What occurs when water in a river or stream transports tiny rock particles or sediment downstream. it also happens when these transported particles grind away the rock and when water dissolves chemical elements in the rock?

This is called the surface runoff. It is part of the water cycle when the water from the mountains flow down to the rivers and empty out to the seas and oceans. Or it could also flow down through the soil. As it flows, it carries nutrients like nitrogen and sulfides, so that's why the oceans are salty because of surface water runoff.