Run-of-river hydro projects use the natural downward flow of rivers and micro turbine generators to capture the kinetic energy carried by water. Typically water is taken from the river at a high point and diverted to a channel, pipeline, or pressurised pipeline (or penstock). The technology is applied best where there is a considerably fast moving river with steady seasonal water. How much electrical energy can be generated by a hydroelectric turbine depends on the flow/quantity of water, and the height from which it has fallen (the head). The higher the head, and the larger the flow, the more electricity can be generated.
Run-of-river hydro projects use the natural downward flow of rivers and micro turbine generators to capture the kinetic energy carried by water. Typically, at a high point along the river a dam is constructed to create a headpond in front of the dam. From the dam water is diverted from the river through a pipeline ('penstock') which leads to a downstream powerhouse. The water level in the headpond is to ensure that the intake to the penstock remains under water. This is illustrated by the above picture which shows how the diverted water takes an almost straight line through the penstock from the dam ('head') to the powerhouse, while the river follows its natural current. The water in the penstock is pressurized so that the power is strong enough for driving the turbines in the power house and produce electricity. From the powerhouse the water is led back to the river through a channel, which is called 'tailrace' (Renewable Energy UK, 2006).
The production rate of run of river projects is more stable than those of wind or solar power systems since power is generated continuously (Renewable Energy UK, 2006), although the amount of electricity that stations generate varies depending on the volume of water in the river. Thus, the technology is applied best where there is a considerably fast moving river with steady seasonal water (Pollution Probe, 2003).
The environmental conditions of the section of the river from which the water is diverted ('the diversion reach') could be affected when significant quantities of water are diverted from the river without precautionary measueres (see below ).
Similar to what was said in the description of large scale hydro for electricity production  in this ClimateTechWiki, the potential for hydro power in the world is considerable. According to the IEA Technology Perspectives 2008 (IEA, 2008), by 2050 over 5000 TWh could be produced annually from 1700 GW of capacity. However, the main challenge for further development is that competition for scarce water and land resources will increase globally, while large scale hydro (both run of river and dams) face increasing concerns about environmental and social impacts.
A large-scale run-of-river hydro plant mostly consists of more than one generating unit and the combined discharge depends on the plant’s scale. The best geographical areas for exploiting run-of-river hydro power are those with steep rivers flowing all year round, such as, e.g., the hill areas of countries with high year-round rainfall, or the great mountain ranges and their foothills, like the Andes and the Himalayas. Islands with moist marine climates, such as the Caribbean Islands, the Philippines and Indonesia are also suitable (IEA, 2008). Low-head turbines have been developed for small-scale exploitation of rivers where there is a small head but sufficient flow to provide adequate power (California Energy Commission, 2001).
To assess the suitability of a potential site, the hydrology of the site needs to be assessed through a site survey, to determine actual flow and head data. Hydrological information can be obtained from the meteorology or irrigation department usually run by the national government. This data gives a good overall picture of annual rain patterns and likely fluctuations in precipitation and, therefore, flow patterns. Flow data should preferably be gathered over a period of at least one full year in order to ascertain the fluctuation in river flow over the various seasons.
During the implementation of run-of-river hydro projects, the following implementation chain aspects may have to be taken into consideration:
- Run-of-river hydro power projects have relatively long operational life-times (up to 80 years, EUSUSTEL, no date) so that their overall life cycle costs could remain relatively low. Moreover, larger scale run of river projects could achieve lower electricity production costs than smaller scale hydro projects due to economies of scale (Pollution Probe, 2003).
- Although run-of-river plants can produce electricity constantly, the output levels depend on the weather. For example, in Romania hydro power output in 2005 (a wet year) took a share of 37% of the country's electricity production, whereas in 2007 (a dry year) this share was only 25% (ANRE, 2008).
- In some countries there is a lack of legislative support for hydropower projects with subsequent problems with gaining permission to use water from rivers, and also due to perceptions that plants might adversely affect fishing.
- In addition, there could be difficulties in gaining affordable connections to the grid.
