Answer: the supply of the output from the hydroelectric power plants to increase.
Explanation:
The hydroelectric power operators are incurring the cost of cleaning the river before they can use it. This cost is money that they could be investing in the business in order to produce more power.
If the government corrects the externality and limits the amount of waste being thrown into the river, the hydroelectric company would be able to invest the expenses it was incurring in clean up into generating more power which would lead to a higher supply of output from the power station.
