This chapter focuses on the processes, character, and significance of hyperpycnal floods and their deposits (hyperpycnites) in the geologic record based on selected examples from modern environments, outcrops, sediment cores, and seismic reflection profiles.

For bed-load- or suspended-load-dominated hyperpycnites, the rating curve of a flood event can be predicted and is represented by a power-law relationship between discharge and load. Because of reconcentration processes in marine waters, rivers other than those considered dirty (high sediment concentration) may form hyperpycnal flows over long distances on the sea floor. Hyperpycnal floods generally have a meteorologic origin, but catastrophic hyperpycnal floods can also occur because of dam outburst in the catchment area of a river. In some cases, catastrophic floods are associated with earthquakes or volcanic activity. Both in marine and lacustrine environments, frequent and powerful hyperpycnal flows develop meandering channel levee systems and more distal basinal fan lobes.

Hyperpycnites are characterized by a coarsening-upward basal sequence formed during the rising limb of the flood (waxing flow) and a fining-upward upper sequence formed during the falling limb of the flood (waning flow). The lower sequence is not always preserved, so hyperpycnites may be difficult to distinguish from classical turbidites. The common occurrence of organic debris of continental origin at the base of deposits is an indicator of hyperpycnites.

In glacial valleys of mountain ranges, a large amount of clastic material can be deposited in large valley lakes or in fjords by frequent hyperpycnal flows. Both in lacustrine and marine environments, large hyperpycnal floods, which are sometimes catastrophic, are documented in association with significant environmental changes such as deglaciation of the continent or volcano-glacier interactions (jokulhlaup).