In autumn year 2000 and winter-spring 2001 the precipitation in the catchment area of Lake Vänern was higher than normal. During spring 2001, the flow in river Göta älv was around 1200 m3/s, nearly three times higher than the average indicating extreme conditions. The flow in the smaller rivers entering the Bohus coast is minor compared to river Göta Älv. To investigate possible effects on the marine environment in the area close to the river mouth an investigation with weekly sampling at four locations was initated by SMHI. Physical and chemical variables in the water was measured as well as phytoplankton composition and abundance. The Swedish Environmental Protection Agency and the Water Quality Association of the Bohus Coast co-funded the investigation together with SMHI. Effects on the water quality such as low surface salinities, high concentrations of dissolved inorganic nitrogen and dissolved phosphate and silicate compared to monthly averages 1990-99 was observed close to the river mouth but not off shore. Effects on the Secchi depth were not observed. On a few occassion high phytoplankton biomass, measured as chlorophyll a, was observed. At other locations along the Bohus coast effects where absent or small. Primary productivity measurements at the mouth of the Gullmar Fjord, showed no effects from the river outflow. During the investigation a bloom of the harmful alga Chattonella sp. occurred in the Kattetat and the Skagerrak.. The bloom of this small flagellate, which is described in the report, is probably not connected to the river input. A unique or unusual feature of the Chattonella-bloom is that it occured in cold water right after the diatom spring bloom in early March. In conclusion the effects of the extreme flooding were less than expected, the fresh water from the river were quickly mixed with the water in the sea and only small effects were seen. No connection between the flooding and the Chattonella bloom was detected.