The Arnold 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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8.26 inches of rain fell in two days, the most for this week of the year since records began in 1929. The next wettest, 7.35 inches, fell in 1950.
Arnold’s center stands about 4,320 ft above sea level. Within three miles the land runs from 3,170 ft to 4,860 ft, a difference of 1,690 ft: steep hill country. The highest named summit within 15 miles is McKee Hill, about 6,360 ft, 13 miles east.
Under the town center the state geologic map shows Tertiary pyroclastic and volcanic mudflow deposits, unit 9 (Cascade Range) (andesite), from the Late Oligocene to Pleistocene, 2 to 24 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 4×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Argillite, quartzite, phyllite. Age: Ordovician to Triassic, 202 to 487 million years old. About 10 mi of the line.
Granodiorite. Age: Permian to Tertiary, 2.6 to 260 million years old. About 7 mi of the line.
Diorite, gabbro. Age: Triassic to Cretaceous, 66 to 252 million years old. About 4 mi of the line.
Andesite. Age: Late Oligocene to Pleistocene, 2 to 24 million years old. About 3 mi of the line.
Limestone, marble. Age: Paleozoic to Mesozoic, 66 to 542 million years old. About 0.4 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 Arnold reaches San Francisco Bay and the Pacific Ocean by way of Moran Creek, Mill Creek, the Stanislaus, the San Joaquin and New York Slough: about 170 river miles.
95 miles of the trip are on the Stanislaus.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Arnold; river miles along the main stem, rounded.
Arnold has a warm-summer Mediterranean climate. Summer afternoons average 81° in July; winter nights fall to about 30° in January. A normal year brings about 105 inches of snow; with the rain, that comes to about 56 inches of water.
On November 14 Arnold normally reaches 52° and falls to 35°. The record high is 72°, in 1932; the record low 15°, in 1964. The most rain on this date, 5.79 in, fell in 1981.
Normals 1991–2020 and records since 1929 at Calaveras Big Trees, 3 miles northeast, 380 ft higher. NOAA, through RCC-ACIS.
