Mapping Land Cover at Willapa Bay

Estuaries are unique locations as they provide a “transition zone” between freshwater rivers that come from inland and the open sea. The resulting brackish ecosystem allows for the growth of unique plants like Japanese eel grass and the cultivation of wildlife like Pacific Oysters. Willapa Bay is a major estuary in the state of Washington that holds 25% of the oysters grown in the United States and supports various tidal ecosystems. This is due to it’s flat topography and muddy benthic layer. Although it may seem that estuaries like Willapa Bay are mainly comprised of marshy mud flats, several more types of land cover are actually present. What actually comprises “land cover” at Willapa Bay? To answer this question, we downloaded spatial data and utilized Arc GIS software to create a map of the various types of land cover here at Willapa Bay.

To create this map, we downloaded spatial data from Data.gov, a national online data source. Upon acquiring this data set, it was uploaded to Arc GIS where we converted it into a map showing the different types of land cover. The map categorized land cover using a legend that listed the following categories: unconsolidated sediments, submersed aquatic vegetation (SAV), tidal marsh, land, and unknown benthic habitat. According to SCHEME (System for Classification of
Habitats in Estuarine and Marine Environments), there are seven categories of estuarine land cover: unconsolidated sediments, submersed aquatic vegetation (SAV), tidal marsh, land, corals and hard bottom, tidal marsh and unknown benthic habitat. It is interesting to note that corals and hard bottom as well as tidal swamp are not present in Willapa Bay.

Figure 1

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The map above illustrates land cover in Willapa Bay. Several data layers on Arc GIS allows the system to compare the entire area of the bay to the amount of land taken up by different habitats. Once we found statistic totals for the land cover at Willapa Bay, we were able to compare it with land cover totals of the area. We used the attribute table tool on Arc GIS to export data to excel and created a graph showing the statistics. See that graph below:

Figure 2

According to the pie chart shown above, it is clear that unconsolidated sediments (mud) make up over half (53%) of Willapa Bay’s land cover. Discontinuous SAV is the second most significant land cover at 31%. Land (8%) and tidal marsh (6%) cover similar areas of land. Unknown benthic sediment and continuous SAV take up marginal plots of land at around 1% each. These results are somewhat surprising as I expected the cover of tidal marsh to be much more significant. Such a result could also stem from an error in the data set or a mishap in how I extracted it. Also, the data provided for this analysis is from 1995. While general land cover in the area probably hasn’t changed too significantly, change is inevitable so this map does not accurately reflect the current make-up of land cover in and around Willapa Bay. Despite these probable sources of error, I’ve come away from this activity feeling more familiarized with Arc GIS and the land cover in the Willapa Bay area.

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