spring/23 training 2

training spring/23

 

 

West Fork Complex fire

from 40 miles away.

Spent the day up high yesterday (much cooler!) and watched the afternoon heat amp up the West Fork fire. It was barely visible in the morning but as the afternoon wore on we could watch the cloud grow larger. What is needed is a good rain storm and the forecast doesn't look all that favorable just yet.

Studying stream velocity in the Grand Canyon

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. 

 Rowing through the Muav Gorge, looking serious and trying to avoid obstacles.

 Navigating the famous "Black Rocks" of the upper Colorado River, again looking serious and trying to avoid obstacles.



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.

Thank you CSU Long Beach


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?

yet another example of a cap rock

Last week we were camping along the Colorado River above Moab Utah when we saw these pillars showing how a more resistant chunk of sandstone will protect the less resistant shale below. The last blog post showed how the more resistant White Rim sandstone was left standing even after the softer Organ Rock shale had all eroded away leaving a really fun arch to visit.

Differential erosion of the Cutler formation along the Colorado River

Regime change

What a fun place, Musselman Arch in Canyonlands National Park.  The arch is composed of the White Rim sandstone a Permian aged wind blown classic sandstone in a near coast environment. Sandstone is fairly well cemented together making a rock unit somewhat resistant to erosion. Right below the White Rim you can see a reddish (isn't almost everything in the Canyonlands red?) brown silt-mud stone formation. This shaley material is poorly cemented making it fairly easily eroded. You can see how the sandstone in the arch is lasting much longer than the missing shale/silt/mud stone found below the arch. 

So the White Rim sandstone is acting as a cap rock to the softer Organ Rock found below.

Musselman Arch is found along the White Rim mountain bike trail. You can ride down the Shafer trail to the arch and back in a day, but the 1300 vertical foot climb back up is an interesting way to end the day's ride.