HOW TO IDENTIFY AN ALLUVIAL FAN

A landform that is important to cultural resources compliance.

Field office personnel need to know how to recognize alluvial fans Because cultural

resources are so common on alluvial fans, field office personnel need to be able to recognize this land form in the field.

An example of a fan is illustrated on the right.

Because of the high probability of encountering cultural resources on an alluvial fan:

No ground disturbing activity is to be conducted on alluvial fans unless it is confined to either post settlement alluvium (PSA), or hydric deposits.

Otherwise, assistance will be terminated, or an exception will be made by the State Conservationist (e.g. PL-566).

How Alluvial Fans Form

Alluvial fans occur where a tributary empties into a larger valley. As the grade becomes less, sediment drops out.

This causes a fan shaped deposit to form at the mouth of the tributary.

How to Identify an Alluvial Fan

Check the mouths of tributaries in larger valleys while in the field.

Check topographic maps, and look for fan shaped elevation lines at the mouths of tributaries.

Check soils maps for soils designated as “local alluvium.”

We are not interested in small fans that resulted from modern cultivation Modern agriculture has resulted in massive

erosion. This has led to the creation of numerous small fans. These obviously are not going to include prehistoric camp sites.

These are composed of Post Settlement Alluvium (PSA)

Caution: Ancient alluvial fans are often covered by a veneer of PSA resulting from the same erosion events. They are usually larger. Take great care in dealing with them.

PSA ALLUVIAL FANS

The 1 Acre Rule

Often, alluvial fans that are less than 1 acre have formed from historic erosion resulting from modern agriculture. If they had been forming for thousands of years, they would likely be larger.

Confirm for PSA by examining channels in the fan for stratification, by using the core from a soil probe and looking for banding, or by seeing if the bases of trees are buried.

If still in doubt, contact a soil scientist.

Low Angle Alluvial Fans Low angle alluvial fans are

the single most likely topographic feature to contain significant cultural resources.

They commonly contain buried sites.

Large low angle fans on major streams may have subtle enough topographic changes that they are hard to spot in the field, such as the one pictured here. One should also use topographic and soils maps in identification.

Why Do Fans Have Such a Concentration of Significant Cultural Resources?

Factors may include the proximity to water, to food resources in wetlands, to streams that could be canoed, and to flood protection from the stream in the main valley.

Spring run-off from the tributary would annually bury with sediment the campsites occupied during the other seasons, thus preserving a stratified record valuable to scientists.

The large fan shown to the left has 4 known prehistoric sites, 4 farmsteads (over 50 years of age), and 1 cemetery.

Lidar sometimes will reveal fans

A village site is present on this low angle fan, and is overlooked by burial mounds

Multiple fans shown on Lidar

Mistakes can be costly

The most expensive archeological dig in Iowa history, was on an alluvial fan. The cost was approximately $1.5 million.

Accidental damage to a site on a fan is not only damage to our cultural heritage, but also very embarrassing to the agency.

Some ground disturbing activities that might be contemplated on a fan

Diversion ditches Grassed waterways Ponds Wetland developments Tile outlets for terraces

Soils that may indicate alluvial fans

Alluvial fans are often found in these soil map units: Ankeny, Cantril, Castana, Coppock, Creal, Deloit, Ely, Gravity, Judson, Martinsburg, Moingona, Napier, Olmitz, Spillville, Terril, Vesser, Volney, and Worthen.

If these soils are present, you should be suspicious that you may be dealing with an alluvial fan. Check the topographic position. These soils can also be associated with colluvial slopes, upland drainages, and stream terraces.

Facts about fans

Many alluvial fans had a period of major formation between approximately 2,500 and 9,000 years ago. Geologically, these fans are known as the Corrington Member of the DeForest Formation.

These are usually low angle fans found in medium to large size valleys.

Sometimes there may be varying amounts of young overwash known as Post Settlement Alluvium (PSA) covering the Corrington Member.

REMEMBER!

Significant cultural resources are often found on alluvial fans. If you are uncertain about the identification of a fan, contact a

soil scientist. Even if you don’t see cultural resources on the surface, they

may well be lurking just below the surface. Always contact the State Office if you plan any ground

disturbance on an alluvial fan and you are not on hydric deposits or PSA. You should use a soil scientist to determine whether or not the project will be confined to hydric deposits or PSA. If the project is not confined to such deposits and you can not avoid the fan or substitute a non ground disturbing practice, then terminate assistance (or in exceptional situations ask for guidance from the State Office).