But why come up river to these open sand flats?但为何要逆流而上到这片开阔的沙滩来呢

come up升起；靠近，走进

His new book did not come up to expectation.

他的新作没有达到预期水平。

You've come up in the world, haven't you?

你的地位已经上升了,是吗?

But this nursery will not exist for long.但这个托儿所无法长期存在

exist有时有“在逆境中生活，仅能活命”的意思。

Some people in some poor countries receive wages that allow them to exist but not to live.

有些贫困国家的人民的收入只能勉强维持生计。

exist与subsist区别：subsist指“继续存在”，而exist指“存在”。

While they wait for it to grow,they do their best to learn the skimming technique.等待下喙长长的同时他们会努力学习捕食技巧

wait for 等候； 等待

It irks us to wait for people who are late.

等候迟到的人使我们厌烦。

He was then sitting on thorns and waiting for you.

他当时如坐针毡，焦急地等待着你。