The Ontario 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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23° on Nov. 11, 1950, the coldest reading for this week of the year since 1893. Next is 25° in 1958.
Ontario’s center stands about 1,010 ft above sea level. Within three miles the land runs from 800 ft to 1,300 ft, a difference of 500 ft: hilly country. The highest named summit within 15 miles is Telegraph Peak, about 8,920 ft, 14 miles north-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 6 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 3×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 13 mi of the line.
Sandstone, mudstone. Age: Oligocene to Pliocene, 2.6 to 34 million years old. About 5 mi of the line.
Gneiss. Age: Paleoproterozoic to Miocene, 5.3 to 2,500 million years old. About 2 mi of the line.
Coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 2 mi of the line.
Granodiorite. Age: Permian to Tertiary, 2.6 to 260 million years old. About 2 mi of the line.
Schist, gneiss. Age: Paleoproterozoic to Cretaceous, 66 to 2,500 million years old. About 0.7 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 Ontario reaches the Pacific Ocean by way of Mill Creek, Chino Creek and the Santa Ana: about 45 river miles.
Often dry. About 60% 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 Ontario; river miles along the main stem, rounded.
Ontario 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 Ontario 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, 15 miles east-southeast, 160 ft lower). NOAA, through RCC-ACIS.
