Based on considerations of tidal height variation and area, Quahog Bay has an estimated flushing rate of 27-36 hours. Despite this apparently rapid exchange of water with eastern Casco Bay, long term water quality measurements show that Quahog Bay approaches hypoxia during the late summer. A combined CTD and ADCP study across the mouth of Quahog Bay, spanning several tidal cycles during spring tides, addressed this discrepancy. These profiles show that the eastern Casco Bay waters entering Quahog Bay do so with the tide, but flow underneath Quahog Bay's resident water, thereby minimizing mixing. Low oxygen surface waters tend to remain in Quahog Bay, leading to hypoxic conditions in late summer. Numerical circulation models are being constructed to better understand these observations and their interpretations.