How do i make a initial assessment of how much hydroelectric power can
be taken from a river? The only data I have available is the river flow and width.

power generated on hydel power plant mainly depneds on volume of pamer
and the hight of water.hight of water means the level difference
between the turbine and the top level of water.
power produced is propotional to hight. if hight is high water need is
less and if hight decreses volume required will increse exponently.
ref:power system operation and control by A.J.Wood.& B.Woolenberg.