No energy source is perfect, and hydroelectric dams come with challenges:

One unavoidable impact is the alteration of a river’s natural flow. While beneficial for human use, this can disrupt ecosystems. Slower-moving waters may allow invasive species to take hold, and changes to the riverbed can affect aquatic life.
Dams without proper fish passage pose challenges for migratory fish species.
Many Northwest dams—including those on the lower and mid-Columbia and lower Snake River—have undergone extensive modifications to minimize their environmental impact.
Beyond ecological concerns, hydroelectric dams displaced Native American communities by flooding important cultural and fishing sites. Many of these sites, once abundant with salmon, were lost. Today, hydroelectric operators recognize the importance of collaborating with tribal nations to restore fish populations and protect cultural heritage.