An Introduction

to

Alluvial Fans

(The Panamint Mountains draped by alluvial fans. Photo by Marli Miller)

RELATED LINKS

http://www.science.smith.edu/departments/Geology/dv/dv.html

http://wrgis.wr.usgs.gov/docs/paks/deva/rfan.html

http://www.earlham.edu/~parkero/

http://www.visualsunlimited.com/browse/vu959/vu95953.html

http://www//wrgis.wr.usgs.gov/ docs/parks/deva/galfan.html

(Fan spreads out from canyon cut into the Black Mountains. Photo by Marli Miller)

(Alluvial Fan. Photo by Marli Miller)

WHAT IS AN ALLUVIAL FAN?

"An alluvial fan is a gently sloping apron of sediment dropped by an ephemeral stream at the base of a mountain in arid or semi arid regions"

(Badwater alluvial fan. Photo from NPS archieves)

How Do Alluvial Fans Form?

Alluvial fans form because material is constantly being eroded in narrow valleys above the fan. During large rain storms, debris choked floods may was large boulders, and a mixture of sand, silt, and clay down toward the base of the slope. As the area at the base of the slope flattens out and the debris is no longer constrained by a channel it spreads out into the fan like shape.

Where Do Alluvial Fans Form?

Alluvial Fans normally form at the base of topographic features where there is a marked break in the slope. Thus, alluvial fans tend to be coarse-grained, especially at their mouths. At their edges, however, they can be relatively fine-grained. - Marli Miller

Where are Alluvial Fans Most Likely to be Found?

Alluvial fans are most likely to be found in desert areas subject to periodic flash floods from nearby thunderstorms in local hills. Alluvial fans are very common around the margins of the sedimentary basins of the Basin and Range province of southwestern United States and northern Mexico.

- Alluvial Fans are mostly in arid & semi arid regions with fault block

- Some in Humid Regions such as the Himalayas and Japan

- Some in arctic regions of glacial outwash

Depostional Processes

Debris Flows: high density, high velocity flow, poorly sorted, oversize clasts, structureless or graded

Mud Flows: debris flow that lacks gravel, poorly sorted, structureless

Channel Flows: typical braided stream deposits

Sheet Flood: thin flow of water

Sheet-like sand and silt: little clay

Can Alluvial Fans Be Stacked?

Absolutely! Alluvial fans can be stacked on top of one another in basins. Stacked sequence of fan deposits contains buried soils, but preservation is discontinuous.

Alluvial Fan Hazard Areas

Alluvial fan hazard areas are those areas on a fan that have the potential to damage or harm the health or welfare of the community. They generally correspond to the path of recent and potential future stream flooding, boulder flooding, and/or debris torrents.

BE WISE AND DON'T BUILD YOUR HOME IN THESE AREAS!

(Glenwood Springs, Colorado. Photo by Ron Parker)

Literature Cited

Articles

"Death Valley National Park Alluvial Fans." Geology of Death Valley National Park. USGS. 29 Nov. 2005 <http://wrgis.wr.usgs.gov/docs/paks/deva/rfan.html>.

"Physiographic Features." USGS: Science Topics: alluvial fans. 23 Nov. 2005. USGS. 29 Nov. 2005 <http://www.usgs.gov/science/science.php?>.

C.P North, S.P. Todd, and J.P. Turner. "Introduction; alluvial fans and their tectonic controls." Journal of the Geological Society May 1989: 507-508.

Books

Marshak, Stepen. Earth: Portrait of A Planet . 2nd ed. New York and London: W.W. Norton & Company, Inc., 2005.

Nichols, Gary J. Sedimentology and Stratigraphy. Malden: Blackwell Publishing, 1999.

Bridge, John S. Rivers and Floodplains: Forms Processes and Sedimentary Record. Malden: Blackwell Publishing, 2002.

WebPages

Parker, Ron. "Ron Parker's Web Page." 01 Dec. 2005. Earlham College. 03 Dec. 2005 <http://www.earlham.edu/~parkero/>.

Hendrix. "G432 Lecture 16- Continental Deposts." 03 Dec. 2005 <http://www2.umt.edu/Geology/faculty/hendrix/g432/g432_L16.htm>.

Miller, Marli. "Alluvial Fans." University of Oregon. 03 Dec. 2005 <http://darkwing.uoregon.edu/~millerm/fan.html>.

Encyclopedia

"Alluvial Fans." Wikipedia: The Free Encyclopedia. Wikipedia. 03 Dec. 2005 <http://en.wikipedia.org/wiki/alluvial_fan>.

Link to other Student Webpages for 2005 Fall Earlham Physical Geology

Copyright © 2005 Earlham College. Revised December 3, 2005 . Send corrections or comments to Shenita V. Piper