The Big Arm 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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71° on Nov. 12, 1999, the hottest reading for this week of the year since 1951. Next is 66° in 1990.
Big Arm’s center stands about 2,950 ft above sea level. Within three miles the land runs from 2,890 ft to 4,870 ft, a difference of 1,980 ft: steep hill country. The highest named summit within 15 miles is Kerr Mountain, about 6,110 ft, 13 miles north.
Under the town center the state geologic map shows Ravalli Group: St. Regis, Revett, and Burke Formations; or Empire and Grinnell Formations; or Empire and Spokane Formations (quartzite, argillite, metasiltstone), from the Mesoproterozoic, 1 to 1.6 billion years old.
The section below crosses 2 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 8×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Quartzite, argillite, metasiltstone. Age: Mesoproterozoic, 1 to 1.6 billion years old. About 14 mi of the line.
Sedimentary. Age: Tertiary, 2.6 to 66 million years old. About 1 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 Big Arm reaches the Pacific Ocean by way of Flathead Lake, the Flathead, the Clark Fork, the Pend Oreille and the Columbia: about 1,060 river miles.
Lakes on the way: Flathead Lake, 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 Big Arm; river miles along the main stem, rounded.
Big Arm has a warm-summer humid continental climate. Summer afternoons average 83° in July; winter nights fall to about 20° in January. A normal year brings about 34 inches of snow; with the rain, that comes to about 16 inches of water.
On November 14 Big Arm normally reaches 40° and falls to 26°. The record high is 62°, in 1990; the record low −4°, in 1959. The most rain on this date, 0.38 in, fell in 2021.
Normals 1991–2020 and records since 1951 at Skq Dam, 9 miles south-southeast, 220 ft lower. NOAA, through RCC-ACIS.
