Floodplain Soils along Moreau River in South Dakota

Study of soils and flooding along Moreau River in South Dakota in USA.

This study was an investigation of flooding and sedimentation along the Moreau River upstream of the Oahe Reservoir which is located on the Missouri River. The study area covers approximately thirty-five square miles of floodplain lands. The digital photographs were taken of representative soil profiles or vertical slices into the soil. Each of the photographs has a depth of five feet and a width of two and one-half feet. The photographs are grouped according to the various terraces or land surfaces in the floodplain. The upper terraces are generally located further from the river and the lower terraces are located near the river. Floodplain sediments have been deposited by historic flood events. Soil profiles with numerous sediment layers are younger in age, since the rate of deposition exceeds the rate of soil formation. Soil profiles with few sediment layers are older in age, in that the soil profiles have developed with very infrequent sediments being deposited on top of the soil profile.

Sediment layers were correlated to dates of floods using USACE cross sectional surveys for areas of aggradation along the river and radiocarbon dating for sediment deposits away from the stream channel. The weighted average sedimentation rates for three study parcels ranged from 0.0016 to 0.0039 feet/year.

Soil Profiles on Upper Terraces along Moreau River

Profile Depth = 5 feet

The soil profiles on the upper terraces lack any significant subsurface horizons expressing sediment deposition due to the age and weathering of the soils.

Soil Profiles on Middle Terraces along Moreau River

Profile Depth = 5 feet

The soil profiles on the middle terraces show expression of sediment deposition in the subsurface horizons and are considered to be younger in age than the soil profiles on the upper terraces. The sediment deposition consists primarily of couplet layers of a silt over silt loam layer from each flood event.

Soil Profiles on Lower Terraces and Levees along Moreau River

Profile Depth = 5 feet

The soil profiles on the lower terraces show significant expression of sediment deposits in the subsurface horizons and are considered to be younger in age than the soil profiles on the middle terraces. The lower terraces and levees are most frequently flooded. The sediment deposition consists primarily of couplet layers of a silt over sandy loam layer from each flood event.

Soil Profiles in Backwater Depressions along Moreau River

Profile Depth = 5 feet

The soil profiles in the backwater depressions and along sloughs show darker colored subsurface horizons generally contemporaneous in age with the lower terrace soils. The sediment deposition consists primarily of layers of silty clay loam over silt layers.

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