The Portola Valley 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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3.29 inches of rain fell in two days, the most for this week of the year since records began in 1906. The next wettest, 3.22 inches, fell in 1981.
Portola Valley’s center stands about 510 ft above sea level. Within three miles the land runs from 220 ft to 2,140 ft, a difference of 1,920 ft: steep hill country. The highest named summit within 15 miles is Mount Bielawski, about 3,280 ft, 14 miles southeast.
The section below crosses 8 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 9×. 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. Age: Paleocene to Oligocene, 23 to 66 million years old. About 8 mi of the line.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 4 mi of the line.
Sandstone, mudstone. Age: Miocene to Pleistocene, less than 23 million years old. About 3 mi of the line.
Coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 3 mi of the line.
Sandstone, mudstone. Age: Oligocene to Pliocene, 2.6 to 34 million years old. About 2 mi of the line.
Sandstone, mudstone. Age: Jurassic to Cretaceous, 66 to 202 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. 2 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 Portola Valley reaches San Francisco Bay and the Pacific Ocean by way of Corte Madera Creek and San Francisquito Creek: about 17 river miles.
Often dry. About 90% of the way is mapped as streams that run only after rain or snowmelt; the water makes the whole trip in floods.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Portola Valley; river miles along the main stem, rounded.
Portola Valley has a warm-summer Mediterranean climate. Summer afternoons average 81° in July; winter nights fall to about 40° in January. About 19 inches of rain fall in a normal year.
On November 14 Portola Valley normally reaches 65° and falls to 44°. The record high is 79°, in 2010; the record low 29°, in 1935. The most rain on this date, 1.72 in, fell in 1953.
Normals 1991–2020 and records since 1906 at Redwood City, 6 miles north, 480 ft lower. NOAA, through RCC-ACIS.
