The following System-Dynamics model simulates the effects of a development aid to a certain project on social and economic sectors in a development country. The project, which has been started in 1980, treats the damming up of the Mahaweli-Ganga in Sri Lanka. The advantages of this great project can't be assessed applying only classical measures of yield.Using System-Dynamics it is possible to analyse the effects of the project on sectors like the labour market, agriculture and the budget of Sri Lanka and to discuss changes of the values in this fields.An improvement of the values of the 4.5 billion US$ project would create the foundation for further investments.Moreover, the effects from the building of hotels and possible direct investments of foreign enterprises in Sri Lanka are to consider,