increase in sediment also adds to the density of the current, and thus increases its velocity even further. Definition Turbidity currents are traditionally defined as those sediment gravity flows in which sediment is suspended by fluid turbulence. However, the term "turbidity current" was adopted to describe a natural phenomenon whose exact nature is often unclear. The turbulence within a turbidity current is not always the support mechanism that keeps the sediment in suspension; however it is probable that turbulence is the primary or sole grain support mechanism in dilute currents (<3%). Definitions are further complicated by an incomplete understanding of the turbulence structure within turbidity currents, and the confusion between the terms turbulent (i.e. disturbed by eddies) and turbid (i.e. opaque with sediment). Kneller & Buckee, 2000 define a suspension current as 'flow induced by the action of gravity upon a turbid mixture of fluid and (suspended) sediment, by virtue of the density difference between the mixture and the ambient fluid'. A turbidity current is a suspension current in which the interstitial fluid is a liquid (generally water); a pyroclastic current is one in which the interstitial fluid is gas. Triggers Hyperpycnal plume When the concentration of suspended sediment at the mouth of a river is so large that the density of river water is greater than the density of sea water a particular kind of turbidity current can form called a hyperpycnal plume. The average concentration of suspended sediment for most river water that enters the ocean is much lower than the sediment concentration needed for entry as a hyperpycnal plume. Although some rivers can often have continuously high sediment load that can create a continuous hyperpycnal plume, such as the Haile River (China), which has an average suspended concentration of 40.5 kg/m3. The sediment concentration needed to produce a hyperpycnal plume in marine water is 35 to 45 kg/m3, depending on the water properties within the coastal zone. Most rivers produce hyperpycnal flows only during exceptional events, such as storms, floods, glacier outbursts, dam breaks, and lahar flows. In fresh water environments, such as lakes, the suspended sediment concentration needed to produce a hyperpycnal plume is quite low (1 kg/m3). Sedimentation in reservoirs The transport and deposition of the sediments in narrow alpine reservoirs is often caused by turbidity currents. They follow the thalweg of the lake to the deepest area near the dam, where the sediments can