Actually, never mind. It’s just riverbeds, which are flat and low, but not under 50 ft.
Use the National Map viewer ( https://viewer.nationalmap.gov/advanced-viewer/ ) and add layers “Elevation Contours” (in feet) & “Elevation Tinted Hillshade”.
It’s really kind of amazing how much of the country is under fifty feet off elevation, even far into the interior. Eastern Arkansas, western Tennessee, southwestern Illinois, half the old South.
«Flood warnings were issued from eastern Oklahoma into southeastern Kansas, southern Missouri, central Illinois and parts of Arkansas, Kentucky, Tennessee, and the Florida panhandle.»