Abstract
The data of water quality in river mouth zones of lakes at floods of influent rivers is mostly used to be excluded as extraordinary data from ordinary data. But, the data at the floods must be evaluated validly by considerng of their frequency and influence period of floods. Inflowing river water at floods not only supplys a large quantity of nutrients and sediments but rises up the water level and water volume content of lake, and changes the hydraulic residence time of lake and water quality by mixing lake water with influent river water.
Especially primary production by phytoplankton is influenced for a fairly long time in spite of bulk supply of nutrients, for phytoplankton density is decreased by dilution and water temperature in lakes is lowered by influent river water at floods. These facts were made clear as the results of observaticn in Takahama-iri Bay of Lake Kasumigaura.
