Metaphor occurs as one idea or element is defined by mentioning another element of idea that has some similarity. This differs from simile because the comparison does not include explicit words such as "like" or "as". In the case of "The line to get into the concert was a river, snaking through an endless valley" the author uses metaphor because to describe the line to get into the concert as long and moving the author defines it by mentioning a river "The line..was a river", also as the line was long the author defines it as "snaking through an endless valley".
