Event-Specific Flood Extent and Depth Maps for Pennsylvania

After flooding events, the United States Geological Survey (USGS) identifies and surveys high-water marks (HWMs) to inform flood assessments, research, and water resource management. These HWMs are empirical evidence of a flood's maximum water surface elevation in a given location. The documented and surveyed HWMs are subsequently published to the USGS Flood Event Viewer for long-term storage and archive (U.S. Geological Survey, 2025). Using a subset of these HWMs, a water surface elevation raster was interpolated using methods described in Stuckey and others (2023) and event-specific extent and depth maps were produced in a Geographic Information System for flooding events in Pennsylvania. This data release contains flood extent polygons, flood depth rasters, flood boundary extent polygons, HWM points, and supporting data tables organized by year and flooding event. Included in the supporting data tables are approved peak gage height and discharge from the associated flooding event if maps were created in the vicinity of a USGS streamgage. Applicable Federal Emergency Management Agency (FEMA) 1-percent Annual Exceedance Probabilities are also included in the supporting data tables (Weaver and others, 2022). Using the FEMA USA Structures dataset, a geospatial analysis was performed to determine the number of buildings potentially impacted by floodwaters for each map (Federal Emergency Management Agency, 2025).

The flood-documentation maps are intended to be estimates of the areal extent of flooding that corresponds to surveyed HWMs and should not be used for navigation, or for regulatory, permitting, or other legal purposes. These maps only depict a specific flood event and may not be representative of other flooding conditions.

References:

Federal Emergency Management Agency, USA Structures, accessed May 1, 2025, at https://gis-fema.hub.arcgis.com/pages/usa-structures.

U.S. Geological Survey, Flood Event Viewer, accessed May 1, 2025, at https://apps.usgs.gov/fev.

Stuckey, M.H., Conlon, M.D., and Weaver, M.R., 2023, Characterization of peak streamflows and flooding in select areas of Pennsylvania from the remnants of Hurricane Ida, September 1–2, 2021 (ver. 1.1, September 28, 2023): U.S. Geological Survey Scientific Investigations Report 2023–5086, 28 p., https://doi.org/10.3133/sir20235086.

Weaver, M.R., Roland, M.A., Colgin, J.E., and Stuckey, M.H., 2022, USGS-derived 1-percent Annual Exceedance Probability Flood-Flow Estimates at Flood Insurance Study Locations Across Pennsylvania: U.S. Geological Survey data release, https://doi.org/10.5066/P9FG2MF2.