A density current generator was designed for sediment treatment of Lake Kasumigaura. A fresh water shellfish, Ishigai, introduced as a bio-indicator for sediment quality monitoring, showed high survival and growth rate around 400m away the from density current generator. Simulation of water circulation was conducted using MEC (Marine Environmental Committee) model and results showed that although the flow rate was very slow, water discharged from the density current generator could reach up to several kilometers within 6 hours.