The Effects of Weirs on Vegetation Communities in Agricultural Drainage Ditches of the Mississippi Alluvial Delta

Author(s): Shipes, C.; Kröger, R.

Drainage ditches in the Mississippi Alluvial Delta are an essential part of the agricultural landscape of this region. These ditches vary greatly in size, location, and shape, but can aid in mitigation of contaminants from agricultural fields. Historically controlled drainage in the form of slotted or drop pipes, flash board risers, and vegetated drainage ditches have been used to effectively manage nutrients. More recently low-grade weirs have been used as an alternative to traditional controlled drainage structures. Low- grade weirs act to slow water flow, increase residence time, and allow sedimentation. Until recently the relationship between low-grade weirs and the vegetation communities present has not been evaluated. We selected 13 agricultural drainage ditches with weirs present in the Mississippi Alluvial Valley and investigated the vegetation community responses to weirs and weir age. We also selected 4 control ditches with no weirs present to compare our results. Vegetation assessment yielded 41 plant species ranging from obligate wetland plants to facultative upland plants. Simple correlation tests showed a strong positive relationship between weir age and species diversity, and also a positive correlation between total percent vegetative coverage and weir age. Linear regression also showed a positive relationship between weir age and the presence of facultative upland plants and a negative relationship between weir age and the presence of obligate wetland plants. This study shows some interesting trends, but we were limited in our sample size. Future studies should focus on a larger sample size with a wider range of age classes and should take other variables into account such as; vegetation communities upstream, slope of individual ditches, the spatial arrangement of weirs within the ditch system, and association with other weirs within the same ditch system.

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