What clues does Steinbeck give to the mature of the country surrounding the green pool?

Steinbeck's introduction is the best answer for this question. The nature of the country surrounding the green pool is summed up in a few paragraphs.

"Afew miles south of Soledad, the Salinas River drops in close to the hillside bank and runs deep and green. The water is warm too, for it has slipped twinkling over the yellow sands in the sunlight before reaching the narrow pool.

On one side of the river the golden foothill slopes curve up to the strong and

rocky Gabilan Mountains, but on the valley side the water is lined with trees—

willows fresh and green with every spring, carrying in their lower leaf junctures

the debris of the winter’s flooding; and sycamores with mottled, white, recumbent limbs and branches that arch over the pool. On the sandy bank under

the trees the leaves lie deep and so crisp that a lizard makes a great skittering if

he runs among them. Rabbits come out of the brush to sit on the sand in the

evening, and the damp flats are covered with the night tracks of ‘coons, and

with the spreadpads of dogs from the ranches, and with the split-wedge tracks of deer that come to drink in the dark.

There is a path through the willows and among the sycamores, a path beaten

hard by boys coming down from the ranches to swim in the deep pool, and

beaten hard by tramps who come wearily down from the highway in the evening

to jungle-up near water. In front of the low horizontal limb of a giant sycamore

there is an ash pile made by many fires; the limb is worn smooth by men who

have sat on it."