Complex sentence: Small underwater sand dunes that act like dams continually form and break down, and so waders in the stream see surges--which look like waves--of water flowing downstream at intervals of anywhere from just a few seconds to a minute or more.
Split into simpler sentences: Small underwater sand dunes that act like dams continually form, and break down. So waders in the stream see surges--which look like waves--of water flowing downstream at intervals of just a few seconds to a minute or more.