Building more hydroelectric dams is restricted by a combination of severe environmental, geographical, and economic hurdles. Most of the world's optimal river sites have already been developed, and constructing new dams causes massive ecological damage, including the destruction of local aquatic habitats, the disruption of fish migration patterns, and the flooding of vast terrestrial ecosystems. Additionally, large dam projects involve astronomical upfront capital expenses, lengthy construction delays, and the social disruption of relocating communities living near proposed reservoir zones.