Rivers come out as the main link between the whole land surface and the ocean, with a discharge, on annual basis, of approximately 35,000 cubic kilometers of fresh water and 22,000,000,000 tons of dissolved sediment and also solid to the global oceans. They are the determinants of the ecological system of a given coastal area.
Despite the fact that many oceanographers have always postulated that the land-ocean boundary, after research, lies at the mouth of any estuary or even at the head of the latter, but further analyses reveal that river basins can also drain into the estuaries. This is very important when considering the effects of both short and medium changes in land usage, climate and there effects to the coastal and global ocean.
Uneven Global Database
The major task in assessing and quantifying fluvial discharge comes out unevenly in quantity and quality of the river data. Large rivers are much better documented than small rivers since their chances of being utilized for transport; irrigation and damming are very high though small rivers also play a big role during the transfer terrigenous sediments to the global ocean. As much as the database for North America and European have a fifty year or more span, most of them in central and South America, Africa and Asia are lowly documented although they have large waters and sediment input.
Data quality problem is increased by the fact that the database available spans the latter since some information was collected twenty to forty years ago when the flowing patterns were different. Recent information show conditions influenced by natural conditions. The Yellow River in Northern China is considered to have the highest load of sediments in the world, 1.1 billion tones y-1. With the young wet mountains and large anthropogenic influence in the South Eastern part of Asia, it is not a surprise that this region stands for 75% of the suspended sediment released to the global ocean. An alternative way to view river fluxes is to take into consideration the ocean basins they empty to. In most cases, fluvial discharges are lost soon as the river releases to the ocean caused by the mixing, flocculation, and chemical uptake. However, in the past few years, the sediment load has been less than one hundred million tons due to drought and the rapid human removal of river water. Little discharge values for rivers cannot show short term events. Floods that are usually related to El Nino lead to large impacts on smaller or arid rivers in that sediment loads have minute importance to short term values. Due to uneven database and inaccessibility of data, it has only been possible in the recent years to enough quantity and diverse data to allow a quantitative understanding of factors controlling fluvial fluxes to the oceans. Works by Meybeck and Ragu (1996) and Milliman and Farnsworth (2002) have brought up a database for close to 1500 rivers with their drainage basins summed up represent close to 85% of land area that empties to the global ocean.
GAS EXCAHNGE IN ESTUARIES
There are many gases that are always exchanged in the coastal oceans. Discharge from a river is a function of precipitation minus evaporation, meteorological run off, and the drainage basin area. Rives draining Indonesia, Taiwan and the Philippines are good examples of river basins with high runoff but their drainage areas are small. They can have discharges just like rivers with much larger basins. On the other hand, rivers with low runoff have high discharges due to large drainage basin areas for example River Lena and Yenisei. The Amazon River has a large basin taking over 35% of South America and a very high runoff. That is its freshwater discharge can sum up to the combined discharge of the next seven largest rivers. The coastal waters along North Eastern South America are affected highly by this great discharge though The Amazon effects can be seen as far as north of the Caribbean that is 2000 km away.
FLUVIAL DISCHARGE TO THE GLOBAL OCEAN
World rivers at large release about 35 000 cubic kilometers of the fresh water to the ocean. A lot of this comes from South-east Asia and North-eastern South America though they represent less than 20% of the total land area draining into the global ocean. Areas with little precipitation have less discharge just like the large land area they drain. With the young wet mountains and large anthropogenic influence in the South Eastern part of Asia, it is not a surprise that this region stands for 75% of the suspended sediment released to the global ocean. An alternative way to view river fluxes is to take into consideration the ocean basins they empty to. In most cases, fluvial discharges are lost soon as the river releases to the ocean caused by the mixing, flocculation, and chemical uptake.
CHEMICAL PROCESS IN ESTUARINE SEDIMENTS
There is a great variation in both the physical and also the chemical environments of the sediments found in an estuarine.
Oxidation/ Reduction Reactions
Electron transfers are the main components of any chemical environment of an estuarine. These reactions involve the affinity of electronics, whereby that environment with a high affinity for electrons will tend to attract electrons from lower affinity quarters.
- Oxidants and Reductants
Oxidants accept electrons, in line with the sediments, and, on the other hand, reductants donate electrons. Organic matter are deemed the main sources of reductants, while oxidants diffuse from deep waters and reach the sediments by the aid of a certain flux or even formed by a sedimentation process.
- The oxidation of organic matter in estuarine Sediments
Oxidation mainly involves the accepting of electrons. These are components of the estuarine that have a very high affinity for electrons. The composition of the coastal waters is thus influenced by this oxidation process.
Elemental Cycling within Estuarine and coastal sediments
The interchange and transport of different gases within the sediments influence the oxidation process. Heterotrophic respiration always leads to organic matter oxidation hence P, C and N are released into the existing solution and hence oxygen gas, Fe, Mn, and S are reduced.
Contaminant Cycling: Anthropogenic Metals
The chemical environment of an estuarine always has a layered structure which has a lot of implications in line with cycling of contaminants. A good example is when a reduction process is considered where Fe and S lead to the formation of Hydrogen sulphide gas.
Fiords, which refer to oceanic intrusion into majorly land, and that, are glacially curved, have both internally and externally influenced circulations. The temporal variations that are experienced in terms of the water density are always deemed essential for the water exchanges that occur both at the top and below the sill level.
Simple Quantitative Models of Ford Circulation
There are several models in relation to Ford circulation:
- The Surface Layer
This is a baroclinic circulation that is driven by the differences in density that occur between the evident upper layers of the given fiord and also the coastal region.
- The intermediate Layer
This is mainly prompted by intermediary circulations, which cause a change in composition of the coastal water depending on the sill depth.
- The Basin Water
It comes about between the filling and stagnation time, where the density in a given basin is affected by the two durations.
They are features that occur in high altitude areas for example in Greenland, the USA, Northern hemisphere, approximately 50 degrees Celsius of Canada. The climate experienced in these regions ranges from boreal to temperate, and many oceanographic processes are a common occurrence in these regions.
- Topography and Estuarine Circulation
They are considered long, deep and narrow, as compared to the continental shelf outside. The separation of the water masses is evident in a three vertical layer form: brakish surface, the intermediate and finally the basin water. They are composed of sills that always prevent any water exchange that may occur between the basin and the dreaded depth that is outside.
- Fresh Water Runoff and Nutrient Cycling
It has a major role of modifying the dynamics of the lower section organisms in any given fiord. Its magnitude greatly varies in relation to the surrounding landmass.