Recommendations

Time limitations prevented digitization of all likely former tidal wetlands. For example, many narrow bands of floodplain were classified as upland in the NWI, but had elevations within our defined 9–11 or 11–13 ft elevation zones. These floodplains also are likely tidal wetlands, former tidal wetlands, and/or tidal waters potentially subject to State and Federal regulations, but these areas were not added to the NWI shapefile. Despite their narrow width, tidal floodplains provide important ecosystem services for aquatic organisms using the main stem rivers. We recommend using LiDAR to map tidal floodplains so they can be included in planning for coastal resource conservation and restoration.

This analysis relied heavily on interpretation of high-resolution aerial photographs acquired in 2005 by Oregon Department of Land Conservation and Development (DLCD) and EPA (Oregon Department of Land Conservation and Development and others, 2007a, 2007b). Field observations in 2010 indicated that changes have occurred at some prioritization sites since production of these 2005 photographs. NAIP (National Agricultural Imagery Program) orthophotographs were flown in 2009, but were not yet available as of the writing of this report. Regular review of field conditions is recommended to update the information provided in this report. Field visits by knowledgeable wetland scientists (after obtaining landowner permission for access) would be particularly useful.

Since completion of the data layers in 2010, GIS inventories of dikes and tide gates in Oregon estuaries have become available (Mattison, 2011a, 2011b). The information in these GIS inventories generally is consistent with this project’s products; additional detail is provided for some dikes and tide gates. Review of the dike and tide gate inventories is recommended when updating this project’s shapefiles, and when planning site-specific actions.

LiDAR data constitute a powerful new tool for studies like this one, with potential utility beyond the present report. For example, the LiDAR data could be used to digitize new boundaries for NWI polygons, provided on-site validation of wetland conditions is conducted. Additional effort is recommended to fully utilize the LiDAR data to assist strategic planning for tidal wetland conservation and restoration.

In some parts of the estuary, particularly the freshwater tidal zone, it is likely that the LiDAR signal did not successfully penetrate dense herbaceous vegetation, resulting in "bare earth" elevations that are higher than the actual land surface. Vegetation interference with the LiDAR signal has been documented (Gopfert and Heipke, 2006). In Oregon, LiDAR-derived elevations are sometimes 1–2 ft higher than actual ground surfaces in areas of dense slough sedge and reed canary grass (Brophy, 2012; Brophy and van de Wetering 2012). Because of this potential inaccuracy, ground-truthing of the LiDAR bare earth model is recommended, particularly for site-specific planning and restoration design.

Intended Uses and Limitations of Mapping

These data are intended to be used for planning purposes only and are non-regulatory in nature. The recommended revisions should be included in the NWI to improve its accuracy, but our methods and suggested wetland classification revisions should be reviewed by NWI staff to ensure they meet NWI standards. As is always the case with NWI mapping, users should be aware that there may be upland areas within mapped wetlands, and there may be unmapped wetlands and tidal waters that are subject to State and/or Federal regulation per State Removal-Fill Law, Federal Clean Water Act or Federal Rivers and Harbors Act. Furthermore, because the NWI uses the Cowardin et al. definition of a wetland, which differs from the definition of a regulatory wetland subject to State and Federal regulations, not all NWI wetlands are necessarily subject to regulation.

First posted May 10, 2013