How does deforestation affect a hydroelectric power plant?

A basin covered with dense vegetation will show 1) less evaporation and 2) less seasonal variability in the rivers flowing to the reservoir. The more the water supply fluctuates through the year, the larger a reservoir needed to maintain a minimum level during dry months.

If deforestation occurs in a basin that supplies water for an existing hydro plant, seasonal variations may start to be observed (i.e., too much water during rainy months and not enough during dry months). Excess water will overflow the reservoir and is useless for power production. Eventually, power production will start to decrease and fuel plants will have to be used to produce the extra power during demand peaks as follows:

If the water level was ever allowed to go below the intake pipe, the turbines can no longer operate. Before that happens, the plant operators would selectively shut down turbines to reduce the rate of depletion of the reservoir, and during demand peaks the power that is not produced by the idle turbines would need to be produced by alternative sources, typically thermoelectric (fuel) plants.