Complex sentence: Material that was created by the maintenance dredging of the Snohomish River was used to create the island, the original dredged material was placed over a 100 years ago and has been added to over time.
Split into simpler sentences: Material that was created by the maintenance dredging of the Snohomish River was used to create the island. The original dredged material was deposited over a 100 years ago and has been added to over time.