# Dam A **dam** is a work of [[Civil_engineering|civil engineering]]: a barrier built across a stream or river to hold back [[Water|water]], raise its level and store it in a reservoir. Dams supply drinking water and irrigation, control floods, make rivers navigable and generate [[Hydroelectricity|hydroelectricity]]. The difference in height between the reservoir surface and the river below is the [[Hydraulic_head|hydraulic head]], and the energy a hydroelectric dam can produce is proportional to that head and to the rate at which water flows through its [[Water_turbine|turbines]].[^earle92] The International Commission on Large Dams counts more than 62,000 large dams in its world register.[^icold] The U.S. National Inventory of Dams documents more than 91,000 dams across the United States and its territories,[^nid] and Minnesota alone has more than 1,150.[^mndnr-dams] On the Thury spine, the dam is where [[Hydropower|hydropower]] gets its head. The pressure that rises with depth behind a dam is the same hydrostatics that sets the design of the structure itself. ## Definition and etymology The International Commission on Large Dams (ICOLD) defines a *large dam* as one with a height of at least 15 m above its foundation, or between 5 and 15 m high and holding more than 3 million cubic metres of water.[^icold] ## Types Dams are classed by how they resist the push of the water. A **gravity dam** is "a solid concrete structure so designed and shaped that its weight is sufficient to ensure stability against the effects of all imposed forces."[^usbr-gravity] An **arch dam** is curved upstream in plan so that it transmits most of the water load sideways to the canyon walls, and a **buttress dam** has a watertight upstream face supported at intervals by buttresses on the downstream side.[^usbr-glossary] **Embankment dams** are built of earth or rock fill. Soil for embankments and dams is chosen partly by its particle size and is compacted to a dense state for strength and low permeability, which makes the work a problem of [[Geotechnical_engineering|geotechnical engineering]].[^hossain] ## Uses Most large dams store water for later use. A reservoir behind a hydroelectric dam lets operators slow or stop the flow through the turbines when demand is low, at night or in summer, and raise it when demand is high. That flexibility is valuable on grids with a large share of wind and solar power, which cannot be dispatched on demand.[^earle92] Earle distinguishes this dam-and-reservoir hydro from [[Run-of-the-river_hydroelectricity|run-of-river hydro]], which diverts part of a stream into a canal or pipe and stores little water.[^earle92] Some dams exist mainly to regulate a river. The federal headwaters dams at Lake Winnibigoshish, Leech Lake and Pokegama Falls were built between 1881 and 1885 "to control flooding and water flow on the Mississippi River."[^mnopedia-headwaters] ## History In Minnesota the first industrial dams were at St. Anthony Falls in Minneapolis. By the end of 1848, two sawmills operated on a new dam there, beginning the waterpower district that made the city.[^nps-ch6] The U.S. Army Corps of Engineers finished Lock and Dam No. 1 on the Mississippi, with the base of a [[Hydropower|hydroelectric]] plant, in 1917.[^fmr-locks] The great concrete dams of the twentieth century followed. Hoover Dam on the Colorado River, begun on April 20, 1931, rises 726.4 feet from foundation rock to the crest road, and its concrete was finished on May 29, 1935.[^usbr-hoover] China's Three Gorges Dam, about 181 m high, is the world's largest hydroelectric plant, with a generating capacity of 22,500 MW.[^usgs-3g] ## Environmental impacts Earle names two significant disadvantages of dam-and-reservoir hydropower: dams block the passage of migratory fish, and reservoirs flood large areas of land. Fish ladders can be built, but they are not usually successful at a high dam.[^earle92] A Minnesota DNR guide to dam removal lists further effects: sediment trapped in reservoirs, degraded river channels below dams, and blocked fish migrations.[^aadland] A dam interrupts the movement of water and sediment through the whole [[Drainage_basin|drainage basin]] downstream of it. ## Socio-economic impacts Reservoirs displace people. The Three Gorges project relocated 1.24 million residents from 13 cities, 140 towns and 1,350 villages.[^earle92] In Minnesota the headwaters reservoirs flooded Ojibwe land. The Ojibwe rejected the government's offer of $15,466.90 in compensation, and there was no final settlement with the Leech Lake Ojibwe until 1985.[^mnopedia-headwaters] ## Design Design begins with the forces. Water pressure rises with depth, so a dam must resist a load that is greatest at its base. A gravity dam does it with weight; an arch dam carries the load into the abutments.[^usbr-gravity][^usbr-glossary] The design also sets the head available for power: at British Columbia's Revelstoke Dam, a 175 m structure has created a reservoir 130 km long and a generating capacity of 2,480 MW.[^earle92] This is the domain of [[Hydraulic_engineering|hydraulic engineering]] and [[Structural_engineering|structural engineering]]. ## Auxiliary structures A dam is more than its wall. A [[Spillway|spillway]] passes floods safely around or over the dam. A [[Penstock|penstock]] carries water from the reservoir down to the turbines, as in the large pipes of the Revelstoke Dam.[^earle92] A lock lifts boats past the dam: the Upper St. Anthony Falls Lock in Minneapolis opened in 1963 and closed to navigation in 2015 under the Water Resources Reform and Development Act of 2014, to slow the spread of invasive carp.[^fedreg2014][^mpr2015] ## Construction Before a dam can be built, the river must be moved out of the way. A cofferdam is "a temporary structure enclosing all or part of the construction area so that construction can proceed in the dry."[^usbr-glossary] Then the dam itself goes up; Hoover Dam holds three and a quarter million cubic yards of concrete.[^usbr-hoover] ## Operation Dams need continuous maintenance. The Minnesota DNR spends about $2 million a year on dam repairs and estimates that $103 million will be needed over the next two decades. About 650 of the state's dams are public, and the state owns over 300 of them.[^mndnr-dams] The American Society of Civil Engineers gave Minnesota's dams a grade of C in 2026, warning that its 1,156 dams "are rapidly aging."[^asce2026] ## Dam failure When a dam fails, the stored water is released at once. The South Fork Dam above Johnstown, Pennsylvania, failed on May 31, 1889, releasing 20 million tons of water; the flood killed 2,209 people.[^nps-johnstown] On June 24, 2024, the Blue Earth River in southern Minnesota overtopped the Rapidan Dam and cut a new channel around its west abutment.[^blueearth] That August the county board directed staff to work with FEMA on projects including the dam's removal; as of June 2026 there was still no timeline for approval.[^blueearth][^keyc2026] ## Society and culture Dams reshape the places around them, and their removal has become a field of its own. Luther Aadland's *Reconnecting Rivers*, published by the Minnesota DNR in 2010, collects 43 case studies of dam removals and fish passages, including the Sandstone Dam on the Kettle River and the Heiberg Dam on the Wild Rice River, which failed in 2002 and was then removed.[^aadland] ## References ### Citations [^earle92]: Earle, Steven (2021). *Environmental Geology*. Thompson Rivers University. §9.2 "Hydro," pp. 329–330. https://environmental-geol.pressbooks.tru.ca/ [^icold]: International Commission on Large Dams. "The History of the World Register of Dams." https://www.icold-cigb.org/article/GB/world_register/history/the-history-of-the-world-register-of-dams [^nid]: U.S. Army Geospatial Center / U.S. Army Corps of Engineers (October 15, 2025). "National Inventory of Dams" fact sheet. https://www.agc.army.mil/Media/Fact-Sheets/Fact-Sheet-Article-View/Article/480923/national-inventory-of-dams/ [^mndnr-dams]: Minnesota Department of Natural Resources. "Dams and Dam Safety." https://www.dnr.state.mn.us/waters/surfacewater_section/damsafety/index.html [^usbr-gravity]: U.S. Bureau of Reclamation (1976). *Design of Gravity Dams*. https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/GravityDams.pdf [^usbr-glossary]: U.S. Bureau