A 16-year monitoring program (1986-2002) determined significant long-term trends among phytoplankton populations and water quality parameters in the tidal waters of several rivers (Rappahannock, James, York, Pamunkey) located in Virginia. There were increasing trends in total phytoplankton abundance and biomass, which included the increasing biomass of diatoms, cyanoprokaryotes, cryptomonads, and chlorophytes in each river. There were also trends of increasing downstream salinity, and decreasing concentrations of TN and DIN. The TP and DIP showed none, or variable trends, except in the York River where both concentrations were increasing. General observations also indicated the common occurrence of dinoflagellates and cyanoprokaryotes blooms in several of these rivers.