Word for crossing the bottom of a valley

Solution 1:

This may not be perfect, but it's at least plausible. How about traverse?

From your question:

He walked down to the traverse of the valley. [analogous to "the crest of the hill"]

From Dictionary.com:

traverse, noun: a place where one may traverse or cross; crossing.

From your question:

He traversed the valley, and surveyed his surroundings.

From Dictionary.com:

traverse, verb: to pass or move over, along, or through.

Acknowledgement: @Mazura mentioned traverse in his/her comment on the OP's question.

Solution 2:

This is perhaps a bit fanciful, but one might conceivably say that "she looked around as she plumbed the valley".

This sense of the word "plumb" is defined by oxforddictionaries.com as

explore or experience fully or to extremes

Solution 3:

From the McGraw-Hill Dictionary of Earth Science:

valley floor The broad flat bottom of a valley. Also known as valley bottom; valley plain. [Italics mine]

So you have a choice of nouns. As a verb, plain won't do, and floor as a verb means either to furnish with a floor or knock to the floor, literally or figuratively. The OED finds "to reach bottom" for a meaning of bottom:

He bottomed with his feet and stood upright [in the pond]

But this is likely to clash with the usage bottom out, which is reserved for more dynamic things like the movement of stock prices. Perhaps

She looked around as she descended to the valley bottom.