The Diamond Bar 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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95° on Nov. 13, 2007, the hottest reading for this week of the year since 1989. Next is 95° in 2016.
Diamond Bar’s center stands about 710 ft above sea level. Within three miles the land runs from 550 ft to 1,280 ft, a difference of 730 ft: hilly country. The highest named summit within 15 miles is Sunset Peak, about 5,770 ft, 15 miles north-northeast.
Under the town center the state geologic map shows Miocene marine rocks (sandstone, mudstone), from the Oligocene to Pliocene, 2.6 to 34 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 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.
Sandstone, mudstone. Age: Oligocene to Pliocene, 2.6 to 34 million years old. About 8 mi of the line.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 7 mi of the line.
Coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 4 mi of the line.
Basalt, andesite. Age: Late Oligocene to Late Miocene, 8 to 25 million years old. About 2 mi of the line.
Gneiss. Age: Paleoproterozoic to Miocene, 5.3 to 2,500 million years old. About 2 mi of the line.
Granodiorite. Age: Permian to Tertiary, 2.6 to 260 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. 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 Diamond Bar reaches the Pacific Ocean by way of Brea Creek, Coyote Creek and the San Gabriel: about 25 river miles.
Often dry. About 100% 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 Diamond Bar; river miles along the main stem, rounded.
Diamond Bar has a hot semi-arid climate. Summer afternoons average 89° in August; winter nights fall to about 48° in December. About 14 inches of rain fall in a normal year.
On November 14 Diamond Bar normally reaches 77° and falls to 53°. The record high is 94°, in 2007; the record low 39°, in 2000. The most rain on this date, 0.86 in, fell in 2025.
Normals 1991–2020 and records since 1989 at Anaheim, 11 miles south, 480 ft lower. NOAA, through RCC-ACIS.
