# Run-of-the-river hydroelectricity **Run-of-the-river hydroelectricity** generates power from a river's flow as it comes, with little or no reservoir: a portion of the river is channelled through a canal or [[Penstock|penstock]] to use the natural fall of the riverbed, often without a large dam.[^doe-plants] Its output follows the river's [[Hydrograph|hydrograph]] — high in the spring melt, low in late summer — so its designer must choose a design flow: a small plant runs near full most of the year but lets the flood spill past, while a large one catches more energy but sits partly idle, with a lower [[Capacity_factor|capacity factor]]. <div class="microsim-player"> <iframe src="https://wikitube-3d-microsims.netlify.app/Run-of-the-river_hydroelectricity.html" width="100%" height="620" frameborder="0" loading="lazy" sandbox="allow-scripts allow-same-origin" allow="accelerometer; gyroscope" title="Run-of-the-river hydroelectricity — p5.js microsim"></iframe> </div> *Microsim (THY-067): pick the design flow for a plant on a snowmelt river and read the trade between energy, spill and capacity factor. ILLUSTRATIVE: a synthetic river year with a mean flow of 100 m³/s.* Videos: [16:9](https://wikitube-3d-microsims.netlify.app/media/Run-of-the-river_hydroelectricity_16x9.mp4) · [9:16](https://wikitube-3d-microsims.netlify.app/media/Run-of-the-river_hydroelectricity_9x16.mp4) ## Concept A diversion or run-of-river facility channels part of a river through a canal and/or penstock to use the natural decline of the river bed, and may not require a dam.[^doe-plants] Power at any moment is P = ηρgH·min(Q, Q_d): the plant uses the river's flow up to its design flow Q_d and spills the rest. Where a stream falls down a hillside, a small private system can collect water above and deliver it through a penstock to the bottom of the hill, gaining head without a dam.[^kerlin-9] ## Major types Run-of-the-river plants are built at the toe of a low dam, behind a diversion weir that feeds a channel, or with a small pond that stores a few hours of flow. ### Dam-Toe A low dam raises the water just enough to feed turbines in a powerhouse at its toe; it creates head without storing much water. ### Diversion Weir A weir diverts part of the river into a channel or penstock that runs along the valley to a powerhouse lower down, returning the water to the river below. ### Pondage A small pond behind the weir stores enough water to shift generation by a few hours within the day, toward the hours when power is most valuable. ## Advantages Without a large reservoir, run-of-river plants flood little land and leave the river's seasonal flow largely intact. ### Cleaner power and fewer greenhouse gases With no large flooded area, they avoid most of the greenhouse gases that decaying vegetation releases from reservoirs. ### Less flooding They do not drown valleys or displace communities as large reservoirs do. ## Disadvantages Their output cannot be stored or dispatched, and it falls when the river is low. ### "Unfirm" power Because generation follows the flow, run-of-river power is "unfirm": it cannot be relied on at a given hour and must be balanced by other sources or storage. ### Environmental impacts The diverted reach between intake and powerhouse carries reduced flow, which regulators address with minimum-flow requirements; in Minnesota the Department of Natural Resources reviews federal hydropower licenses for minimum flows and fish passage.[^dnr-hydro] ### Vulnerable to climate change Changes in snowpack and rainfall shift and can shrink the river flows on which run-of-river output depends. ## Major examples In Minnesota, Minnesota Power operates eleven hydroelectric stations with more than 120 MW of combined capacity on the Mississippi, Kawishiwi, Crow Wing, Prairie and St. Louis rivers.[^mnpower] ## See also - [[Hydroelectricity]] - [[Capacity_factor]] - [[Hydrograph]] - [[Penstock]] ## Notes - Kerlin's *Future Energy* also uses "run-of-river" for hydrokinetic turbines submerged in a free-flowing river, whose power is P = ½ρAv³ like a wind turbine's; that is a different machine from the diversion plants described here.[^kerlin-9] ## References [^doe-plants]: U.S. Department of Energy, Water Power Technologies Office. "Types of Hydropower Plants." https://www.energy.gov/eere/water/types-hydropower-plants [^kerlin-9]: Kerlin, Thomas W. (2013). *Future Energy: Opportunities & Challenges*, ch. 9 "Hydroenergy," pp. 203–216 (§9.4 dams and small systems; §9.5 pumped storage; §9.6 river-flow converters). ISA; CC BY 4.0. https://open.umn.edu/opentextbooks/textbooks/future-energy-opportunities-challenges [^dnr-hydro]: Minnesota Department of Natural Resources. "Hydropower." https://www.dnr.state.mn.us/waters/surfacewater_section/stream_hydro/hydropower.html [^mnpower]: Minnesota Power. "Hydro." Company page. https://www.mnpower.com/Environment/Hydro ### Portal Books - Kerlin, Thomas W. (2013). *Future Energy: Opportunities & Challenges* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/future-energy-opportunities-challenges); on the [[PORTAL_Thury_Hydrodynamics_Apex_Spine]] and [[PORTAL_Energy]] shelves. - Murphy, Tom W., Jr. (2021). *Energy and Human Ambitions on a Finite Planet* — [eScholarship](https://escholarship.org/uc/item/9js5291m); on the [[PORTAL_Energy]] shelf. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Run-of-the-river_hydroelectricity) : [Wikitube](https://en.wikitube.io/wiki/Run-of-the-river_hydroelectricity) · pinned revision [1369927425](https://en.wikipedia.org/w/index.php?oldid=1369927425) · 2026-09-10 ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Energy]]. --- *Thury station wave · 2026-09-10 · drafted · microsim THY-067 (p5.js) · parent [[Hydroelectricity]].*