Because of their rather loose nature and absence of controlled spillway, landslide dams frequently fail catastropically and lead to downstream flooding, often with high casualties. A common failure scenario is overflowing with subsequent dam breach and erosion by the overflow stream.
Landslide dams are responsible for two types of flooding: backflooding (upstream flooding) upon creation and downstream flooding upon failure. Compared with catastrophic downflooding, relative slow backflooding typically presents little life hazard, but property damage can be substantial.
While the dam is being filled, the surrounding groundwater level rises. The dam failure may trigger further catastrophic processes. As the water level rapidly drops, the uncompensated groundwater hydraulic pressure may initiate additional landslides. Those which fall into the dam reservoir may lead to further catastrophic spillages. Moreover, the resulting flood may undercut the sides of the river valley to further produce landslides downstream.
After forming, the dam leads to aggradation of the valley upstream, and dam failure leads to aggradation downstream.
Construction engineers responsible for design of artificial dams and other structures in river valleys must take into an account the potential of such events leading to abrupt changes in river's regimen.
Quake Lake, created in Montana in 1959.
The highest known landslide dam of historic times is the Usoi Dam in modern Tajikistan created by a landslide triggered by an earthquake on February 18, 1911. It dammed the Murghab River to the height of 301 m (987 ft) to impound Sarez Lake deep.
Among the most destructive landslide lake outburst floods in recorded history occurred in the Sichuan province of China on 10 June 1786 when the dam on the Dadu River burst, causing a flood that extended 1400km downstream and killed 100,000 people.
The Tangjiashan Lake, a dangerous "quake lake", was created as a result of the 2008 Sichuan earthquake. It was located in the extremely rugged terrain of Tangjiashan Mountain. Chinese engineers, scientists, and soldiers were involved in the digging of a sluice to alleviate the dangers of this one of 34 landslide dams created by the magnitude 8.0 Sichuan earthquake. On June 10, 2008, the lake started to drain via the sluice, flooding the evacuated town of Beichuan. .
The Riñihuazo begun on 22 May 1960, after a landslide caused by the Great Chilean Earthquake blocked Riñihue Lake's outflow. According to the chronicler Mariño de Lobera a similar event occurred after the 1575 Valdivia earthquake.