I've seen many videos of rivers getting blocked by a strip of sand right where it's about to enter the ocean. (such as https://www.youtube.com/watch?v=psi62O-NHRQ)

I can't comprehend how this possibly happens. If the river flows continuously, shouldn't it be able to easily cut a path through this thin strip of sand? Even if it doesn't, shouldn't there still be a dry river channel to the ocean? How did the sand get there?

I'm currently reading into the discovery of the planets and the theories about our solar system. The ancient Greeks developed the geocentric model in which all the "planets", including sun and moon, revolve around earth. They placed all the planets in the "correct" order, but how did they figure this out? How did they determine that Mars for example is closer to earth than Jupiter?