The Irwin 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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67° on Nov. 14, 2025, the hottest reading for this week of the year since 1960. Next is 66° in 1999.
Irwin’s center stands about 5,320 ft above sea level. Within three miles the land runs from 5,280 ft to 6,910 ft, a difference of 1,630 ft: steep hill country. The highest named summit within 15 miles is Mount Baird, about 9,950 ft, 11 miles east-southeast.
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 11 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.
Sandstone, shale. Age: Cretaceous, 66 to 145 million years old. About 4 mi of the line.
Age: Carboniferous to Permian, 252 to 324 million years old. About 4 mi of the line.
Rhyolite. Age: Late Miocene to Late Pliocene, 3 to 6.6 million years old. About 3 mi of the line.
Limestone, dolostone. Age: Cambrian to Ordovician, 445 to 542 million years old. About 3 mi of the line.
Gravel, sand, silt. Age: Quaternary, less than 2.6 million years old. About 3 mi of the line.
Mixed-clastic, volcanic. Age: Early Miocene to Pleistocene, 2 to 16 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. 5 smaller units along the line are not listed.
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 Irwin reaches the Pacific Ocean by way of Rainey Creek, the Snake and the Columbia: about 1,210 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 Irwin; river miles along the main stem, rounded.
Irwin has a warm-summer humid continental climate. Summer afternoons average 85° in July; winter nights fall to about 11° in January. About 20 inches of rain fall in a normal year.
On November 14 Irwin normally reaches 44° and falls to 21°. The record high is 67°, in 2025; the record low 2°, in 1978. The most rain on this date, 0.68 in, fell in 2010.
Normals 1991–2020 and records since 1960 at Swan Valley, 2 miles north-northeast. NOAA, through RCC-ACIS.
