Mouth-mixing zone

The definition of the sea boundary of the mouth area is related to the term mouth-mixing zone. Water salinity within this zone increases from the salinity inherent in river water (usually 0.2-0.5%o) to the salinity of seawater (usually 10-40%o in different seas). The salt composition of water radically changes within the mixing zone river water of hydrocarbonate class and calcium group transforms into seawater of chloride class and sodium group. 

The mixing zone of river fresh water with salinity less than 0.5%o and sea brackish water with salinity up to 18%> occupies a bend up to 20-30 km in width during high-flow period and western winds and of 3-5 km during the low-flow period and eastern winds [7,30]. The Danube water spreads mainly on the surface with layer from 1-3 to 5 m [9]. The Danube River water runoff and its distribution between delta branches play a very important role not only in hydrological and hydrochemical regime of the delta and mouth nearshore zone but also in formation of ecological conditions in the northwestern part of the Black Sea as a whole [7,9,30]. 

Sometimes semi-enclosed coastal water bodies are situated between a mouth reach of the river and open nearshore zone [20]. These intermediate parts of the river mouth areas can be presented as narrow sea bays, lagoons, limans, and estuaries. These coastal water bodies are often separated from the open nearshore by coastal bars, spits, barrier islands, etc. and connected with it through relatively narrow outlets. These semi-enclosed coastal water bodies are characterized by active interaction and mixing of river and seawater. 

The sea boundary of a river mouth area or the outer boundary of an open nearshore zone of a river mouth is defined by a maximum propagation distance of the outer (marine) part of the frontal zone into the sea, when river and sea waters are mixed in the surface layer. This boundary is arbitrarily defined by the location of the isohaline equalling about 90% of water salinity in the adjacent part of the sea at the river high-flow period. 

Environments ofTerrigenous/Siliciclastic Sedimentation That Are Associated with Mouths of Large Rivers 2,5,1,1,2,1 Estuaries An estuary is a drowned mouth of a river valley forming a funnel-shaped coastal indentation in which seawater can circulate and mix with river freshwater. Estuaries form where the discharged fluvial sedimentary load is small (<16 mg/1), subsidence is greater than sediment accumulation, and in macrotidal zones. Most of the estuaries are located in humid temperate climatic zones where extensive vegetation effectively restricts erosion and riverine sediment transport (Friedman and Sanders, 1978). 

Data captured by the Rapid Underway Monitoring system show the increase in turbidity, and decrease in salinity, as well as the more diffuse increase in chlorophyll-a associated with the discharge from a major wastewater-treatment plant near the mouth of the Brisbane River. One useful application of the system is this capacity to map mixing and impact zones from discharges. (Reproduced from Hodge, ]., Longstaff, B., Steven, A., Thornton, P., Ellis, P., and McKelvie, I., Mur. Pol. Bull, 51,113, 2005. With permission.)... 

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