What are Alluvial Soils and why are they Important?

Alluvial Soils

Alluvial soils are soils transported and deposited during flood events. As such, volume and frequency of deposition varies per event, often resulting in 'stratification' of soils, or distinct layers, visible when looking at a 'soil profile' - or horizontal cross section of soil layers, distinguishable by color differences among the different layers. As time passes, various weather events - the drying out of soils by the sun/wind, freezing and thawing cycles, breaks down larger soil particles, creating opportunities for future flooding events to deposit various nutrients from broken down larger soils further downstream. Alluvial soils in floodplains are nutrient rich soils, resulting in fertile areas highly suitable for crop growth. Often in developed lands, soil deposition occurs at a faster rate and can do more harm than good when this soil deposition is a result of massive amounts of erosion or transports high nutrient loads from agricultural lands (fertilizers and nitrogen) further downstream.

Within the Fourmile Creek Watershed, two main soil regions exist: Loamy Wisconsin Glacial Till and Loess Ridges and Sideslopes. The images below are from USDA NRCS document discussing the various soil profiles in Iowa. The top two images describe the main two soil regions within the Fourmile Creek Watershed.

(Figure 1: Description of Soil Regions)

(Figure 2: Map of Iowa Soil Regions)

Alluvial Soils within Fourmile Creek

Iowa DNR has established an online mapping program to determine if alluvial soils and/or alluvial aquifers exist within an area. They have a very specific reason for making this information available online: Proper Animal Feeding Operation (AFO) Siting Atlas. Iowa State Law prevents livestock and poultry producers wishing to construct or modify an existing feeding confinement area, open feedlots, or a manure storage structure within a 100-year flood plain. (a 100 year flood plain simply means that each year, there is a 1% chance - 1/100 of a flood event occurring within an area; as such a 500 year flood plain means there is a 0.2 % chance of a flood event occurring with an area). The law is intended to prevent contamination of a water source. 

I used their Siting Atlas to clip manually clip my own areas of interest - the Fourmile Creek Watershed (Figure 3). As depicted, land within/adjacent to the stream all contains land within alluvial aquifers/alluvial soils. Additionally, Iowa DNR provides siting information with information including distance to major river, distance to surface water, percent slope, and well distance (Figure 5). In order to obtain this information, you have to select an individual point on the map - this is where the useful ness for agricultural operations comes in. They are able to use this Siting Atlas to find a specific point on their land to preliminarily determine if the lands lie within the alluvial soils area. Figure 4 shows a zoomed in version of a subsection of the Fourmile Creek Watershed, highlighting to areas delineated as having alluvial soils and alluvial aquifer. I chose a section near agricultural lands given the intended utility of this Siting Atlas. 

(Figure 3: Map of alluvial aquifers and soils within the Fourmile Creek Watershed)

(Figure 4: Zoomed-in map of a subsection of the Fourmile Creek Watershed)

 

(Figure 5: Point-specific siting information)

Sources:

Iowa DNR: https://www.iowadnr.gov/Environmental-Protection/Land-Quality/Animal-Feeding-Operations/Mapping/Proper-AFO-Siting

Iowa DNR AFO Siting Atlas: https://programs.iowadnr.gov/maps//afo/

Iowa Soil Associations: https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_006362.pdf

USDA NRCS: https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_006038.pdf

https://soilsmatter.wordpress.com/2020/02/15/what-are-alluvial-soils/

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/alluvial-soils