The Walnut 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.
Walnut’s center stands about 590 ft above sea level. Within three miles the land runs from 410 ft to 1,170 ft, a difference of 760 ft: hilly country. The highest named summit within 15 miles is Monrovia Peak, about 5,350 ft, 15 miles north-northwest.
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 7×. 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 9 mi of the line.
Coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 6 mi of the line.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 4 mi of the line.
Gneiss. Age: Paleoproterozoic to Miocene, 5.3 to 2,500 million years old. About 2 mi of the line.
Basalt, andesite. Age: Late Oligocene to Late Miocene, 8 to 25 million years old. About 2 mi of the line.
Sandstone, mudstone. Age: Miocene to Pleistocene, less than 23 million years old. About 0.5 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 Walnut reaches the Pacific Ocean by way of Lemon Creek and the San Gabriel: about 35 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 Walnut; river miles along the main stem, rounded.
Walnut 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 Walnut 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, 360 ft lower. NOAA, through RCC-ACIS.
