The unexpected inundation of Caledonia Marsh, a previously drained wetland adjacent to Upper Klamath Lake, Oregon, provided an opportunity to observe nutrient release from sediments into the water column of the flooded area and the resulting algal growth. Three sites, with differing proximity to the levee breach that reconnected the area to Upper Klamath Lake, were selected for water sample collection in the marsh. Chlorophyll a concentrations (an indicator of algal biomass) were lowest and dissolved nutrient concentrations were highest at the site farthest from the breach. At the site where chlorophyll a concentrations were lowest, dissolved organic carbon concentrations were highest, and the presence of tannic compounds was indicated by the dark brown color of the water. Both DOC and specific conductance was higher at the site farthest from the breach, which indicated less mixing with Upper Klamath Lake water at that site. Dissolved oxygen concentrations and pH were lowest at the beginning of the sampling period at the site farthest from the levee breach, coincident with the lowest algal growth. Phosphorus concentrations measured in the flooded Caledonia Marsh were greater than median concentrations in Upper Klamath Lake, indicating that phosphorus likely was released from the previously drained wetland soils of the marsh when they were flooded. However, a proportional increase in algal biomass was not measured either in the marsh or in the adjacent bay of the lake. Nitrogen to phosphorus ratios indicated that phosphorus was not limiting to algal growth at the marsh sites, and possibly was not limiting in the adjacent bay either. In terms of nutrient dynamics, wetlands normally function as nutrient sinks. In contrast, the drained wetlands around Upper Klamath Lake cannot be expected to provide that function in the short term after being flooded and may, in fact, be a source of nutrients to the lake instead. The consequences for algal growth in the lake, however, seem to be small.