Simpson Lagoon is a shallow body of water separated from the Beaufort Sea by a chain of low-lying barrier islands. Narrow channels are found between the barrier islands, of of which, the Egg Island Channel, is a major connection between the lagoon and the Beufort Sea. The Kuparuk River produces great quantitites of springtime floodwaters which overflow the still-frozen lagoon. This paper examines the relationships between these floodwaters and the character of the water within the lagoon. Measurements of temperature, salinity, sea level, and currents have been made in Egg Island Channel during 1978 and 1979. Meltwater from the Kuparuk River was observed in satellite images to overflow to 2-m-thick bottomfast ice of the lagoon in late May and early June. These floodwaters flush saline (>40%) water out the 5-m deep channel and replace it with freshwater at 0¿C. Analysis of satellite images for several years shows that overflow occurs on May 31 with a standard deviation of 8 days. Sea level rose 64 cm above mean at peak flood in 1978. Subsequently, channel waters warmed to 8¿C in late June and were flushed by cold (~0¿C, saline (~24%) water from the Beaufort Sea in mid-July. Flooding in 1979 was anomalously early, providing additional insight into the flushing process. The return of seawater to the channel is due to wind-driven current following the clearing of bottomfast ice in the lagoon. Satellite image analysis shows that ice-free conditions occur by July 10 with a standard deviation of 8 days.