In this paper, some results of field observation concerning the anual and short-term behavior of salt intrusion in Lake Abasiri is reported. From the data analysis, we found that this lake has two layers of salt and fresh water vertically and that the lower layer of salt water is gradually ascending in this decade and it has no soluted oxygen. In the field observation in the river, we found that the salt water intruded into the lake in the form of salt wedge. Because of the occurence of record breaking heavy rainfall in this catchement area just before the observation, the thickness of the upper layer became from about 5 to 7 meter.
In order to make clear the mechanism of long-term trends of the lake, we proposed the model of tank-model type. In the final section of this paper, we compared the numerical results with the field data and we found the good agreement.