Grain sizes get larger downstream as slope decreases in a stream channel.

Question

BlackTom AI · Accepted Answer

The question asks whether grain sizes get larger downstream as slope decreases in a stream channel. 
Option: False. 
To evaluate this, consider how sediment transport and deposition respond to changes in slope and energy. 
First, in higher-gradient, steeper reaches, streams typically have higher energy and can move and entrain larger particles, causing coarser material to be transported or reworked. As the channel transitions to gentler slopes downstream, the flow energy generally decreases. 
With lower energy downstream, the river’s capacity to transport larger grains diminishes, leading to deposition of coarser material in upstream or upper-mid reaches and allowing finer material to persist or be deposited downstream. This sorting tends to produce finer-grained sediments downstream as energy drops, not coarser grains. 
Second, abrasion and selective transport dynamics favor removing finer particles at times, but overall, the energy decline with decreasing slope typically reduces the ability to carry large particles, causing coarser material to settle out earlier and leave finer material as the downstream transport-limiting process. 
Third, in many fluvial systems, downstream reaches exhibit finer bed and bank materials due to dispersion of energy over a longer reach and the blocking of large grains by channel boundaries, which further supports a trend toward smaller grain sizes downstream rather than larger. 
Therefore, the statement that grain sizes get larger downstream as slope decreases contradicts the common understanding of fluvial sediment transport and sorting, making it incorrect. 
In summary, grain sizes do not systematically increase downstream as slope decreases; instead, they tend to become finer downstream due to reduced transport capacity and sorting processes.

Grain sizes get larger downstream as slope decreases in a stream channel.单项选择题

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