The following assumptions were made in performing the calculations which predict the ESR signal intensity along the mainstream: (a) sediments from the mainstream and tributaries are sufficiently mixed below junctions, (b) sediment input from small tributaries is negligible, and (c) quartz contents do not change during transport as SPM along the mainstream.
If we consider only K2O/Al2O3, then we can see that the chemical weathering state at the river mouth is similar to that through much of the mainstream, although the sediments in the uppermost Jinshajiang do seem to be slightly more altered than most of the mainstream, despite the fact that they are close to their source regions.
In the mainstream, Tl in TGR sediments showed the accumulation tendency from the upstream to downstream.
Thus, the sediments discharged from Dongting Lake to the mainstream are a mixture of sediments from the upper part of the mainstream and from tributaries flowing into Dongting Lake itself.
Therefore, Dongting Lake receives sediments from the mainstream above the Three Gorges through the Three Outlets, in addition to sediment originating from its own tributaries.
From Fig. 11a, we can also deduce that the tributaries of the Daduhe, Minjiang, Jialingjiang, and Hanjiang contribute more sediment to the mainstream than the Yuangjiang and Ganjiang because the bulk composition of the river changes substantially downstream of each confluence towards the value of the tributary.
Sediments from the mainstream were sampled at locations a few hundred meters to a few kilometers above and below the junctions with major tributaries to evaluate the effects of sediment mixing between the mainstream and the tributaries.
In the calculations, we assume that sediments from the mainstream and the tributary are sufficiently mixed below the junction to apply the simple mixing ratio approach.
The junctions with the Yalongjiang and Dongting Lake were excluded from the analysis, as riverbed sediments in the mainstream are lacking above the junctions with these tributaries.
Using such an approach, we can also estimate changes in ESR signal intensities of the fine-silt fraction of sediments along the mainstream from the uppermost part to the rivermouth.
