The Firth 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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70° on Nov. 15, 1999, the hottest reading for this week of the year since 1895. Next is 69° in 1954.
Firth’s center stands about 4,570 ft above sea level. Within three miles the land runs from 4,540 ft to 4,630 ft, a difference of 90 ft: gently rolling country. The highest named summit within 15 miles is Taylor Mountain, about 7,370 ft, 13 miles east.
Under the town center the state geologic map shows Alluvial deposits (gravel, sand, silt), from the Quaternary, less than 2.6 million years old.
The section below crosses 6 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 10×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Mudstone, sandstone, limestone. Age: Triassic, 202 to 252 million years old. About 8 mi of the line.
Basalt. Age: Late Pliocene to Pleistocene, less than 3 million years old. About 7 mi of the line.
Basalt. Age: Pleistocene to Quaternary, less than 15,000 years old. About 5 mi of the line.
Gravel, sand, silt. Age: Quaternary, less than 2.6 million years old. About 3 mi of the line.
Sand. Age: Quaternary, less than 2.6 million years old. About 2 mi of the line.
Gravel. Age: Quaternary, less than 2.6 million years old. About 0.3 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 Firth reaches the Pacific Ocean by way of the Snake and the Columbia: about 1,090 river miles.
Lakes on the way: American Falls Reservoir, Lake Umatilla, Brownlee Reservoir and Cove Arm Lake.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Firth; river miles along the main stem, rounded.
Firth has a cold semi-arid climate. Summer afternoons average 87° in July; winter nights fall to about 17° in January. A normal year brings about 15 inches of snow; with the rain, that comes to about 12 inches of water.
On November 14 Firth normally reaches 46° and falls to 26°. The record high is 67°, in 2025; the record low 3°, in 1916. The most rain on this date, 0.41 in, fell in 2014.
Normals 1991–2020 and records since 1895 at Blackfoot Fire Department, 11 miles southwest. NOAA, through RCC-ACIS.
