The impact of hydroelectric power plants on the environment is a non-polluting ecological impact. For example, construction of the power plant requires a certain amount of flooding, and the river bed may be exposed downstream. The flooded area may affect the environment. the scope and habitat of plants and animals; exposed riverbed downstream can cause surface damage. This will impact the life, reproduction and migration of aquatic organisms. Restoring vegetation on the borrowed and degraded site can take a long time.
As for the energy, it was initially wasted, but it was used after the power plant was built.
There are certainly hidden dangers. For example, the construction of the Three Gorges Dam could cause landslides, enormous pressure could cause local crustal changes, etc.
Small hydropower plants do lose money, but large hydropower plants generally do not.
The sum of cost depreciation, operating expenses, and maintenance expenses is greater than the electricity sales revenue, and the hydropower plant will lose money. This usually happens in small hydroelectric plants. Due to unstable electricity production, the power grid is not willing to purchase it, leading to losses.
Large hydroelectric power plants have a large electricity production capacity and a stable current, so they do not necessarily suffer losses.