Hello again, Tiff!
You're on the right track. I agree with the points raised by Annie and Disneymom, and I'd like to add a few subjective points of my own.
First, some minor points.
Thoreau's original quote (from "Trip to Saddleback Mountain") is a wee bit different from your citation: "... in more classic shades" instead of "... in more classic shade", and "... that they went to the college" instead of "... that they went to college". (I know this sounds like nit-picking, but accuracy counts for a lot in the shady groves of academia - whether in the middle of the city or by the side of the mountain.)
In your third paragraph, you use the adjective "great" twice. Try for a little variety. In the same paragraph you also give the full name of the Tahoe Center for Environmental Sciences twice; the second time around, I would shorten it to "the Tahoe Center".
In the fourth paragraph, somehow the phrase "questioning the meaning of nature" doesn't sound right. I assume what you are trying to say is that nature is a never-ending source of fascination for you, something that has excited your questioning mind ever since you were a young child.
You end the fourth paragraph by saying that you "can sit there for hours on end, because it simply mystifies me." Leaving aside the point that the idea of just sitting around staring may convey a sense of vacuity, it is not clear what the "there" refers to - "outside" is too general a denominator to be referred to as "there". Also, while I understand that the "it" in this sentence refers to nature (which you mention in the first sentence in the paragraph), there is so much distance between the first and the last sentence in the paragraph that the reader may be confused as to whether the "it" refers to other things mentioned in the paragraph, such as science, physics or in general "the outside". And finally, with this same sentence, perhaps "mystify" is not the word you intend. If a person is mystified, he or she is perplexed, baffled or confused; not at all what you would like to be at a place of learning. (How about one of the following: "fills me with wonder", "fascinates me", or "intrigues me"?)
In the fifth paragraph, you say that you want to "gain the insight of my professors" during your undergraduate years. I applaud your ambition, but aren't you short-changing your potential professors? If I were a tenured professor with many years of research under my belt, I am not all that sure I would be pleased if a student said that he or she would want to learn all I know in just four years! Why not be more modest and say that you want to "benefit from the insights of my professors"? Then in the next sentence, you repeat the word "insight". Why not change this to something like "After those four years, I would like to share what I have learned with the world"?
Also in the fifth paragraph, "teachings" is again perhaps an inappropriate word. It is generally understood to be a synonym for "doctrines" or "precepts". Perhaps the word you are looking for is simply "lessons".
As for more general points, I agree that the second paragraph sounds too much like a tourist guide, and the third paragraph like something out of a brochure for the Tahoe Center. You definitely should inject more of yourself into the essay. The main change I would make is that already in, or soon after, the first paragraph, I would explain why I am interested in the Tahoe Center and Sierra Nevada College - in your case, by inserting what is now your fourth paragraph to become the new second paragraph.
Above all, make the essay as personal as possible - convey a sense of you, and why you want to go to Sierra Nevada College!
All the best!