Master of Science (M.Sc.)
Virginia Institute of Marine Science
Salt marshes along many tidal systems in New England have been ditched since colonial times. These ditches have been thought to help control mosquito populations and increase salt marsh hay production by improving water drainage from the marsh. Although these ditches are prominent geomorphic features, little quantitative work has focused on how these man-made ditches may alter marsh hydrology and geomorphology. This study attempts to quantify the ways in which ditches alter sediment and water transport pathways and how that affects the overall morphology and surface geology. This study also addresses treatment affects on sedimentation from fertilization and fish removal associated with the NSF funded TIDE project.
Short-term sediment deposition rates and relative elevations were determined for ditched and non-ditched marsh areas in four tidal creeks along the Rowley River in the summers of 2003, 2004, 2005, and 2006. Total suspended solids samples were also collected in the creek channels adjacent to the marsh areas sampled. Marsh surface samples for grain size analysis and organic content were collected as well. To quantify flow patterns, a grid system of stakes was set up on the marsh platform of two creek systems and water height was measured relative to these stakes over the course of a tidal cycle. This allowed for a better determination of the areas that flooded first and how long water stayed on the marsh platform in ditched and non-ditched areas.
Measurements of marsh platform elevation indicated that the interior regions of ditched areas stand significantly lower than non-ditched areas. In ditched marsh areas, the hydrologic data demonstrated that the interior regions of the marsh were typically flooded first and stayed flooded longer. Non-ditched regions instead were flooded only after the water had topped the creek bank. As a result of these modified flow pathways, classic patterns of sedimentation and of organic matter and grain size distribution occurred less often in ditched marsh areas relative to non-ditched areas. In contrast to non-ditched marsh platforms, ditched platforms commonly had areas of increased deposition, decreased organic matter and increased grain size in innermost marsh areas. Although the absolute magnitude of deposition was similar with or without ditches, deposition in ditched areas was somewhat less responsive to the classic control of hydroperiod.
Possible explanations for lower platform elevation in ditched marshes include (i) trapping of suspended sediment in ditches and (ii) enhancement of sediment export associated with the short distance from platform to ditch. The former is consistent with observations of lower concentration in ditches relative to natural creeks, and the latter is consistent with observations of coarser grain size and lower organic content observed on ditched marshes relative to non-ditched marshes. A longer hydroperiod and lower elevation in ditched areas would then be required at equilibrium to enhance net deposition so that accretion could keep up with relative sea level rise. The similar observed magnitudes of deposition in ditched and non-ditched areas supports the conclusion that the lower elevation of ditched platforms is in quasi-equilibrium with the higher elevations of non-ditched areas. Other results of this study include a demonstration of the high (1-2 cm) accuracy of the tide stick method for measuring marsh elevation, the finding that short term source of marsh sediment is primarily internal cycling, and the finding that fertilizing creeks systems and removing fish have no significant effects on short term sediment concentration, sediment delivery, sedimentation, inorganic sediment properties or marsh elevation.
© The Author
LeMay, Lynsey E., "The Impact of Drainage Ditches on Salt Marsh Flow Patterns, Sedimentation and Morphology: Rowley River, Massachusetts" (2007). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539617854.