Why is a beach more likely to change rapidly if it is placed along a river mouth where there are many winter storms than if it is found along a quiet stretch where there are no rivers and few storms?

Rivers carry sediment, which when deposited at the beach, increases the land. Similarly, rivers can carry sediment away from a beach, also changing its shape. These two forces can work together to effectively move sediment from one area of the beach to another. Similarly, storms also move sediment.