The San Dimas 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.
Mostly sunny
Mostly sunny
Showers likely
Chance of showers
Partly cloudy
Showers likely
Mostly sunny
23° on Nov. 11, 1950, the coldest reading for this week of the year since 1893. Next is 25° in 1958.
San Dimas’s center stands about 960 ft above sea level. Within three miles the land runs from 690 ft to 2,190 ft, a difference of 1,500 ft: steep hill country. The highest named summit within 15 miles is Ontario Peak, about 8,630 ft, 13 miles northeast.
Under the town center the state geologic map shows Older Quaternary alluvium and marine deposits (coarse-detrital), from the Pleistocene, less than 2.6 million years old.
The section below crosses 7 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.
Sandstone, mudstone. Age: Oligocene to Pliocene, 2.6 to 34 million years old. About 7 mi of the line.
Gneiss. Age: Paleoproterozoic to Miocene, 5.3 to 2,500 million years old. About 6 mi of the line.
Coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 5 mi of the line.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 3 mi of the line.
Granodiorite. Age: Permian to Tertiary, 2.6 to 260 million years old. About 2 mi of the line.
Basalt, andesite. Age: Late Oligocene to Late Miocene, 8 to 25 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. 1 smaller unit 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 San Dimas reaches the Pacific Ocean by way of Walnut Creek and the San Gabriel: about 35 river miles.
Often dry. About 80% 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 San Dimas; river miles along the main stem, rounded.
San Dimas has a hot semi-arid climate. Summer afternoons average 95° in August; winter nights fall to about 43° in December. About 9 inches of rain fall in a normal year.
On November 14 San Dimas normally reaches 75° and falls to 48°. The record high is 95°, in 1906; the record low 27°, in 1915. The most rain on this date, 0.54 in, fell in 1972.
Normals 1991–2020 and records since 1893 from the Riverside Area threaded record (Riverside Municipal Airport, 24 miles east-southeast, 120 ft lower). NOAA, through RCC-ACIS.
