What Are Dams?

Dams are engineered barriers constructed across watercourses to impound water, control flow, or raise the water level for one or more purposes including hydroelectric power generation, irrigation, water supply, flood control, and navigation. They rank among the largest civil engineering structures ever built, ranging from small earthen embankments retaining a few thousand cubic meters to concrete gravity and arch dams holding tens of billions of cubic meters behind walls hundreds of meters high. The reservoir formed upstream of a dam is itself an engineered system whose capacity, operation, and ecological consequences are inseparable from the dam's function.

Dams draw on structural and geotechnical engineering, hydraulics, hydrology, materials science, and environmental science. The design of a dam must account for hydrostatic loads, flood surcharge, seismic forces, foundation conditions, and the consequences of potential failure, which for large structures can be catastrophic for downstream populations. Regulatory bodies such as the U.S. Army Corps of Engineers and the Federal Energy Regulatory Commission (FERC) establish safety standards and inspection programs for dams in the United States.

Dam Types and Structural Design

Dams are classified by construction material and load-carrying mechanism. Embankment dams, the most numerous type worldwide, use compacted earth or rockfill to form a watertight structure whose weight resists sliding and overturning; a central impermeable core of clay or concrete prevents seepage. Concrete gravity dams resist hydrostatic pressure through sheer mass, with a cross-section designed so that the resultant of all forces passes within the base. Concrete arch dams transfer load horizontally into the canyon walls by arch action, allowing much thinner profiles than gravity structures. Concrete-face rockfill dams use a reinforced concrete slab on the upstream face as the waterproof element while the rockfill body provides structural mass. Foundation conditions, seismicity, and available construction materials govern the choice among these types for any given site.

Hydroelectric Power Generation

A dam's reservoir stores potential energy that is converted to electricity by directing water through penstocks to turbine-generator units housed in a powerhouse at or near the dam's base. The power output is proportional to both the head (the vertical drop from reservoir surface to turbine) and the volumetric flow rate through the turbines. The U.S. Department of Energy's overview of hydropower basics notes that hydropower supplies roughly 6 percent of total U.S. electricity generation. The U.S. Army Corps of Engineers operates 75 hydropower facilities with a combined installed capacity exceeding 20,000 megawatts, as detailed in its hydropower program overview. Pumped-storage hydropower, a variant in which water is pumped back to the upper reservoir during periods of low electricity demand, provides the largest form of grid-scale energy storage currently deployed.

Reservoir Management and Environmental Considerations

The reservoir impounded behind a dam serves multiple functions that must often be balanced against one another: water supply storage, flood attenuation, irrigation diversion, recreation, and ecological habitat. Reservoir operation involves decisions about release timing, drawdown rates, and maintenance of minimum environmental flows to support downstream aquatic species. Sediment accumulation in reservoirs reduces storage capacity over decades and must be managed through flushing, sluicing, or dredging. Dam construction and operation alter downstream hydrology, temperature regimes, and sediment transport, with effects on fish migration, floodplain ecology, and coastal sediment budgets. FERC Part 12 inspection requirements, administered through programs detailed by organizations such as GEI Consultants on dam safety and hydropower oversight, mandate periodic structural assessments to maintain public safety.

Applications

Dams have applications in a wide range of water resources and energy contexts, including:

• Hydroelectric power generation, including pumped-storage for grid balancing
• Municipal and agricultural water supply reservoir storage
• Flood control and peak flow attenuation in river basins
• Irrigation diversion for crop production
• Navigation channel maintenance and recreational impoundments