Complex sentence: The Missouri River (The Big Muddy), dumped millions of cubic feet of soil into the Mississippi River every year, which, in turn deposited this silt into the gulf and formed a string of barrier islands.
Split into simpler sentences: The Missouri River (The Big Muddy), dumped millions of cubic feet of soil into the Mississippi River every year. Which, in turn deposited this silt into the gulf and formed a string of barrier islands.