This investigation into the nature of dissolved oxygen dynamics in the lower Fraser River/Estuary has made use of the application of two mathematical water quality models - a tidally averaged dissolved oxygen model and a tidally varying dissolved oxygen model. The tidally averaged model analyzes the inter-tidal behaviour of the river/ estuary, giving estimates of steady-state dissolved oxygen response. The tidally varying model, on the other hand, analyzes conditions within the tidal cycle, thereby describing the "real-time", intra-tidal behaviour of the river/estuary. Both dissolved oxygen models are one-dimensional and make the assumption that the only operative dissolved oxygen source/sink processes are deoxygenation due to the oxidation of discharged organics and reoxygenation due to atmospheric reaeration. The present high dissolved oxygen levels in the lower Fraser preclude the accurate calibration of the dissolved oxygen models. However, an analysis of model sensitivities is presented, in lieu of verification, to document model responses. Dissolved oxygen predictions made using the unverified models indicate that the assimilative capacity of the lower Fraser River/ Estuary is considerable, mainly because of the large freshwater inflows which afford extensive dilution as well as rapid flushing. The "critical period" is likely to be in late summer when the combined effects of water temperature and freshwater flows result in the lowest dissolved oxygen levels. Future water quality impairment in the main channels of the lower Fraser, at least insofar as dissolved oxygen is concerned, is considered by this study to be unlikely, providing that existing pollution control policies are adhered to. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate