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A mudflow, also known as mudslide or mud flow, is a form of mass wasting involving fast-moving flow of debris and dirt that has become liquified by the addition of water.^[1] Such flows can move at speeds ranging from three to five metres per second (9.8 to 16.4 ft/s).^[2] Mudflows contain a significant proportion of clay, which makes them more fluid than debris flows, allowing them to travel farther and across lower slope angles. Both types of flow are generally mixtures of particles with a wide range of sizes, which typically become sorted by size upon deposition.^[3]

Mudflows are often called mudslips, a term applied indiscriminately by the mass media to a variety of mass wasting events.^[4] Mudflows often start as slides, becoming flows as water is entrained along the flow path; such events are often called mud failures.^[5]

Other types of mudflows include lahars (involving fine-grained pyroclastic deposits on the flanks of volcanoes) and jökulhlaups (outbursts from under glaciers or icecaps).^[6]

Heavy rainfall, snowmelt, or high levels of groundwater flowing through cracked bedrock may trigger a movement of soil or sediments in landslides that continue as mudflows. Floods and debris flows may also occur when strong rains on hill or mountain slopes cause extensive erosion and/or mobilize loose sediment that is located in steep mountain channels. The 2006 Sidoarjo mud flow may have been caused by rogue drilling.

The point where a muddy material begins to flow depends on its grain size, the water content, and the slope of the topography. Fine grained material like mud or sand can be mobilized by shallower flows than a coarse sediment or a debris flow. Higher water content (higher precipitation/overland flow) also increases the potential to initiate a mudflow.^[7]

After a mudflow forms, coarser sediment may be picked up by the flow. Coarser sediment picked up by the flow often forms the front of a mudflow surge and is pushed by finer sediment and water that pools up behind the coarse-grained moving mudflow-front.^[8] Mudflows may contain multiple surges of material as the flow scours channels and destabilizes adjacent hillslopes (potentially nucleating new mudflows).^[9] Mudflows have mobilized boulders 1–10 metres (3 ft 3 in – 32 ft 10 in) across in mountain settings.^[10]

Some broad mudflows are rather viscous and therefore slow; others begin very quickly and continue like an avalanche. They are composed of at least 50% silt and clay-sized materials and up to 30% water. Because mudflows mobilize a significant amount of sediment, mudflows have higher flow heights than a clear water flood for the same water discharge. Also, sediment within the mudflow increases granular friction within the flow structure of the flow relative to clear water floods, which raises the flow depth for the same water discharge.^[11]

Landslide is a more general term than mudflow. It refers to the gravity-driven failure and more subsequent movement downslope of any types of surface movement of soil, rock, or other debris. The term incorporates earth slides, rock falls, flows, and mudslides, amongst other categories of hillslope mass movements.^[12]

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The world's largest historic subaerial (on land) landslide occurred during the 1980 eruption of Mount St. Helens, a volcano in the Cascade Mountain Range in the US state of Washington.^[13] The volume of material displaced was 2.8 cubic kilometres (0.67 cu mi).^[14]

The largest known of all prehistoric landslides was an enormous submarine landslide that disintegrated 60,000 years ago and produced the longest flow of sand and mud yet documented on Earth. The massive submarine flow travelled 1,500 kilometres (930 mi) – the distance from London to Rome.^[15][16]

By volume, the largest submarine landslide (the Agulhas slide off South Africa) occurred approximately 2.6 million years ago. The volume of the slide was 20,000 cubic kilometres (4,800 cu mi).^[17]

• Quick clay – Type of glaciomarine and unstable clay prone to landslides
• Osceola Mudflow – Volcanic mudflow in Washington, USA

• Dingle, R. V. (December 1977). "The anatomy of a large submarine slump on a sheared continental margin (SE Africa)". Journal of the Geological Society. 134 (3): 293–310. Bibcode:1977JGSoc.134..293D. doi:10.1144/gsjgs.134.3.0293. S2CID 129229469.
• Fletcher, Lara; Hungr, Oldrich; Evans, S G (1 February 2002). "Contrasting failure behaviour of two large landslides in clay and silt". Canadian Geotechnical Journal. 39 (1): 46–62. Bibcode:2002CaGJ...39...46F. doi:10.1139/t01-079.
• Hungr, Oldrich; Leroueil, Serge; Picarelli, Luciano (1 April 2014), "The Varnes classification of landslide types, an update", Landslides, 11 (2): 167–194, Bibcode:2014Lands..11..167H, doi:10.1007/s10346-013-0436-y, S2CID 38328696, archived from the original on 27 July 2014, retrieved 16 July 2014. On-line publication 30 November 2013.
• Hungr, Oldrich; Leroueil, Serge; Picarelli, Luciano (4 January 2013), The Varnes classification of landslide types, an update. Draft of Hungr, Leroueil & Picarelli 2014, with page numbers.
• Iverson, RM; Reid, ME; LaHusen, RG (May 1997). "Debris-flow mobilization from landslides". Annual Review of Earth and Planetary Sciences. 25 (1): 85–138. Bibcode:1997AREPS..25...85I. doi:10.1146/annurev.earth.25.1.85.
• Kean, Jason W.; McCoy, Scott W.; Tucker, Gregory E.; Staley, Dennis M.; Coe, Jeffrey A. (December 2013). "Runoff-generated debris flows: Observations and modeling of surge initiation, magnitude, and frequency: RUNOFF-GENERATED DEBRIS FLOWS". Journal of Geophysical Research: Earth Surface. 118 (4): 2190–2207. doi:10.1002/jgrf.20148. S2CID 130762677.
• Kean, Jason W.; Staley, Dennis M.; Cannon, Susan H. (5 November 2011). "In situ measurements of post-fire debris flows in southern California: Comparisons of the timing and magnitude of 24 debris-flow events with rainfall and soil moisture conditions". Journal of Geophysical Research. 116 (F4): F04019. Bibcode:2011JGRF..116.4019K. doi:10.1029/2011JF002005.
• Stock, J. D.; Dietrich, W. E. (1 September 2006). "Erosion of steepland valleys by debris flows". Geological Society of America Bulletin. 118 (9–10): 1125–1148. Bibcode:2006GSAB..118.1125S. doi:10.1130/B25902.1.
• Talling, P. J.; Wynn, R. B.; Masson, D. G.; Frenz, M.; Cronin, B. T.; Schiebel, R.; Akhmetzhanov, A. M.; Dallmeier-Tiessen, S.; Benetti, S.; Weaver, P. P. E.; Georgiopoulou, A.; Zühlsdorff, C.; Amy, L. A. (November 2007). "Onset of submarine debris flow deposition far from original giant landslide". Nature. 450 (7169): 541–544. Bibcode:2007Natur.450..541T. doi:10.1038/nature06313. PMID 18033295. S2CID 4373921.

• Hungr, Oldirch; Evans, S. G.; Bovis, M. J.; Hutchinson, J. N. (August 2001), "A review of the classification of landslides of the flow type", Environmental & Engineering Geoscience, 7 (3): 221–238, Bibcode:2001EEGeo...7..221H, doi:10.2113/gseegeosci.7.3.221.

• United States Department of Homeland Security: Facts about Mudflows/Landslides.