Dissolved load is the portion of astream's total sediment load that is carried insolution, especiallyions fromchemical weathering. It is a major contributor to the total amount of material removed from a river'sdrainage basin, along withsuspended load andbed load. The amount of material carried as dissolved load is typically much smaller than thesuspended load,[1] though this is not always the case, particularly when the available river flow is mostly harnessed for purposes such asirrigation or industrial uses. Dissolved load comprises a significant portion of the total material flux out of a landscape, and its composition is important in regulating the chemistry and biology of the stream water.
The dissolved load is primarily controlled by the rate ofchemical weathering, which depends onclimate andweather conditions such asmoisture andtemperature.[2] Dissolved load has many useful applications within the field ofgeology, includingerosion,denudation, and reconstructing climate in the past.
Dissolved load is typically measured by taking samples of water from a river and running various scientific tests on them. First, thepH,conductivity, andbicarbonatealkalinity of the sample are measured. Next, samples are filtered to remove any suspendedsediments and preserved withchloroform to prevent growth ofmicroorganisms, while the others are acidified withhydrochloric acid added to keep dissolved ions fromprecipitating out of solution. Then, various chemical tests are applied to determine the concentration of eachsolute. For example, the concentrations ofsodium andpotassium ions can be determined byflame photometry, while thecalcium andmagnesium ion concentrations can be determined byatomic absorption spectrophotometry.[3]
Dissolved load can provide valuable information about the rate ofsoil formation and other processes of chemicalerosion. In particular, the mass balance between the dissolved load and solid phase is helpful in determining surface dynamics. In addition, dissolved load can be used to reconstruct theclimate of the Earth in the past. This is because chemical weathering is the major contributor to the dissolved load of a stream. The chemical weathering ofsilicate rocks is the primary sink forcarbon dioxide in the atmosphere, because atmospheric carbon dioxide is converted intocarbonate rocks in thecarbonate–silicate cycle.Carbon dioxide concentrations are the primary control of thegreenhouse effect, which determines the temperature of the Earth.[4]
Denudation is the process of wearing away the top layers of Earth'slandscape. Because the denudation rate is normally too low to measure directly, it can be indirectly determined by measuring thesediment load of the streams that drain the area in question. This is possible because any material that passes through a certain point on a stream is guaranteed to have come from somewhere in the stream'sdrainage basin upstream of that point. Astopographic relief increases, the dissolved load's contribution to the total stream load decreases because on steeper surfaces, rain is less likely toinfiltrate the rocks, leading to less chemical weathering, which decreases the dissolved load.[5]
The process of carryingsalts by water to the sea or aland-locked lake from a river basin is called salt export. When adequate salt export is not occurring, the river basin area gradually converts intosaline soils and/oralkali soils, particularly in lower reaches.[6]
| River | Drainage area, 106 km2 | Discharge, 109 m3/yr | Total dissolved solids (TDS), 106 tonnes/yr |
|---|---|---|---|
| Xijiang | 0.35 | 30 | 10.14 |
| Changjiang | 1.95 | 1063 | 226 |
| Huanghe | 0.745 | 48 | 84 |
| Ganges-Brahmaputra | 1.48 | 1071 | 129.5 |
| Lena | 2.44 | 532 | 50.6 |
| Amazon | 4.69 | 6930 | 324.6 |
| Orinoco | 1.00 | 1100 | 51.3 |
| Krishna | 0.251 | 30 | 10.4 |
| Godavari | 0.31 | 92 | 17 |
| Kaveri | 0.09 | 21 | 3.5 |
| Ganges | 0.75 | 493 | 84 |
| World total | 101 | 37000[9] | 3843.0 |