ON THE NATURE AND FREQUENCY OF FLOOD-GENERATED HYPERPYCNAL FLOWS INTO THE COASTAL OCEAN

Scientists have been chasing down the significance of hyperpycnal discharge from rivers into marine environments for most of the 20th century. Many turbidite sequences in the rock record have been attributed to marine hyperpycnal flows. While common in modern lacustrine, particularly glacial, environments, marine observations of hyperpycnal plumes are rather rare. Buoyancy considerations suggest that rivers would need to carry much sediment to overcome the ambient density of seawater. This then limits the marine generation of hyperpycnal flows to rivers that can generate super-elevated concentrations during in-frequent high-energy flood events: small to medium size rivers draining mountainous terrain able to rapidly shed sediment. The energetic nature and short duration of these flows do not lend themselves to observations, thus leading to the rarity of observations. This has led to a situation where modern observations have so far not been able to constrain the interpretation of the rock record. A series of significant advances across the last decade have shed light on this issue and include: 1) a spatial temporal analysis of which rivers are likely to generate hyperpycnal flows; 2) experimental flume studies on the generation of hyperpycnal flows with sub-critical sediment concentrations; 3) observations of hyperpynal flows at the field scale; 4) within-day (hourly) analysis of river discharge for the identification of hyperpycnal concentrations in small dirty rivers; 5) modeling advances of predicting the plunge and flow of hyperpycnal plumes; 6) the identification of hyperpycnites from field observations of recent deposits associated with hyperpycnal discharges.