The Hidden Wetlands of Providence

My project examined a 1894 US Geological Survey (USGS) topographic map of Greater Providence, Rhode Island to determine where wetlands have been drained/filled and built upon since that time.

My objectives were:

• To identify Greater Providence area wetlands that have been drained or filled since 1894;

• To identify within the most urbanized section of this map the structures and sites that have been established on these drained or filled wetlands;

• To identify which of these structures and sites are not located within FEMA Special Flood Hazard Areas (and therefore may be less prepared for extreme weather-related flooding should it occur).

• 1894 USGS topographical map of the Greater Providence, RI area (Source: USGS Historical Topographic Map Collection)
• 2021 National Wetlands Inventory watershed maps for Narragansett Bay and the Blackstone River (Source: US Fish and Wildlife Service)
• 2021 E-911 map of Rhode Island building structures and sites (Source: RIGIS)
• 2016 Rhode Island Special Flood Hazard Areas acquired from county Digital Flood Insurance Rate Map databases (Source: RIGIS)
• 2021 World Topographic Map basemap (Source: ESRI)

Study Area 1, defined on this present-day map by the solid black line, covers the entire area of the 1894 USGS map of the Greater Providence area (seen below).

Study Area 2, defined by the smaller rectangle within the dashed line, is a more urbanized subset of Study Area 1, comprising Providence and neighboring cities.

Utilizing ArcGIS Pro, I created this geoprocessing workflow to produce the maps that are to follow:

1. Source online data and convert to Rhode Island State Plane projection.
2. Clip data to define Study Area 1 as the area within the entire 1894 topo map.
3. Create unique shapefile of 1894 wetlands by tracing polygons over 1894 topo map (Approx. 200 polygons created, conservatively traced when wetland boundaries were ambiguous; lakes, ponds, rivers, marshes, harbor included but not streams; hilltop reservoirs not included).
4. Erase National Wetlands Inventory wetlands from 1894 wetlands to identify drained/filled wetlands within Study Area 1.
5. Clip data to define Study Area 2 as Providence and neighboring cities.
6. Erase "E-911" structures and sites from areas that are not drained/filled wetlands to identify structures and sites within drained/filled wetlands in Study Area 2.
7. Erase Special Flood Hazard Areas (SFHAs) from Study Area 2 drained/filled wetland structures and sites to identify the structures and sites that are not within SFHAs.

A case could be made for working from an earlier map of the Providence area for the purposes of this project. As this 1858 painting of Providence Harbor illustrates, the city was already highly industrialized by the mid-19th century, with portions of its natural wetlands already significantly altered.

The 1894 USGS map was the earliest downloadable geoTIFF area map that I could easily access that had a coordinate system convertible to the Rhode Island State Plane. While it bears a topographical accuracy and attention to detail likely surpassing Providence area maps made prior to the 1884 inception of the USGS mapping program, the map is not error-free: I observed certain sub-sections diverging by more than 350 feet from the 2021 ESRI basemap.

As such, the choice of the 1894 map was a compromise between topographical accuracy and pre-existing wetland draining or filling.

The drained or filled wetlands indicated here in bright blue within Study Area 1 include area lakes, ponds, rivers, marshes, and sections of Providence Harbor. Streams and hilltop reservoirs were not included this assessment.

Note the large drained/filled wetland at Pawtuxet Village in Cranston, the landfill-expanded Field's Point on Providence Harbor, and the significant wetland shrinkage along the Woonasquatucket, Blackstone, and Providence Rivers. Many of these areas have experienced flooding or threats of flooding in recent years due to extreme weather events.

"Water will go where water has always gone."--Eric Sanderson

The footprints of building structures and sites within the purple areas shown within Study Area 2 indicate those structures and sites that have been established on these drained or filled wetlands.

Note the extensive structures at the Port of Providence along Allen's Avenue, which in 1894 was a shoreline road. Major flooding of this petroleum and gas distribution port could prove environmentally catastrophic to the neighborhood of South Providence, a community of color residing near this facility.

Of additional concern are the subset of structures and sites within the yellow areas shown within Study Area 2, that are established on these drained or filled wetlands but not located within Special Flood Hazard Areas (SFHAs), areas which predict a 1 percent annual flooding event. FEMA, which publishes some of these data, has been criticized for underestimating the increased risks of extreme weather-related flooding due to climate change.

Note the numerous homes in the Mt. Pleasant neighborhood between Providence College and Rhode Island College, and in the Auburn neighborhood of Cranston near and along the once larger Fenner's Pond (located southwest of Roger Williams Park). These homes are not federally required to own flood insurance, but they are sited on former wetlands nonetheless.