Hydrology of Reservoirs

Among reservoirs, those built for generating hydro-electricity usually have the most pronounced fluctuations in water level (Figure 3). These fluctuations result from variations in the demand for electricity.

Free-flowing river

Free-flowing river

Storage reservoir
Run-of-river impoundment

Figure 3 Typical annual water-level fluctuations in boreal free-flowing rivers and in the two major types of impounded waters. Note that the range of fluctuations differs between water bodies and that the storage reservoir has reversed hydrological conditions during summer, with early low-water and a late flood. From Jansson et al. (2000b) Fragmentation of riparian floras in rivers with multiple dams. Ecology 81: 899-903, with permission form Ecological Society of America.

Figure 3 Typical annual water-level fluctuations in boreal free-flowing rivers and in the two major types of impounded waters. Note that the range of fluctuations differs between water bodies and that the storage reservoir has reversed hydrological conditions during summer, with early low-water and a late flood. From Jansson et al. (2000b) Fragmentation of riparian floras in rivers with multiple dams. Ecology 81: 899-903, with permission form Ecological Society of America.

Principally, there are two kinds of reservoir operation: storage reservoirs and run-of-river impoundments. Storage reservoirs are built primarily to sustain flow in the river downstream and level out ordinary fluctuations in discharge. In regions exhibiting seasonal climate variations and where rivers show large natural fluctuations in flow within or between years, storage reservoirs need to be large to fulfill this task. For example, in Norway there are two storage reservoirs with maximum legislated water-level fluctuations of 125 and 140 m, respectively.

Run-of-river impoundments are built primarily to balance daily variations in the demands for water for electricity production, and to provide hydraulic head. A river completely developed for hydropower production forms stairs of dams and impounded water surfaces without leaving any runs or rapids in between. Normally, power stations process more water during the day than during the night, implying that impounded water-levels are lowered during the day and raised during the night. This variation is commonly rather small to avoid loss of hydraulic head and thus reductions in power production.

Therefore, on a monthly or annual basis water levels could become more stable, but on a daily basis there could be frequent variation.

The numbers and locations of reservoirs vary between rivers. Many impounded rivers have big reservoirs in the upstream reaches, but long unimpounded reaches downstream. The flow hydrograph of such downstream reaches is affected by the reservoir. It has been estimated that, on average, about 5% of the water in a reservoir seeps into the ground and another 3.5% evaporates each year. The rest may be extracted or released to the river downstream. Usually, annual variations in flow are reduced so that floods and low-water periods become less dramatic. Additionally, the timing of the flood events that do occur is frequently changed. For example, in northern regions in which the spring floodwater is stored in reservoirs, floods may occur during summer or fall if reservoirs are filled in the spring and heavy rains continue through summer. In winter, storage reservoirs usually freeze over at a high water-level. Along with the emptying of the reservoir during winter, ice settles on the shorelines. Reservoir operation does not only change flow patterns, reservoirs also affect temperature, dissolved gases, and concentrations of waterborne material in the river downstream of the reservoir.

In recent years, an increasing number of ecologists advocate modified flow releases from such reservoirs, in order to attain more natural hydrologic conditions in the river downstream. This concept of dam reoperation attempting to find a compromise between human and environmental needs without sacrificing one or another is called environmental flows. It has been developed in water-poor areas such as South Africa and Australia where wise sharing of available water resources has become a chief issue, but the spirit of this concept is applicable to regulated rivers all over the world.

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