I have done an example to calculate the actual max efficiency of a power plant given the temperatures of both the hot and cold reservoirs

I have done an example to calculate the actual max efficiency of a power plant given the temperatures of both the hot and cold reservoirs. My book, however, tells me nothing about how to calculate any of the following questions.

A power plant produces 1 GW of electricity, at an efficiency of 40%.

(a) At what rate does this plant expel waste heat into its environment?

(b) Assume first that the cold reservoir for this plant is a river whose flow rate is 100 m^3 /s. By how much will the temperature of the river increase?

(c) To avoid this "thermal pollution" of the river, the plant could instead be cooled by evaporation of river water. At what rate must the water evaporate?What fraction of the river must be evaporated?