The San Jacinto 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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93° on Nov. 15, 1999, the hottest reading for this week of the year since 1948. Next is 92° in 1989.
San Jacinto’s center stands about 1,570 ft above sea level. Within three miles the land runs from 1,480 ft to 3,400 ft, a difference of 1,920 ft: steep hill country. The highest named summit within 15 miles is Castle Rocks, about 8,540 ft, 14 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 4 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.
Tonalite. Age: Middle Jurassic to Late Cretaceous, 66 to 175 million years old. About 12 mi of the line.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 8 mi of the line.
Sandstone, conglomerate. Age: Miocene to Pleistocene, less than 23 million years old. About 4 mi of the line.
Plutonic, gneiss. Age: Paleoproterozoic to Late Cretaceous, 66 to 2,500 million years old. About 0.4 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 San Jacinto reaches the Pacific Ocean by way of the San Jacinto, Temescal Wash and the Santa Ana: about 90 river miles.
Often dry. About 70% 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 Jacinto; river miles along the main stem, rounded.
San Jacinto has a hot semi-arid climate. Summer afternoons average 97° in August; winter nights fall to about 39° in December. About 12 inches of rain fall in a normal year.
On November 14 San Jacinto normally reaches 73° and falls to 45°. The record high is 91°, in 2021; the record low 30°, in 2000. The most rain on this date, 0.29 in, fell in 1997.
Normals 1991–2020 and records since 1948 at San Jacinto, here. NOAA, through RCC-ACIS.
