such that in some cases, the dry season flows of rivers are largely made up of effluent discharges.

The alteration and conversion of natural streams to drainage channels enhances the natural capacity of freshwaters to assimilate floodwaters and drainage, but can result in water quality impacts. In addition, channelisation results in a loss of ecological capital stock since drainage channels (which are usually constructed from flat concrete) cannot be colonised by normal stream assemblages of flora and fauna. Unless more advanced channel designs which allow for some ecological mitigation are used, this process is simply substituting ecological capital stock for assimilative capital stock. There are examples of EIA studies which have resulted in drainage channel designs which provide for ecological mitigation, such as the Main Drainage Channels for Yuen Long and Kam Tin which feature specially constructed mangrove platforms.