Delta branch mouth

The term mouth is recommended to be used either in a wide sense as a reduced version and absolute synonym if the term mouth area (in this case, the use of the term river mouth is quite necessary) or in a narrow sense as applied to the point of inflow of any watercourse to another watercourse (tributary mouth) or to a receiving basin (delta branch mouth) as well as to the outlet to the open sea from a semi-enclosed coastal water body (liman mouth, lagoon mouth, estuary mouth). 

River mouth Delta area, km2 Number of delta branch mouths Length of the main delta branch, km Length of delta coastline, km Area of the semi-endosed coastal water body, km2 Area of open nearshore zone km2 Refs. 

According to the morphological characteristics, all the mouth reaches of the river can be subdivided into mouth reaches without deltas (single-branch) and deltaic mouth reaches. The latter can be multi-branch (the number of delta branches exceed 5) or with few branches (the number of delta branches does not exceed 5). 

The boundaries of a river mouth area are defined by the intense manifestation of mouth processes (Fig. 1). The river boundary of a mouth area or the head of a mouth area are defined either by a maximum propagation distance of water level fluctuations of marine origin (tides, storm surges) into a river during low-flow period or by the point, where the river channel is divided into delta branches (a delta head or a delta apex), if water level fluctuations of marine origin do not reach this point. In the first case, there is a part of the river mouth reach above the delta head (or a near-delta reach) between the river boundary of the mouth area and the delta head. In the second case, these two components coincide. The first principle of defining the upper boundary of a river mouth area is usually applicable to river mouths with small deltas or... 

The RSLR substantially affects the river mouths. It results in flooding in coastal areas, increase in coastal water bodies, backwater in delta branches, and a certain intensification of the wave erosion of delta coastline. 

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]. 

The Rioni River mouth area belongs to the open deltaic type (Fig. 10) and comprises a protruding delta with a few branches and a deep nearshore zone. The delta head is near the water division sluice constructed at 7 km from the sea in 1959 for distributing river flow between the two main branches of the delta. The channel network of the Rioni mouth includes delta branches, the City Canal (left) and the Rioni-spillway (right). The City Canal, 7.5 km long, is divided into Northern and Southern branches separated by the Bol-shoy Island to form the old delta. The Rioni-spillway, 7 km long, is also divided into Northern and Southern branches separated by an island to form the new delta. The total area of the present-day delta of the Rioni River is about 20 km2. 

Fig. 10 Scheme of the mouth area of the Rioni River. 1 - old delta, 2 - new delta, 3 - port, 4 - sea approach canal, 5 - submarine canyon. Delta branches 6 - Northern ( old delta) 7 - Southern ( old delta) 8 - Northern ( new delta) 9 - Southern ( new delta). 10 - Water division sluice... 

Fig. 7 Scheme of the Dniester River mouth. 1 - the Dniester delta, 2 - the Glubokiy Turunchuk branch, 3 - The Dniester Branch, 4 - the Dnestrovskiy Liman, 5 - the Tsaregradskoye outlet. Settlements 6 - Majaki, 7 - Belgorod-Dnestrovskiy, 8 - OvidiopoT... 

In modern times, the development of the protruding delta of the Rioni River has been as follows [15,28] the old55 delta was formed at the Rioni mouth before the diversion of river flow in 1939 (the data on the progradation of the old55 delta are very scarce) and a new55 delta was formed at the mouth of the Rioni-spillway after 1939 the old55 delta virtually inherited the distinctive hydrographic features of the ancient times (two branches, Northern and Southern), divided by the Bolshoy Island. 

The flow bifurcated into the Northern and Southern branches of the new delta and, later, an island was formed (Fig. 11c). By its dimensions and outlines, this island closely resembles Bolshoy Island, which was formed at the mouth of the main channel of the Rioni River in the old delta. 

In the last 30-35 years, delta coast has experienced abrasion. The retreat of the delta coastline has consisted of 3-5 myear-1. The main reasons of this process are reduction of the river sediment load after reservoir construction and sea level rise. Slight progradation of the delta is observed only near the mouths of the main branches. 

---