Final answer:
A stream entering a flat valley plain from a steep mountain slope would likely deposit poorly sorted sediments due to the energy of the water decreasing quickly, thus dropping a mix of sediment sizes.
Step-by-step explanation:
The main answer to what type of sediment a stream would likely deposit when entering a flat valley plain from a steep mountain slope is that it would deposit poorly sorted sediments. This happens because as the stream flows from a steep slope to a flatter area, the energy of the water decreases, causing it to deposit a mix of different-sized particles, from large boulders and pebbles to fine sand and silt, rather than being uniform in size or well compacted. Particle sorting occurs when the transportation energy of the stream is able to segregate particles by size, which is less likely to occur in this scenario due to the abrupt change in gradient and potential presence of a variety of sediment from upstream areas or rock types.Graded bedding is associated with a settling of particles of varying sizes in a more defined pattern; however, the scenario described indicates a rapid, rather than gradual, energy decrease which would not allow for graded settling, resulting in poorly sorted sediments. Additionally, these sediments are not exclusively fine-grained, nor are they strictly chemical precipitates, and they may not be well compacted due to the sudden drop in sediment-transport energy.