[en] Wildfires occur naturally in many countries and continents. However, they can have serious impacts on human infrastructure, including the provision of fresh water supply. Worldwide, many cities have water supply reservoirs that are forested, prone to wildfire, and thus potentially susceptible to the negative impacts of fires on reservoir water quality. Climate change is also likely to increase the prevalence of fires in those countries that will suffer reduced rainfall patterns. Consequently, it is very important to know in what ways wildfires will impact on reservoirs. The CSIRO team used fallout radionuclides and geomorphologic methods to investigate the impact on water quality of sediments and nutrients released following a catastrophic wildfire in Warragamba reservoir, which is the principal water supply for Sydney (with a population of 3.5 million) in NSW, Australia. The project was carried out in three phases: 1) Quantifying the amount of sediments and nutrients transferred from burnt hillslopes to streams after the fire, 2) Characterizing the nature, extent and timeframe of the sediment impact on the downstream streams and, 3) Assessing the impact of this fire event on the overall amount of sediment and nutrients already stored in the reservoir. Tracer budgets showed that significant amounts of sediment and attached nutrients were eroded from burnt hillslopes in the months following the fire event. Concentrations of fine fused aggregates with high nutrient concentrations were prevalent. Comparison of concentrations of ¹³⁷Cs and 210Pbex on eroded surface soil and river sediments showed that a high proportion of river sediment was derived from surface erosion of hillslopes after fire, in contrast to the pre-fire sediment sources which were dominated by erosion of sub-surface material from rivers and gullies. The post-fire nutrient concentration of river sediment was many times that of pre-fire. The size of the area burnt was shown to control the amount of surface erosion and delivery of high nutrient materials. The total suspended solids, Nitrogen and Phosphorus concentrations in post-fire runoff events were one to two orders of magnitude larger than in prefire conditions. The characteristics of post-fire river sediment only returned to pre-fire conditions over four years. The post-fire FRN data indicates that nutrient concentration in river sediment was many times that of pre-fire, resulting from surface erosion of burnt hillslopes. Given the potential for nutrient-fuelled negative water quality impacts such as algal blooms, it is imperative to undertake monitoring after fires and to manage for post-fire sediment and nutrient delivery to reservoirs