How Do People Affect Instream Flow?

Run-of-the-River Withdrawals

Run-of-river withdrawals skim the ambient stream flow without regulating downstream flows through storage or augmentation. A run-of-river withdrawal may be a farmer's temporary irrigation pipe, a concrete encased pipe to water a golf course, a concrete wing-wall diversion for a trout hatchery, or a municipality's water treatment plant intake. Some withdrawals are upstream of a weir, or low head dam, to provide more depth for the withdrawal.

Impoundments

People build dams for flood protection, recreation and to provide a reliable water supply for drinking, power generation, fire protection, industry and irrigation. Placing a dam in a river also has environmental consequences for the riverine system.

A dam can influence the natural fluctuation of stream flow. The available storage capacity behind a dam can hold the surge of water during a storm and release it at a more constant rate. During drought conditions, the water stored behind a dam can provide more water downstream than would normally be available, this is known as flow augmentation. A solid dam with an impoundment with no offstream uses tends to mimic stream flow, i.e. inflow equals outflow; however, multi-purpose reservoirs--especially those with offstream uses--behind dams with sub-surface release gates can alter downstream flows drastically.

Dams can affect downstream water chemistry. Dams release water downstream either over the crest, known as a spillway, or through the dam by way of a gated tower. Water over the crest comes from the surface of the impoundment, which is well-oxygenated and, in the summer, warmer than the bottom water. A bottom release is cooler but is depleted of oxygen. A gated tower can draw from various water depths to modulate temperature and oxygen.

Dams alter the transport of materials downstream. Impoundments capture bedload, e.g. silt, sand, gravel, and detritus that is transported in the channel. The dampening of high flood flows downstream of dams reduce the force required to move finer particles that do pass the dam and accumulate downstream.

Dams block the movement of fish and other aquatic organisms. All fish range within a reach of river. Some fish travel great distances within a river. Anadromous fish, like striped bass, move from the ocean into the upper reaches of rivers or to the first dam to lay their eggs. The eggs develop as they are transported downstream. High spring flows are needed to both direct the adults upstream and transport the eggs downstream. Fish ladders and barges allow some fish to circumvent dams.

Hydropower

One function of dams is to capture water and convert the kinetic energy of falling water into electricity. Terrain, drainage area, the amount of water available, budget and imagination determine the shape and size of a hydropower facility.

In the North Carolina Piedmont, broad, shallow river valleys with small drainage areas have low head dams with the powerhouse either located at the dam (see example above) or a short distance downstream and the water is transported to the powerhouse by a canal or pipe (View example: High Falls Hydro ). In the same shallow and broad river valleys with large drainage areas the dams are higher and the powerhouse sits at the dam (View example: Gaston Hydro ).

In the mountains of North Carolina, hydropower facilities located at lower elevations on rivers of moderate drainage areas have moderate size dams with the powerhouse located on site (View example: Sharpe Falls Hydro ). At these same low elevations on rivers with large drainage areas, the powerhouse is still on site but the dams become much taller (View example: Cheoah Hydro ). Dams at higher elevations store and supply water to the powerhouse, located on the valley floor, through a network of pipes (View example: Queens Creek Hydro ).

Water that is transported by canal, pipe or other conduit from the dam to the powerhouse causes a reduction in flow in the stream channel from the dam to the point where the water returns to the stream at, or some distance below, the powerhouse. Although the intervening drainage area supplies water to the bypassed reach, it can't replace the lost volume, and, during drought conditions, the bypassed reach may be reduced to a trickle.

Peaking hydropower facilities, those that generate only during high power demand hours, can cause stream flow to fluctuate greatly on a daily basis.