A coal$-$fired power plant converts one$-$third of the coal energy into electrical energy. The electrical power output of the plant is $1000MW($mega watt$).$ The other two$-$third of the energy content of the fuel is rejected to the environment as waste heat. About $15\%$ of the waste heat goes up the smokestack and the other $85\%$ is taken away by cooling water that is drawn from a nearby river. The river has an upstream flow of $100\frac{m^3}{s}$ and temperature of $20C.$

iii$)$ How it would impact the river water quality downstream with respect to dissolved oxygen level in the water. Explain your answer quantitatively.