How much fun is a Grand Canyon raft trip for a geologist? Well...plenty! I had the opportunity to float the Colorado River through the famous canyon a few weeks ago. I had plenty to look at and to explore but one thing literally took the oar right out of my hand.
While rowing even the easy sections I could easily feel the current push and pull my oar with practically every push I made. Frankly I am used to sections of the upper Colorado where there is much less force. Was I being just lazy, or was there something else at work?
The week we were on the river, the discharge at Lee's ferry, (mile 0 of the Grand Canyon) was a steady 8,000 cfs (cubic feet per second) for a study the NPS was conducting. Discharge is simply a measurement of how much water is flowing past a certain point. here in the US we measure discharge in cubic FEET/second, while the rest of the world measures cubic METERS pr second...oh well.
Often when I am on the upper Colorado, the discharge is similar 5,000-9,000 cfs. This could account for some of the extra force I was feeling on the oars, but not all of it.
The diagram below, borrowed from California State University, shows that the velocity is some how proportional to the wet area (depth times width). Looking through some old notes when I would have students measure the width and depth of the upper Colorado to calculate discharge, I could see that there is little appreciable difference in these two physical parameters between the upper Colorado and the Grand Canyon.
However, the one physical parameter I found that was different was river gradient. Looking through my river guides and doing some simple math I found that the upper Colorado stretch has a gradient of 3+ feet/mile while the section of the Grand Canyon has a gradient of almost 7.5 feet/mile.
Could this be the reason I found the oars being snatched out of my hands?