Spring floods carry Alaskan river water north to a frozen Beaufort Sea. A plume of water from the Sagavanirktok River (SR) was identified and traced by measuring salinity, δ18O, and dissolved silica in discrete water samples collected beneath landfast ice in the coastal Alaskan Beaufort Sea from late May to early June 2004 during high river flow. An Optimum Multiparameter analysis was used to calculate the fractions of SR water from the measured geochemical parameters. The SR plume followed the northwestward flowing local circulation and moved ~17 km north and ~15 km west under ice from the river mouth. The river plume was ~1--1.5 m thick beneath the ice and flowed above a persistent halocline in the top 2.5 m of the water column. The calculated volume of SR water beneath the ice on 2 June was ~0.5 km3, approximately 50% of the river discharge during the study period. The volume of water that was not accounted for was assumed to flow above the ice canopy or was not captured during the study. Interactions of SR water with another under-ice plume from the Kuparuk River resulted in increased northward transport of both freshwater discharges.