Wanderlust World Wanderlust World Contrary to a common assumption, large dams are typically sited on rivers with moderate to low flow velocities, not on fast-flowing, steep-gradient streams. Here's why:
Fast-flowing rivers have high stream power—they aggressively erode their beds and banks. A dam in such a setting faces constant scour at its foundation and spillway, threatening structural integrity.
High velocities make cofferdam construction, foundation excavation, and concrete placement extremely hazardous and expensive. Slow water allows safe, precise work.
Dams create reservoirs for storage, hydropower, or flood control. Low-gradient valleys (broad, flat) yield large storage volumes per meter of dam height—far more cost-effective than narrow, steep canyons where fast rivers usually run.
Fast rivers carry heavy sediment loads. Reservoirs on such rivers silt up rapidly, losing capacity and requiring costly dredging.
Run-of-river hydropower projects and small diversion weirs are sometimes built on steeper reaches, but they have minimal storage and different design criteria.
In short, engineers seek the "Goldilocks" reach: enough flow for power/yield, but gentle enough slope for a stable, economical structure.