The technology is commercially and technically mature. Innovations in design, equipment and control/instrumentation would improve performance and increase access to export markets, as would systems to mitigate environmental impact. As explained in the Introduction , many of world’s hydro reserves still remain unexploited (IEA, 2008). Roughly half of the world's hydroelectricity production takes place in OECD countries, but most of the remaining potential for the technology remains in developing countries. Especially, the potential for small-scale hydropower has been largely unexploited with only 5% of the world's potential used (IEA, 2008).
Run-of-river hydroelectricity plants can have different capacity levels varying from roughly 15 to 75 MW.
In developing countries several run-of-river hydroelectricity projects have been implemented. Below, some examples are described.
The World Bank assists the Government of India in meeting its targets for hydropower expansion in a financially, economically, and technically sound manner. It also aims to ensure that such projects meet the good environmental and social practices which have been developed by the industry in recent years. The Bank has been engaged in hydropower in India since the late 1950s. The two most recent Bank engagements, Nathpa Jhakri and Koyna IV – both approved in 1989 – have been successfully completed with the help of Bank finance (World Bank, 2005).
The Khimti I hydropower project  in Nepal is a run-of-river hydropower generation plant with an installed generating capacity of 60 MW and an annual production of 350 million kWh of electrical energy. The plant has increased Nepal’s installed capacity by approximately 25%. The civil design and construction works of the project were carried out under a contract by a consortium of NCC Tunnelling, formly Statkraft Anlegg (Norwegian company) and Himal Hydro (Nepalese company). A consortium of Alston Power, formly ABB Kraft and Kvarner Energy along with Nepal Hydro & Electric (Pte) Limited carried out the electro-mechanical works. Similarly a consortium of Statkraft Engineering and BPC Hydroconsult had carried out the project management on behalf of HPL (Himal Power Limited, 2001).
As per June 2010, 413 run of river hydroelectricity projects have been registered by the Executive Board for the Clean Development Mechanism of the Kyoto Protocol (CDM). Together, these projects are excepted to contribute to an emission reduction of greenhouse gases of 127 Mtonnes CO2-eq (http://cdm.unfccc.int/Projects/registered.html  and below ).
Run of river hydroelectricity production has the advantage of being a cost-effective and reliable energy technology. Its output can be predicted relatively well as this varies mainly with annual rainfall patterns and only gradually varies from day-to-day instead of minute-wise. Output is also possitively correlated with demand, i.e. output is maximum in winter when there is more water available (British Hydropower Association, 2005). The technology could have a strong impact on improving energy security of supply and reducing poverty alleviation (International Hydropower Association, 2002).
In comparison with the other major type of hydroelecticity production (facilities with large storage reservoirs based on dams ) run-of-river plants have a relatively small environmental ‘footprint’ (Renewable Energy Access, 2005). Good design could mitigate the stresses placed on the environment. For instance, a fish ladder can allow fish to swim around the station (Pollution Probe, 2003).
Potential disadvantages of run-of-river hydroelectricity plants relate to the environmental impact in the 'diversion reach' of a plant and the social costs related to recreation and tourism values (such as anglers, hunters, kayakers and hikers affected by project construction and infrastructure, Watershed Watch, 2007) and relocation of communities. As explained above, the diversion reach is the section of the river where the dam is placed and the point where the water comes back into the river through the tailrace from the power house. For example, the aquatic habitat in this section could be affected through reductions in water velocity, higher water temperature during the summer and earlier formation of ice during the winter (Watershed Watch, 2007). Diverting water out of the stream channel can dry out streamside vegetation. Moreover, hydropower projects can also affect aquatic organisms directly; downstream-moving fish may be drawn into the power plant intake flow and pass through the turbine. These fish are exposed to physical stresses (pressure changes, shear, turbulence, strike) that may cause disorientation, physiological stress, injury, or death (Sternberg, 2008).
An example of how negative socio-economic and environmental impacts of run-of-river projects can be mitigated can be found in the HPP Freudenau run-of-river power plant in Austria (annual output 1.037 GWh). The construction of this facility stops the erosion of the river bed and the resulting drop of the groundwater level. Equally, the ‘Old Danube’ and the ‘Lobau’, two specious former branches of the river, which are popular suburban recreation areas, are supplied with sufficient fresh water and thus revitalised. Carefully observing some 500 environmental regulations, the Freudenau facility is in full compliance with the Viennese Environmental Protection Act (Poyry, 2006).
