A model for water exchange of a basin connected to the open sea by two channels has been worked out. The model is applied on the Gulf of Riga, where the water exchange processes versus strong landbased inflow of nutrients are the most important factors in formation of the trophic status of the Gulf.

The model's response both to stationary external conditions and nonstationary impulses (changes in the sea level, wind and riverine inflow) is analysed and possibilities for matter exchange calculations are discussed. It appeared that the water exchange depends strongly on the configuration and position of the straits. The water exchange in such a two-channel system is considerably different (and stronger) than in the case of a one-channel version. The main external force for the Gulf appeared to be the wind.