N 36 04.300, W 112 11.990
The Power of Mechanical Weathering: Cutting it Deep and Wide
The Colorado River has been has been winding its way through this area and eroding away the bottom of the Grand Canyon for over 5 million years. The river has been able to cut this canyon in a relatively short period of time in terms of geologic history (the age of the Earth -4.6 billion years). There are 3 main attributes that have helped the river cut the Grand Canyon so rapidly. First is the gradient or downhill slope ofthe river. The headwaters of the Colorado are high in the Rocky Mountains, which are over 14000 feet/ 4200 meters high, and in a short 1400 miles/ 2250 km it reaches the Gulf of California at sea level. The gradient of the Colorado River on average is a loss of 10 feet/ 3 meters for every mile/ 1.6 meters of downstream travel. This means the river is moving swiftly and carrying debris along with it. Second, the Colorado River for most of its life has been the most turbid river in the world. The more turbid the river the more sediments that it is carrying, such as rocks, soils, sands, etc., and therefore the ability for grinding away rocks the river flows over. The more turbid the river is the more brown it will appear. Finally, the Colorado River has been very dynamic for most of its life growing to 10 to 20 times its normal size during annual periods of snow melt in the spring and monsoonal rains in the summer. The extreme growth in the size of the river allowed it to carry much larger sediments, like large boulders. During these times of greater flow the river did more than grind away at the rocks on the river bed and along its edges, it cracked, pulverized, and destroyed rocks that were in its path.
As the Colorado River, cut through the rock layers that we now see exposed, the widening of the canyon took place through annual rain and snow eroding away the rims of the canyon. The canyon rims are at a high elevation. High enough to increase precipitation levels drastically enough to wash down the sides of the canyon down to river level on an annual basis. In the colder months, snow accumulates along the rims and as the snow melts during warmer days it fills in the cracks and fissures along the canyon walls. Conversely, at night temperatures drop down to freezing and as the water freezes it expands and causes the rocks to crack and shear along the canyon walls. In the warmer months, especially in July and August, the canyon areas experience monsoonal weather patterns that brings a lot of needed rain fall. But again, the vast majority of this precipitation falls along the rim. The rainfall washes down the sides carrying away sediments, soil, rocks, and boulders down to the Colorado River to wash out of the area.
At this EarthCache site you can see the Colorado River below and Granite Rapids, but even more impressive, listen- you might even be able to hear the river. What you are seeing and hearing below is the powerful river cutting down through the basement rock layers. Additionally, you are witnessing a side stream depositing sediments and rocks into the rivers path. This side stream is called Monument Creek. The stream feeds rocks, sediments, and soil into the Colorado River channel causing a debris field and a narrowing of the river. This process speeds up the flow of the river and causes more damage along the river bed and edges which erodes away rocks quicker.
- What color is the Colorado River?
- Is the color of the river indicative of high or low levels of turbidity (sediment load)?
- What do you think the cutting power of the river is today?