Final answer:
Rivers sort and transport sediment based on the flow's energy and the sediment's size, with coarser particles requiring higher energy to move. Braided rivers, characterized by shifting channels and evidence of strong flow, are indicated by sand and gravel deposits, while fine materials denote swamps or wetlands. Depositional environments and sedimentary structures reveal ancient river dynamics.
Step-by-step explanation:
Rivers sort and transport sediment based on the energy of the water flow and the size of the sediment particles. Sediment transport in rivers is generally downstream, where the flow velocity dictates the size of the sediment that can be carried. Larger particles like cobbles and pebbles require high energy to move, which is typically found in steeper and faster-flowing parts of the river. As it proceeds downstream and the gradient decreases, the river loses energy, and hence the capability to carry larger sediment particles diminishes, leading to the deposition of these particles. Sand, silt, and clay, being finer particles, can be transported much further and are often deposited in slower-moving parts of the river.
Environments such as braided rivers are typified by multiple channels and are formed in areas where the river carries a significant load of sediment. They are more likely to have sand deposits with gravel and exhibit cross-bedding structures indicating directional flow of sediment. Swamps or wetlands, on the other hand, are characterized by finer sediments and lower energy water flow.
Depositional environments and sedimentary structures, such as point bars in meandering rivers or deltas at river mouths, provide essential clues as to the direction and energy of ancient river systems. Determining the facies, such as marine, coastal, or terrestrial, is essential for analyzing sedimentary structures and understanding past environments.