Simplify the following text for a Grade 4 native English-speaking person.
Original: The amount of plastic in the seas has increased dramatically since the first record in the 1960s , coming from waste discarded by ships , offshore dumping , litter on beaches and waste washed to sea by rivers .
Simplified: This is because of the waste thrown out by ships and trash dumped into the ocean . Sometimes the litter on beaches can also be washed into the sea by rivers .