What a Geologist Sees - Part 5

Sometimes we can learn about large geomorphic (land-shape) features by observing small analogs. In this case, by observing small-scale erosion and deposition at a construction site, we can see how alluvial fans are formed (and then later eroded). 

When rain water moves in a uniform flow across an inclined surface, we call that sheetwash.  Sometimes small surface irregularities concentrate the flow into small rivulets which may erode small rills and when rivulets combine, they may produce gullies, as has happened at this construction site.

Differences in soil compaction on this slope may have facilitated the gully erosion seen in these three gulley examples. Though these gullies are short, they still illustrate how sediments are carried in flood conditions. While the storm water is in the channel (in this case the gully), it is confined by the walls of the channel (or in a larger example, the walls of a canyon), which keeps the water velocity higher. When the flood waters leave the confinement of the gulley (or canyon), the loss of lateral confinement results in the rapid slowing of the water and the deposition of most sediments carried by the waters.

Over time, when unrestricted by the presence of vegetation, this builds a fan-shaped deposit at the mouth of the gulley (or canyon).  Of the three gullies in this photo, the one of the left has the best preserved alluvial fan. The middle gully probably had a similar-sized alluvial fan at one time, but it was "dissected" (eroded) by subsequent waters from the largest gulley (on the right). 

Generally, alluvial fan growth in the Appalachians is hindered by the presence of trees and other vegetation during the ongoing erosion of the mountains.  If alluvial fans are present, their visibility is further hindered by said trees and vegetation.  Also, ongoing, routine rain events tend to erode the alluvial fans, which are generally formed by short-term, high-intensity storm events.  The more arid Western United States is a much better place to view alluvial fans, large and small.  Subsequent photos (in later posts) will show more examples of alluvial fans. The largest alluvial fans are generally at the mouths of mountain canyons, where the canyons empty into broad valleys, particularly in the Basin and Range Province.

In some western cities, e.g., El Paso and Albuquerque, large alluvial fans may be favored sites for building fancy homes - with a good view of the broad valley below - without regard as to how the alluvial fans formed.  It is not generally an issue until the occurrence of a 100-year flood, a 500-year flood, or especially a 1000-year flood.  Thus if you can afford a "million dollar view" of a valley and behind you lies the mouth of a mountain canyon, just give it a little thought before you sign the papers.