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Thursday
Mar042010

An innovative approach for monitoring tidal wetland restoration success

Heath Bohman with USGS collects field data with the echosounder system in a wetland on the northern edge of the San Francisco Bay. Image credit, USGS.Researchers from the US Geological Survey at the San Francisco Bay Estuary Station have created a new, cost-effect, and more accurate method for measuring early sediment accretion in restored, tidal marshes.

Their innovative method involves using sound waves to determine water depth. The echosounder system is mounted onto a shallow draft kayak and includes an acoustic profiler, GPS unit, and laptop computer.

Salt marsh restoration often involves returning tidal flow to wetlands that had been previously diked and drained for agriculture, development, or other uses. Sediment accretion is an important metric that reveals the early stages of success in tidal wetland restoration - particularly at sites that have seen substantial subsidence below the march plain elevation.

Conservation practitioners strive to gain enough sediment at their tidal marsh restoration sites to support native vegetation that will only grow at certain elevations. John Callaway et al. (2001) identified sediment accretion as an essential element of tidal marsh restoration monitoring. If the site is not collecting enough sediment, changes in the restoration plan such as breaching more dykes or creating more channels may be in order to help drive the sediment source.

Methods for measuring sediment accumulation typically include: sediment pins, marker horizons, sediment erosion tables (SET), and LiDAR systems. But these approaches all face limitations. Sediment pins and marker horizons are problematic in the sense that they provide limited sample points and thus inherently lose spatial variation within the site. SETs require fixed locations presenting the same problem, while LiDAR systems tend to be quite expensive.

John Takekawa tested the echosounder system in a new study in the journal Wetlands Ecology and Management.  Their method provided accurate and repeatable measurement over a 20-ha restored marsh in San Pablo Bay, CA. Their system cost $12,000 to construct, but since the initial study, the team has mapped over 35 sites. They state,

“Unlike methods such as sediment pins or marker horizons that provide a limited set of sample points, grid maps created from the echosounder system provided a more detailed, comprehensive, and accurate view of the entire project.”

--Reviewed by Evyan Borgnis

Takekawa, J., Woo, I., Athearn, N., Demers, S., Gardiner, R., Perry, W., Ganju, N., Shellenbarger, G., & Schoellhamer, D. (2010). Measuring sediment accretion in early tidal marsh restoration Wetlands Ecology and Management DOI: 10.1007/s11273-009-9170-6

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