Answer to Question 1
You would need an understanding of
climate and hydrology, in order to know the amount of water that could be confined behind the dam.
geomorphology, in order to know the dimensions of the river valley and the stability of the hillslopes near the dam (to evaluate the landslide potential).
structural geology, in order to assess the risk of movement from faults that may be located at or near the dam site.
petrology, in order to evaluate the strength of the rocks that the dam will be built on top of and alongside of.
environmental geology, to evaluate the impact of the dam and reservoir on the stream and surrounding landscapes and people.
Answer to Question 2
The scientific method is the process of inquiry that examines and explains a problem or observed phenomenon. The scientific method is not a step-by-step, cookbook recipe for doing science. Instead, the method describes how scientists measure natural features and processes and rigorously test new ideas about how the natural world works. There is no single recipe for good science. Scientists undertake many activities that collectively further our knowledge. Scientific studies involve many activities and even repeating steps during the course of a research project. There are various paths that can be followed. Unlike the steps of a recipe, these steps do not need to be taken in any particular order; scientific studies commonly take unexpected turns and can be triggered by any number of initial observations or problems.