Flood Prone region

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The portion of the greater watershed that I have focused on in this study is bounded on the east and west by levees.  Although Federal Emergency Management Agency flood zone maps are only partially available digitally, the area is entirely within the 100 year floodplain (1% chance of flooding in any given year). This is more than appropriate, as the majority of the area floods seasonally every late winter through early spring.  The current water management plan between the USACE and the AGFC has a goal of keeping the maximum water levels below 214' msl, as at this level, nearly 100% of the area is inundated, which displaces upland animals (especially whitetail deer and wild turkey) into the adjacent cropland.  These flood events particularly impact wild turkey, as they often occur during the nesting season in the spring.

Although the hydrology of the surrounding region has been seriously altered by ditching, channelizing, and levees, with the goal of getting the water quickly into the St. Francis River (and holding it there), floods have always been a part of this watershed.  Observing the soils in the watershed can paint a picture of the environmental conditions throughout time that lead to their development.  One attribute of particular importance to wetland habitats is hydric soils.  Hydric soils are defined by the National Resource Conservation Service as soils that formed "under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part."  In the field, these soils display observable traits, such as color changes, oxidized root channels, and iron-manganese concretions. The US Department of Argiculture produces maps of soil series, along with descriptions of their attributes (see map above).  Within the focused study area of the Sunken Lands, the vast majority of the soils are hydric (Table 1).  Other than a few small areas of high ground, more than 95% of the area in the Sunken Lands is on hydric soils, or is open water.

Map unit name	Hydric Rating	Acres in AOI	Percent of AOI
Sharkey soils, 0 to 1 percent slopes, frequently flooded	97	23,065.40	55.80%
Mhoon soils, frequently flooded	100	4,701.70	11.40%
Hayti soils	100	3,366.80	8.20%
Commerce soils, 0 to 1 percent slopes, frequently flooded	100	2,254.60	5.50%
Kobel silty clay loam, 0 to 1 percent slopes, frequently flooded	85	1,546.70	3.70%
Water	0	2,697.20	6.50%
Dundee fine sandy loam	10	519.10	1.30%
Dundee silt loam, 0 to 2 percent slopes	10	456.60	1.10%
Amagon fine sandy loam	95	419.80	1.00%
Misc Minor components		2273.47	5.50%
	Total Acres	41301.37

Table 1, Soil Series within the Sunken Lands

Invasive animals in the watershed

Watershed Organization

Common trees in the watershed

Water sampling ideas for the Sunken Lands

Additional Threats and Challenges

As the Sunken Lands regions has been channelized, and leveed, the surrounding land use has been converted to nearly exclusively row crop agriculture.  Very little consideration has been given to protection of topsoil in the region, as the rich alluvial soils are very deep, the result of thousands of years of alluvial deposits.  This culminates in most fields being left as exposed soil post-harvest, and susceptible to erosion through the winter and early spring, which sees the highest rainfall amounts of the year.  Sediment that is carried through artificially straightened ditches and canals is allowed to flow at a high rate, carrying this load until it gets to the wider, slower, braided areas, and the St. Francis Lake area of the Sunken Lands.

Sedimentation is a concern for both the natural and recreational uses of the Sunken Lands.  Suspended sediment increases turbidity, and limits the penetration of sunlight into the water column.  It also comes with increased nutrients from excess fertilizer, further jeopardizing water quality.  Benthic macroinvertebrates (insect larva and crustaceans that live on the bottom) are often used as a surrogate measure for water quality over long periods, as they are susceptible to extreme events that might not be observed in a one time sample of a particular area.  A study of the benthic communities of the Sunken Lands in 1990 found that the areas with the richest species diversity were those old river channels and oxbow lakes that had the least amount of sedimentation and disturbance from artificial flow regimes.  The channelized sections, and the St. Francis Lake, both had reduced diversity and quantity of sampled species.  (The Aquatic Macroinvertebrates of the St. Francis Sunken Lands in Northeast Arkansas, https://scholarworks.uark.edu/cgi/viewcontent.cgi?article=2234&context=jaas).  This study confirmed that the Sunken Lands is a unique remnant of habitat that serves as a refugium for species that were likely common across the Mississippi Alluvial Valley, but have been extirpated due to habitat loss.  Several species of benthic macroinvertibrates were found that had not been reported in Arkansas, although they would be expected in area.

Sediment deposited in the channels and the lake area creates hazards to boating, and limits fishing and other enjoyment of the St, Francis Lake.  Dr, Neal Vickers of the St. Francis Lake Association recounted that as children the depth in the lake even at water elevations of 208-210' msl were sufficient for swimming and safe boating year round, but the sedimentation has been slowly filling in the lake, and increasing the need for responsibly managed water levels by the AGFC and and USACE.

Summertime kayaking on the proposed paddling trail
St. Francis Lake Association
https://www.facebook.com/StFLake/photos/a.1031775026897651/2581717808570024/?type=3&theater

The popularity of waterfowl hunting on flooded fields within the region offers a potential solution to limit erosion on fallow agriculture fields.  Most fields in the area have infrastructure to hold water for rice production, which allows a shallow flood (6-12 inches) to be held on the fields.  This creates habitat for waterfowl and other migrating birds, replicating some of the natural wetlands that have been lost.  But most importantly, erosion is significantly decreased, as sediment is captured within the fields, and not allowed to run into ditches and make its way into the Sunken Lands.  It also mimics the buffering capacity of wetlands to peak flow rates, by extending the time it takes runoff to work through the watershed, smoothing out peaks in water flow after large storm events.