Author’s Note: This part of a weekly series on geology for a class I am taking this semester at BYU-Idaho. This week’s prompt required students to go outside, take a photo showing an example of sediment transport and explain what is going on. Other posts from this assignment can be found under the “Geology 111” category.
In February, my wife and I took her sisters from our home near Reno down to southern California. I convinced Alisha to let me cross something off my bucket list – visiting Badwater Basin in Death Valley National Park. Badwater is the lowest point in the United States, at an elevation of 282 feet below sea level. It was definitely well worth the adventure to me.
At Death Valley National Park, there are numerous examples of sediment transport. There are sand dunes that are transported by the wind, rocks that meander around on thin sections of ice blown by the wind, and numerous alluvial fans. I’ll be focusing on alluvial fans for this post.
Alluvial fans form at the base of mountains where canyons empty into an adjacent valley. During periods of heavy rainfall or high volumes of snowmelt, the force of the water erodes the canyon, sending the debris out onto the valley floor. The debris found on the fan range in size from sand to large boulders, with the larger debris generally being closer to the canyon itself.
These fans form most frequently in dry climates, such as the one found in Death Valley. Furnace Creek, about 20 miles north of Badwater Basin, lies at an elevation of 190 feet below sea level. The weather station there reports an average annual rainfall of 2.36 inches (compare to 7.94 inches in Pasco, Wash., 16.1 inches in Salt Lake City, Utah, and 37.49 inches in Seattle, Wash.). On top of this extremely low rainfall, Furnace Creek is home to the hottest temperature ever recorded by an official weather station – 134F.
To show how arid the region is, here’s the current forecast (as of 11am Monday):
Notice how temperatures are expected to reach 90F by the end of the week, and it’s still March! The average high for this time of year is in the mid-to-upper 80s, with the record high for the month of March being 102F. Furthermore, the Weather Prediction Center expects no precipitation for the next seven days.
When it does rain, though, a lot of times it pours – hence the massive debris flows that created these alluvial fans. This region, which is at the transition between the Mojave and Great Basin Deserts, can see monsoonal moisture during the summer months.
The North American Monsoon, which is also responsible for the legendary flash floods in southern Utah and Arizona, is a weather pattern that sets up between July and September. The location where monsoon rains fall can range from Death Valley in the west to near Abilene, Texas in the east depending on where important weather patterns (like a thermal low) form that year.
While driving across these fans, it was amazing to me the size of some of the boulders, which were easily located a quarter mile from the mouth of the canyon. One of these was about a quarter the size of my CR-V. Fans have been forming for so long in this part of Death Valley that the lower portions of them seem to have melded into one, making it hard to determine where one ends and the next begins in some instances.
Also found within these fans are dry streambeds. While almost always dry, these streams probably flow with great force during debris flows. These streambeds will change from time to time – a response to being clogged up by sediment rushing downhill. I imagine that if you were to map each fan’s drainage network, the result would be similar to a map of the waterways in Egypt’s Nile Delta.
The entirety of Death Valley National Park is an excellent example of how climate, geology, and geography are inseparably linked. It is well worth a visit.
