Simplify the following text for a Grade 10 native English-speaking person.
Original: On 1 April 1998 , the existing local government districts of Rochester-upon-Medway and Gillingham were abolished and became the new unitary authority of Medway .
Simplified: On April 1 , 1998 , Rochester-upon-Medway and Gillingham joined together to became the new unitary authority of Medway .