How does reservoir sedimentation affect downstream environments and how does the sediment size and shape change downstream?
Answer on question depends on environment in which reservoir was deposited. Sand (that will be converted to sandstone) can accumulates and be preserved in alluvial fan, river, deltas, deep water fans and so on. For example in deltas size of grains became smaller from proximal part of delta to distal part. In fans (alluvial and subwater) size of grains bigger in proximal and smaller in distal. All because of gravitation: when slope (in which stream of sediment transports) become gentle or when river fall into delta the velocity of stream decreases and stream can‘t anymore transport biggest grains and they sediment first. In rivers situation somewhat different, but also explainable. Size of grains change lateral to stream, not down or upstream. It is all because of strength of stream. If we imagine cross section of river we notice that river has 2 banks - one with steeper slope and another one with gentle. The strength of stream more powerful beside steeper band and less powerful beside gentle one. More powerful stream transport sediment and erode bottom of river, meanwhile less powerful stream led to sedimentation of material. Lateral decrease of stream strength toward gentle bank lead to decrease of grain size from sand to silt to clay. For carbonate rocks story is more interesting and complicated because beside gravitation their sedimentation are influenced by insolation, salinity and so on.
What happens below the reservoir will be more influenced by the tributary streams downstream than prior to reservoir construction. If they drain significantly different terrains, a shift in grain size and mineralogy should occur. If the tributaries drain similar geology perhaps the only change will be in the volume of sediment. A study of the outcrop geology of the entire watershed should allow an estimate of pre- and post-reservoir sediment distribution. Another factor to consider is disruption of the ecosystem downstream of the reservoir. If the riparian vegetation changes, the amount of sediment retained along the river might be less. This has several implications including impacts to fauna. This has been studied in the Grand Canyon downstream from Glen Canyon dam so you might find some useful research from those studies.
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