The flux of fine sediment within agricultural watersheds is an important factor determining the environmental quality of streams and rivers. Human activity has significantly altered the hydrological and biogeochemical cycles within terrestrial and aquatic environments through agricultural intensification, tile drainage installation, and urban development. The study of watershed-scale sediment dynamics is of great value for understanding and predicting the response of sediment dynamics to intensive human impact and is crucial to developing management strategies for reducing the vulnerability of the ecosystem to future changes. The primary objective of this dissertation is to investigate sediment sources, sediment transport, and sediment yield in an intensively managed agricultural landscape. This objective was accomplished by combining of field sampling and measurements, laboratory analysis, sediment fingerprinting study, statistical analysis and modeling exploration in the Upper Sangamon River Basin, Illinois.
The relative contributions from cropland, grassland, forested floodplain, upper grazed floodplain, and lower grazed floodplain to the suspended sediment in the stream are evaluated by sediment fingerprinting techniques. The grazed areas of the floodplain are identified as the primary source of fine suspended sediment within the headwaters of the Sangamon River. Erosion of the floodplain both by surface runoff and by streambank erosion contribute to the production of almost all fine sediment sampled within the stream system. The results are consistent both for event and aggregated samples and for large and small events. The fingerprinting results are also consistent with visible and historical evidence of active erosion of grazed areas of floodplain upstream from the in-stream sampling location. Evidence from field reconnaissance and inspection of aerial photography supports the conclusion that cattle grazing plays an important role in accelerating floodplain and streambank erosion.
The relationships between rainfall, discharge, and suspended sediment concentration are examined by sediment rating curve approach and hysteresis analysis. Sediment rating curves developed for three sites along the Sangamon River all have a peaked pattern with a transition point at geometric mean of discharge, indicating suspended sediment load in the stream is far below the stream transport capacity during high flows. Spatially, suspended sediment concentrations tend to become more coincident with the seasonality of rainfall and discharge with increasing watershed size and the mean suspended sediment concentration decreases as drainage area increases. Temporally, the SRCs developed for the rising and falling limbs of hydrographs and the four sampling seasons also exhibit the same trends, suggesting that these trends are not scale-dependent. The peaked pattern of sediment rating curve is most apparent in sediment rating curve developed on discharge and sediment data collected in summer, which means the limitation of sediment supply is most significant in summer.
Sediment fluxes in modern times and before European settlement is investigated by using a semi-distributed, coupled hydrologic and sediment model. Intensive agricultural activities since European settlement have increased sediment supply and enhanced suspended sediment load in stream, and also influence re-distribution of detached sediment within the system. The percent of sediment supply from each source to the total amount of mobilized sediment significantly changed from 1840s to 2000s, and the agricultural uplands have become the major source of suspended sediment in the stream. The model estimates that sediment supply from uplands increased 11-fold from the 1840s to 2000s, and sediment yield in 2000s is 9 times of that in 1840s. A higher percent of sediment is transported out of the system and deposited in the channel in 2000s than in 1840s. Suspended sediment load has increased more rapidly than floodplain sedimentation. The re-distribution of detached sediment is also influenced by the presence of built levees and extended channel network. With the increased sediment supply and decreased percent of floodplain sedimentation, sediment delivery ratio for the entire watershed only increased 4%.
In conclusion, the integrated results from field, statistical and modeling studies advance the knowledge and understanding of sediment supply, delivery, and export in intensively managed landscapes. The findings also inform management strategies aimed at reducing the vulnerability of this landscape to ongoing human impact
