Combined Sewer Overflows (CSOs) are widely recognised as being a primary source of faecal pollution indicator bacteria and pathogens to urban receiving waters. The discharge and in-stream accumulation of sediments from CSOs have also been considered to constitute a bacterial reservoir yielding a continuous inoculation to the receiving water. This paper reports the findings of a microbiological study of CSO discharges and sediments within a N London urban catchments focusing on bacterial indicators (Total and Faecal Coliforms, Faecal Streptococci and pathogens) and the role of somatic coliphages in typing sewage pollution. Evidence is provided for both extended in-stream bacterial survival and bacterial generation in the presence of nutrified organic sediment.