Dave, I'm on the road, but in the meantime, maybe Roman could interject here and give us some help.

Here's a simple link that has some very good, albeit basic information, but it is a good start!

http://www.nps.gov/grca/grandcanyon/faq.htm

here's a GREAT site to get some additional information
http://www.kaibab.org/misc/gc_faq.htm

I'll be back later!