When I want to create the three surfaces for simulating dam break (surfaces for reservoir, dam & remain region downstream the dam) I choose from (Geometry >> Creation >> Straight line) to surround the surfaces by lines and after that I convert the lines to polygons. But the problem is when I want to create the line when I click by mouse the first point near the reservoir the point is fixed outside the image not in the actual location I choose. 

I dont know if this method is correct or not to create the surfaces. 

Thank you very much

MSc. student Mustafa Salim