We needed to cross the foaming river to get back to camp, the river had washed the bridge away, all that was left was a few pieces of broken wood.

But, and.

Explanation:

Conjunctions are the parts of speech that join sentences or act as the 'glue' to put two or more similar sentences together. They help in linking words, phrases, and clauses and help make the sentence better and clearer.

In the given example, the correct conjunctions to use are "but, and". This is because, in the first blank, "but" will provide the link for the contrast or difference in the sentences before and after the blank. And the use of "and" in the second blank will help connect the previous sentence with the adjoining sentence.

Thus, the correct sentence will become-

We need to cross the foaming river to get back to camp, but the river had washed the bridge away, and all that was left was a few pieces of broken wood.

the mending wall was written by robert frost