Preliminary screening only. Verify every value against its authoritative source before acting on it. Not for design, permits, insurance, or regulatory filings. The two drainage areas shown are different on purpose: the SnapBasin value (top) is NHDPlus HR’s per-reach published total — constant within a reach segment; the StreamStats value (below) is an independent modern flow-accumulation delineation at the click point. Where they disagree, the modern DEM does not confirm the legacy NHD basin definition.
Click any point on the map. First SnapBasin tries to snap to a nearby NHD flowline (green). If NHD doesn't have it, SnapBasin pulls a 10m 3DEP elevation tile and walks steepest-descent downhill through the terrain (purple dashed) until it finds a drainage — the same physics StreamStats uses server-side. Then it delineates from there.
Zoom in to tighten the NHD snap.FEMA + gauges always run from your original click.
Toggle FEMA Flood Map (FIRM) in the layers panel to see cross-section letters and BFE elevations rendered directly on the map (visible at z15+).
Toggle USA Structures to overlay building footprints (occupancy / height / address) from the FEMA + Oak Ridge dataset (visible at z14+). Click any building for details.