The Potomac plate. Relief from Copernicus GLO-30; rivers and lakes from USGS NHD; watersheds from the USGS Watershed Boundary Dataset; roads and city limits from Census TIGER 2024.
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72° on Nov. 13, 1999, the hottest reading for this week of the year since 1964. Next is 65° in 1990.
Potomac’s center stands about 3,630 ft above sea level. Within three miles the land runs from 3,530 ft to 5,330 ft, a difference of 1,800 ft: steep hill country. The highest named summit within 15 miles is Sheep Mountain, about 7,600 ft, 12 miles west-northwest.
Under the town center the state geologic map shows Alluvium (gravel, sand, silt), from the Pleistocene to Holocene, less than 1.8 million years old.
The section below crosses 5 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 5×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Metasiltstone, argillite, quartzite. Age: Mesoproterozoic, 1 to 1.6 billion years old. About 9 mi of the line.
Sedimentary. Age: Tertiary, 2.6 to 66 million years old. About 5 mi of the line.
Quartzite, argillite, metasiltstone. Age: Mesoproterozoic, 1 to 1.6 billion years old. About 5 mi of the line.
Gravel, sand, silt. Age: Pleistocene to Holocene, less than 1.8 million years old. About 3 mi of the line.
Limestone, shale, sandstone. Age: Middle Cambrian to Cambrian, 487 to 521 million years old. About 2 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale.
Heights from Copernicus GLO-90 (a surface model: it includes buildings and tree canopy); summits from USGS GNIS; rivers from USGS NHDPlus HR; rock from the USGS State Geologic Map Compilation.
A raindrop that lands in Potomac reaches the Pacific Ocean by way of Union Creek, the Blackfoot, the Clark Fork, the Pend Oreille and the Columbia: about 1,100 river miles.
Lakes on the way: Lake Pend Oreille, Lake Umatilla and Noxon Rapids Reservoir.
460 miles of the trip are on the Columbia.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Potomac; river miles along the main stem, rounded.
Potomac has a warm-summer humid continental climate. Summer afternoons average 82° in July; winter nights fall to about 8° in January. A normal year brings about 52 inches of snow; with the rain, that comes to about 15 inches of water.
On November 14 Potomac normally reaches 40° and falls to 17°. The record high is 65°, in 1999; the record low −6°, in 1978. The most rain on this date, 0.49 in, fell in 1986.
Normals 1991–2020 and records since 1964 at Potomac, here. NOAA, through RCC-ACIS.
