Hydropower conjures images of massive, landscape-shattering dams, such as the Three Gorges on upper tributaries of the Yangtze River in China. Large hydroelectric dams produce enormous amounts of electricity, but they also swallow up vast swaths of natural and human habitat. They impact water movement and quality, sediment patterns, and fish migration.
Smaller in-stream turbines are different. Placed within a free-flowing river or stream, they capture water’s kinetic energy without creating a reservoir and its repercussions. The underwater analogue to wind turbines, their blades rotate as water moves past, generating relatively continuous electricity. No barriers, diversions, or storage are required, only limited structural support. No emissions ensue.
In remote communities from Alaska to Nepal, this technology is expanding electrification and replacing expensive and dirty diesel generators. In urban environments, in-stream turbines can be placed within city water mains (called conduit hydropower).
As in-stream hydro grows, it is important to note that not all “run-of-river” projects actually let the river run. Some have diverted waterways, caused floods, and impeded fish migration. Careful design and installation can ensure clean energy that is also ecologically sound.
Three Gorges…displaced 1.2 million people: Watts, Jonathan. “Three Gorges Dam May Force Relocation of a Further 300,000 People.” The Guardian. January 22, 2010.
native communities in rural Alaska: Mooney, Chris. “Alaska’s Quest to Power Remote Villages—and How It Could Spread Clean Energy Worldwide.” Washington Post. August 14, 2015.
Waterways fed by Himalayan snowmelt: Lee, Amy. “Microhydro Drives Change in Rural Nepal.” New York Times. June 20, 2012
city water mains; Portland, Oregon: Profita, Cassandra. “Portland Now Generating Hydropower In Its Water Pipes.” Oregon Public Broadcasting. January 20, 2015; Slavin, Terry. “From Oregon to Johannesburg, Micro-Hydro Offers Solution to Drought Hit Cities.” The Guardian. September 18, 2015.
[potential of] U.S. hydrokinetic resources: National Research Council. An Evaluation of the U.S. Department of Energy’s Marine and Hydrokinetic Resource Assessments. Washington, D.C.: National Academies Press, 2013.
Correction: If in-stream hydro grows to supply 3.7 percent of the world’s electricity by 2050, it can reduce 4 gigatons of carbon dioxide emissions and save $568.4 billion in energy costs.
Project Drawdown defines in-stream hydro as: small-scale hydropower technologies under 10 megawatts, including in-stream hydrokinetic systems. This solution replaces conventional electricity-generating technologies such as coal, oil, and natural gas power plants.
In-stream hydro, sometimes referred to as run-of-river or simply small hydro, is similar to large reservoir-based hydroelectricity but does not divert and store large amounts of water. Another type of in-stream hydro is modeled after tidal energy, where underwater turbines are anchored to the riverbed and spin from the flowing river current. This report focuses almost exclusively on the former.
This analysis models small-scale hydropower technologies under 10 megawatts.
Total Addressable Market 
The total addressable market for in-stream hydro is based on projected global electricity generation in terawatt-hours from 2020-2050, with current adoption  estimated at 2.43 percent of generation (IRENA, 2016).
Adoption Scenarios 
Impacts of increased adoption of in-stream hydro from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the solution’s market share was fixed at the current levels.
- Plausible Scenario: Due to the uncertainty associated with the development of these technologies, the Plausible Scenario follows a customized high-growth adoption. Using the 2030 projection from IRENA (2016), it is assumed that electricity generation from small hydro will double by 2050. In this scenario, in-stream hydro is projected to capture 3.7 percent of the electricity generation market in 2050.
- Drawdown Scenario: This scenario is derived from the evaluation of the ambitious scenarios of three energy systems models,  following a low-growth trajectory. None of the models explicitly identify the evolution of small hydro systems for electricity generation; therefore, a conservative assumption was adopted that, in the future, the current share of 14 percent of all hydroelectricity would continue to come from small systems. This adoption results in a market share of 2.67 percent in 2050.
- Optimum Scenario: This scenario is identical to the Plausible Scenario, and likewise results in a 3.7 percent share of the electricity generation market in 2050.
The financial inputs used in the model assume an average installation cost of US$2,722 per kilowatt  with a learning rate of 2 percent,  reducing the cost to US$2,680 in 2030 and to US$2,626 in 2050. An average capacity factor of 44 percent is used for in-stream hydro, compared to 55 percent for conventional technologies such as coal, natural gas, and oil power plants.
Through the process of integrating in-stream hydro with other solutions, the total addressable market for electricity generation technologies was adjusted to account for reduced demand resulting from the growth of more energy-efficient technologies,  as well as increased electrification from other solutions like electric vehicles and high-speed rail. Grid emissions factors were calculated based on the annual mix of different electricity generation technologies over time. Emissions factors for each technology were determined through a meta-analysis of multiple sources, accounting for direct and indirect emissions.
The results for the Plausible Scenario show that the net cost compared to the Reference Scenario would be US$202.53 billion from 2020-50, with nearly US$568.36 billion in savings over the same period. Increasing the use of in-stream hydro from about 2.43 percent in 2014 to 3.7 percent of world electricity generation by 2050 would require an estimated US$884.95 billion in cumulative first costs. Under the Plausible Scenario, this increased adoption could avoid 4 gigatons of carbon dioxide-equivalent greenhouse gas emissions from 2020-2050. The Drawdown Scenario results in 1.7 gigatons of avoided greenhouse gas emissions, while the Optimum Scenario estimates 3.7 gigatons. The lower emissions impact in the Drawdown and Optimum Scenarios is due to the lower total electricity demand resulting from the increased uptake of energy efficiency solutions.
Small hydropower systems impose a smaller impact on aquatic ecosystems and local communities; but, like all forms of electricity generation technologies, they cannot completely prevent stresses on ecosystems and human well-being.
Small-scale hydropower has a wide range of designs, equipment, and material. In-stream hydrokinetic solutions, for example, might play a crucial role in remote mountainous regions in need of electrification where it is uneconomical to install power transmission lines. In-stream hydro offers the best, most reliable, and most economical method of generating electricity in these places. Instead of building expensive electric transmission networks or transporting diesel to fuel generators, the natural flowing rivers adjacent to so many of these villages can be harnessed to provide a clean, nearly endless supply of electricity.
 For more about the Total Addressable Market for the Energy Sector, click the Sector Summary: Energy link below.
 Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.
 To learn more about Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Energy Sector-specific scenarios, click the Sector Summary: Energy link.
 IEA ETP 2°C Scenario, Greenpeace Energy [R]evolution Scenario, AMPERE MESSAGE 450 Scenario.
 All monetary values are presented in US2014$.
 A similar learning rate was applied to conventional technologies for electricity generation (coal, natural gas, and oil power plants).
 For more on Project Drawdown’s Energy Sector integration model, click the Sector Summary: Energy link below.
 For example: LED lighting, heat pumps.