The Foresthill 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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4.75 inches of rain fell in two days, the most for this week of the year since records began in 1905. The next wettest, 4.70 inches, fell in 2017.
Foresthill’s center stands about 3,230 ft above sea level. Within three miles the land runs from 1,000 ft to 3,600 ft, a difference of 2,600 ft: mountain country. The highest named summit within 15 miles is American Hill, about 5,130 ft, 15 miles northeast.
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 6 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.
Andesite. Age: Late Oligocene to Pleistocene, 2 to 24 million years old. About 7 mi of the line.
Argillite, quartzite, phyllite. Age: Ordovician to Triassic, 202 to 487 million years old. About 7 mi of the line.
Schist, gneiss. Age: Paleoproterozoic to Cretaceous, 66 to 2,500 million years old. About 4 mi of the line.
Peridotite, serpentinite. Age: Neoproterozoic to Early Jurassic, 175 to 1,000 million years old. About 3 mi of the line.
Mafic-volcanic. Age: Jurassic, 145 to 202 million years old. About 2 mi of the line.
Slate, graywacke. Age: Triassic to Late Jurassic, 145 to 252 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 Foresthill reaches San Francisco Bay and the Pacific Ocean by way of Dardanelles Creek, the Middle Fork American, the North Fork American and the Sacramento: about 130 river miles.
Lake on the way: Folsom Lake.
55 miles of the trip are on the Sacramento.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Foresthill; river miles along the main stem, rounded.
Foresthill has a hot-summer Mediterranean climate. Summer afternoons average 92° in July; winter nights fall to about 36° in December. A normal year brings about 7 inches of snow; with the rain, that comes to about 46 inches of water.
On November 14 Foresthill normally reaches 62° and falls to 41°. The record high is 82°, in 1906; the record low 20°, in 1916. The most rain on this date, 2.68 in, fell in 1953.
Normals 1991–2020 and records since 1905 at Colfax, 8 miles northwest, 860 ft lower. NOAA, through RCC-ACIS.
