Designing large wood placement in tidal marshes

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Project Description

Large wood influences river geomorphology and ecology, so similar function is presumed in tidal channels. Consequently, restoration ecologists and engineers often recommend large wood supplementation for tidal marsh restoration, but there is no guidance on how much large wood is appropriate or where it should be located. GIS analysis of high resolution aerial photos was used to map the distribution of individual tree logs > 2 m length in reference tidal marshes of eight Puget Sound river deltas. Statistical analysis showed that distributary networks, marsh size, fetch, topography, and woody vegetation affect large wood distribution on the marsh surface and in tidal channels. River delta marshes exposed to high fetch had high densities of large wood on marsh surfaces and in blind tidal channels; deltas with low fetch had comparatively low densities of large wood. Large wood on marsh surfaces was most abundant along the banks of large river distributaries and on the banks of large blind tidal channels, suggesting these channels are an important route for wood delivery to the marshes. Large wood also tended to accumulate on marsh surfaces at elevations between mean high water and mean higher high water, as well as against topographic discontinuities, e.g., low, 0.5-m scarps distinguishing newly prograded marsh from older marsh. Channel and log size also affected large wood distribution within channels. Large wood tends to accumulate in channels that are 1-3 m wide; larger channels are generally areas of wood transport rather than wood accumulation. Large wood densities were 28 to 50 times lower in Puget Sound tidal channels than in Western Washington streams, but this may be partially explained by the smaller minimum diameters of stream wood (10 cm) classified as “large” wood, compared to the estuarine wood in this study (30-60 cm). These results provide an initial foundation for further studies on the ecological and geomorphological significance of tidal marsh large wood, and some initial guidance to engineers and planners for large wood placement in marsh restoration projects.

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