Hydrographic measurements made during the US Southern Ocean Global Ocean Ecosystem Dynamics cruises, which took place from April to June and July to September 2001, provide a description of changes in water-mass distributions and circulation patterns in the Marguerite Bay region of the west Antarctic Peninsula continental shelf that result from seasonal variability and offshore forcing by the southern boundary of the Antarctic Circumpolar Current (ACC). The primary seasonal change in water-mass properties is the reduction in Antarctic Surface Water and replacement by a thick Winter Water layer. The primary effect of the ACC is to pump warm (>1.5°C), salty (34.65–34.7), and nutrient-rich Circumpolar Deep Water (CDW) onto the continental shelf below 200m at specific sites that correspond to bathymetric features, such as Marguerite Trough. The CDW intruded onto the continental shelf, moved across shelf, and entered Marguerite Bay. This flow pattern was observed during both cruises, suggesting that onshelf intrusions of CDW are a regular occurrence on this shelf. The number of CDW intrusions observed suggests that 4–6 events can occur in a year. Regions where CDW intrusions occur are characterized by surface waters that are above freezing in winter. Diffusive heat fluxes based on estimated diffusivities are insufficient for the observed rate of temperature decay of CDW intrusions. Localized, bathymetrically controlled vertical mixing is suggested as the primary heat transfer mechanism. The hydrographic and Acoustic Doppler Current Profiler measurements show a southwesterly flowing coastal current along Adelaide Island that enters the north side of Marguerite Bay and exits around Alexander Island. This current, which may result from seasonal, coastal buoyancy forcing, was present in fall and winter, but was better developed in fall. This current may be part of a larger cyclonic gyre that overlies the northern part of the area surveyed during the two cruises.