Time is but the stream i go a-fishing in. i drink at it; but while i drink i see the sandy bottom and detect how shallow it is. its thin current slides away, but eternity remains. in the excerpt, thoreau mostly uses imagery to illustrate?

Thoreau mostly uses imagery to illustrate time in the excerpt. The second sentence is saying that he is in the midst of time/living, and he can "detect how shallow it is," meaning that he is aware that life is short and that the end is inevitable. Thoreau also describes time as being fleeting, but ever-present.
I hope this helps!