Why do shorelines change?
The short answer
Waves and currents move sand and stones, so some places lose sediment while others receive it and shorelines shift.
How it works
Waves arriving at an angle can move sand along the shore. Storms may remove sediment, while rivers and currents add it elsewhere. Seawalls and other structures can change these flows.
A familiar example
The same beach may have a different amount of sand before and after a storm or between seasons.