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Geomorphometric Indicators of Tidal Marsh Condition

Vinton J. Valentine 1 and Charles S. Hopkinson, Jr. 1

1 The Ecosystems Center, Marine Biological Laboratory (MBL), Woods Hole, Massachusetts

We are developing geomorphometric indicators of tidal marsh condition as part of the Atlantic Coast Environmental Indicators Consortium (ACE INC), funded as an Estuarine and Great Lakes Environmental Indicators (EaGLe) program through a U.S. Environmental Protection Agency (USEPA) Science to Achieve Results (STAR) grant. Our premise is that the configuration of tidal creek networks differs greatly between marshes differing in ability to maintain elevation relative to sea level rise and that geomorphometric indicators can be used to quantify these differences. Our work focuses on the Plum Island Estuary marshes in northeastern Massachusetts, a NSF Long Term Ecological Research (LTER) site. We examined regions that differ in response to sea level rise along gradients of salinity, sediment supply, vegetation community, and human impact. Using a geographical information system (GIS), tidal channel networks, including human-made mosquito ditches, were digitized from spring 2001 color orthophotography with wetlands cover data developed for the MA Department of Environmental Protection Wetlands Conservancy Program serving as a starting point. With the networks as input, we generated "catchments" for all stream order segments. We calculated drainage density and fractal dimension for all "catchments" and analyzed the measures by tidal "creekshed" and by stream order. We propose drainage density and fractal dimension as indicators of marsh condition, stability, and, when incorporating time series data, development state, in the face of rising sea level and other stressors. The indicators can offer landscape, as well as site, information when assigning priorities and developing strategies for wetland conservation and restoration efforts.

Keywords: tidal marshes, geomorphology, channel networks, indicators, drainage density, fractal dimension, Plum Island Estuary, PIE-LTER, ACE INC.

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