My wife and I are walking along the banks of the river.我和妻子正沿河岸走着.

My wife and I are walking along the banks of the river.我和妻子正沿河岸走着.
请问bank为什么要加s呢?前后文是Here is another photograph of the village.My wife and I are walking along the banks of the river.We are on the left.There is a boy in the water.He is swiming across the river,