A coastal reservoir (CR) is any structure designed to capture fresh river flow before entering the sea and mixing with salt water. CRs are vulnerable to catchment and water quality deterioration as they are at the mercy of the water quality processes that occur upstream. Several potential contaminants have been identified that affect the water quality of CR intake water. These include salts, suspended sediments, organic matter, nutrients, organic and inorganic toxic substances and potentially a number of emerging pollutants at very low concentrations. Based on a number of international standards and guidelines, recommended water quality levels for both river water and CR water for water supply purposes have been identified. It is recommended that in situ water quality measurement should be undertaken over a period of several years to establish base line levels, and selected water quality parameters need to be measured in real time so that CR gate operations can be undertaken.