of Reclamation. "Glossary." https://www.usbr.gov/projects/glossary.php [^hossain]: Hossain, MD Sahadat; Islam, Md Azijul; Badhon, Faria Fahim; Imtiaz, Tanvir (2021). *Properties and Behavior of Soil – Online Lab Manual*. Mavs Open Press. Practical-application notes to the particle-size analysis and compaction tests. On the Compendium's Southern Agriculture book shelf. [^mnopedia-headwaters]: Cooper, Philip (2012). "Lake Winnibigoshish, Leech Lake, and Pokegama Falls Dams." *MNopedia*, Minnesota Historical Society. https://www.mnhs.org/mnopedia/search/index/thing/lake-winnibigoshish-leech-lake-and-pokegama-falls-dams [^nps-ch6]: National Park Service, Mississippi National River and Recreation Area. "River of History — Chapter 6." https://home.nps.gov/miss/learn/historyculture/river-of-hisory-chapter-6.htm [^fmr-locks]: O'Connor Toberman, Colleen (2022, updated 2026). "Meet our Twin Cities locks and dams." Friends of the Mississippi River. https://fmr.org/updates/land-use-planning/meet-our-twin-cities-locks-and-dams [^usbr-hoover]: U.S. Bureau of Reclamation, Lower Colorado Region. "The Colorado River and Hoover Dam Facts and Figures." https://www.usbr.gov/lc/region/pao/faq.html [^usgs-3g]: U.S. Geological Survey, Water Science School. "Three Gorges Dam: The World's Largest Hydroelectric Plant." https://www.usgs.gov/water-science-school/science/three-gorges-dam-worlds-largest-hydroelectric-plant [^aadland]: Aadland, Luther P. (2010). *Reconnecting Rivers: Natural Channel Design in Dam Removals and Fish Passage*. Minnesota Department of Natural Resources. https://files.dnr.state.mn.us/eco/streamhab/reconnecting_rivers_intro.pdf [^fedreg2014]: U.S. Army Corps of Engineers (December 19, 2014). "Closing Upper St. Anthony Falls Lock to Navigation, Located in Minneapolis, MN." *Federal Register*. https://www.federalregister.gov/documents/2014/12/19/2014-29716/closing-upper-st-anthony-falls-lock-to-navigation-located-in-minneapolis-mn [^mpr2015]: "Upper St. Anthony lock closing after half a century; blame the carp." *MPR News*, June 8, 2015. https://www.mprnews.org/story/2015/06/08/upper-st-anthony-lock [^asce2026]: American Society of Civil Engineers, Minnesota Section (April 30, 2026). *2026 Minnesota Infrastructure Report Card*. https://infrastructurereportcard.org/minnesota-infrastructure-report-card-2026/ [^nps-johnstown]: National Park Service. Johnstown Flood National Memorial. https://www.nps.gov/jofl/index.htm [^blueearth]: Blue Earth County. "Rapidan Dam." https://www.blueearthcountymn.gov/rapidandam [^keyc2026]: Berg, Evan (June 9, 2026). "Blue Earth County Commissioners review plans for Rapidan Dam removal, bridge replacement." KEYC. https://www.keyc.com/2026/06/09/blue-earth-county-commissioners-review-plans-rapidan-dam-removal-bridge-replacement/ ### Sources - Earle, Steven (2021). *Environmental Geology* — on the Compendium's center-circle shelf (WT!Space_Mining_In_Minnesota). - Hossain, MD Sahadat, et al. (2021). *Properties and Behavior of Soil – Online Lab Manual* — on the Compendium's Southern Agriculture shelf. <!-- COMPENDIUMLINK:BEGIN g19 — generated from _registry/plans/THURY_COMPENDIUM_SECTIONS.md; do not hand-edit inside --> **Part of the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]]** — main article for section 30, *Dams and reservoirs*. Related sections: [[Hydrostatics]] · [[Hydropower]] · [[Hydroelectricity]] · [[Hydraulic_jump]]. <!-- COMPENDIUMLINK:END --> <!-- THURYSIM:BEGIN g21 — Thury Compendium microsim (framework build, specs/sims/Dam.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Dam* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/thury/Dam.html" data-title="Dam"></div> *Built from `MICROSIM_GUIDE/specs/sims/Dam.json`; part of the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]] set.* <!-- THURYSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Dam) : [Wikitube](https://en.wikitube.io/wiki/Dam) · pinned revision [1373812173](https://en.wikipedia.org/w/index.php?oldid=1373812173) · 2026-09-10 ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Energy]]. --- *Thury main articles, wave 2 · 2026-09-10 · drafted · Compendium section 30 · sim pending THY-071.*