The Simi 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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4.43 inches of rain fell in two days, the most for this week of the year since records began in 1949. The next wettest, 3.50 inches, fell in 2025.
Simi Valley’s center stands about 770 ft above sea level. Within three miles the land runs from 660 ft to 1,490 ft, a difference of 830 ft: hilly country. The highest named summit within 15 miles is Oat Mountain, about 3,740 ft, 11 miles east-northeast.
Under the town center the state geologic map shows Quaternary alluvium and marine deposits (coarse-detrital), from the Pleistocene to Holocene, less than 2.6 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 6×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Basalt, andesite. Age: Late Oligocene to Late Miocene, 8 to 25 million years old. About 11 mi of the line.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 7 mi of the line.
Mudstone, sandstone. Age: Paleocene to Oligocene, 23 to 66 million years old. About 4 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: Miocene to Pleistocene, less than 23 million years old. About 0.8 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 Simi Valley reaches the Pacific Ocean by way of the Arroyo Simi, the Arroyo Las Posas and Calleguas Creek: about 30 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 Simi Valley; river miles along the main stem, rounded.
Simi Valley has a hot-summer Mediterranean climate. Summer afternoons average 98° in August; winter nights fall to about 42° in December. About 16 inches of rain fall in a normal year.
On November 14 Simi Valley normally reaches 77° and falls to 46°. The record high is 96°, in 2021; the record low 25°, in 2000. The most rain on this date, 1.10 in, fell in 1972.
Normals 1991–2020 and records since 1949 at Woodland Hills Pierce College, 13 miles east-southeast. NOAA, through RCC-ACIS.