Hydropower can achieve significant GHG emission reductions as it, depending on the energy mix of the country concerned, could replace fossil based technologies for electricity production. For calculation of GHG emission reduction of a large-scale run-of-river hydroelectricity plant, it is recommended to apply the Approved Consolidated Methodology ACM0002, which has been developed for grid connected renewable energy production projects under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html 
The capital required for run-of-river hydro plants depends on the effective head, flow rate, geographical features, the equipment (turbines, generators, etc.) and civil engineering works, and whether the flow of water is constant throughout the year. The capital investment cost varies from € 900 per kWh to € 4000 per kW, while generation costs vary from € 0.025/kWh to € 0.125 per kWh (EUSUSTEL, no date). Once established, hydropower plants can have long and productive lives (even up to 80 years). For example, the Bhakra Nangal plant in India , now more than 40 years old, has operating costs of only USDcent 0.002/kWh (ENTTRANS, 2008).
Small projects are usually privately financed, with partial recourse to different kinds of loans. Bigger projects are mostly financed by corporations but there are also third party financing models. The main project risk for hydro power plants lies in varying electricity prices. Therefore, in countries with stable price agreements (with feed-in tariffs) projects are easier to be financed than in countries where energy prices oscillate.
Financing run-of-river hydro power plants can be sometimes difficult due to the unforeseeable power production in the short term. Financing institutes could interpret the uncertainty about short-term output as low reliability, even though long term data is often very reliable (EUSUSTEL, no date). Consequently, often short payback periods (5 years or less) are levied requiring high returns during the early years of operation. This approach places the operator at risk, particularly if a drought is experienced early in the repayment schedule. Longer term finance is more appropriate to the nature of the asset, which carries low technical risk and long life, albeit with energy yield subject to annual variation.
[this information is kindly provided by the UNEP Risoe Centre Carbon Markets Group ]
Project developers of wind projects in the CDM pipeline mainly apply the following two CDM methdologies:
CDM projects based on hydro represent 27.4% of all CDM projects in the pipeline and, as such, are the most common project type in the pipeline. The geographical distribution of hydro projects is concentrated in Asia, particularly in China. [media:image:2]
ANRE, 2008. Annual report Date Statistice Aferente Energiei Electrice, 2004-2007.
British Hydropower Association, 2005. A guide to UK mini-hydro developments , London, the UK.
EUSUSTEL, no date, EUSUSTEL: European Sustainable Electricity; Comprehensive Analysis of Future European Demand and Generation of European Electricity and its Security of Supply, EU - FP6. Available at: http://www.eusustel.be/ 
Himal Power Limited, 2001. Khimti 1 Hydropower Project. Available at: http://www.hpl.com.np/khimti_1_hydropower_project.htm 
IEA, 2008. Energy Technology Perspectives 2008, International Energy Agency (IEA), Paris, France.
International Hydropower Association, 2002. Hydropower - a Key Tool for Sustainable Development. Available at: http://www.hydropower.org/downloads/F3%20Hydropower%20A%20Key%20Tool%20for%20 Sustainable%20Development.pdf 
Pollution Probe, 2003. Primer on the technologies of renewable energy. Available at: http://www.pollutionprobe.org/Reports/renewableenergyprimer.pdf 
Poyry, 2006. HPP Freudenau - Run-of-River Power Plant. Available at: http://www.verbundplan.at/hydropower/hydropower_3.html?Id=69 
Renewable Energy UK, 2006. Run of River Hydro Power. Available at: http://www.reuk.co.uk/Run-of-River-Hydro-Power.htm 
Sternberg, R., 2008. Hydropower: Dimensions of social and environmental coexistence, Renewable and Sustainable Energy Reviews, 12(6), pp.1588–1621.
Watershed Watch, 2007. Run-of-River Hydropower in BC - a citizen's guide to understanding approvals, impacts and sustainable of independent power projects. Available at: http://www.watershed-watch.org