If a Side Stream Feeds Coarse Gravel into a River

If a side stream feeds coarse gravel into a river with a sandy bottom,what does the river do about it?

A) The river flushes it down valley.
B) The slope of the riverbed builds up until it is steep enough to transport the size of gravel supplied.
C) The pebbles work themselves down through sediments in the riverbed until they reach bedrock.
D) The pebbles dam the river channel.
E) The water slows down because the larger sediment gets in the way.