Answer
$W_{g}=443.7MW$
Work Step by Step
In this case we are working with kinetic and potential energy:
$e_{t}=e_{k}+e_{p}=\frac{V^2}{2}+gz=\frac{(3\frac{m}{s})^2}{2}+(9.81\frac{m}{s^2})*(90m)=887.4\frac{J}{kg}$
For the flux of mass:
$m=\rho V=(1000\frac{kg}{m^3})*(500\frac{m^3}{s})=500000\frac{kg}{s}$
Then the power generation potential is:
$W_{g}=E_{t}=m*e_{t}=500000\frac{kg}{s}*0.8874\frac{kJ}{kg}=443700\frac{kJ}{s}=443700kW$
$W_{g}=443.7MW$
