The St. Helena 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; warmer
Sunny
Partly cloudy
Partly cloudy
Partly cloudy; cooler
Showers likely
83° on Nov. 14, 2025, the hottest reading for this week of the year since 1932. Next is 77° in 2021.
St. Helena’s center stands about 1,250 ft above sea level. Within three miles the land runs from 1,140 ft to 1,430 ft, a difference of 290 ft: rolling country.
Under the town center the state geologic map shows Niobrara Formation (limestone, shale), from the Late Cretaceous, 66 to 100 million years old.
The section below crosses 5 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 50×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Limestone, shale. Age: Late Cretaceous, 66 to 100 million years old. About 11 mi of the line.
Shale. Age: Late Cretaceous, 66 to 100 million years old. About 9 mi of the line.
Shale. Age: Late Cretaceous, 66 to 100 million years old. About 2 mi of the line.
Siltstone-mudstone, conglomerate-sandstone. Age: Miocene, 5.3 to 23 million years old. About 1 mi of the line.
Chalk, marlstone, shale. Age: Late Cretaceous, 66 to 100 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 St. Helena reaches the Gulf of America by way of the Missouri and the Mississippi: about 1,950 river miles.
1,170 miles of the trip are on the Mississippi.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of St. Helena; river miles along the main stem, rounded.
St. Helena has a hot-summer humid continental climate. Summer afternoons average 87° in July; winter nights fall to about 10° in January. A normal year brings about 38 inches of snow; with the rain, that comes to about 29 inches of water.
Normals 1991–2020 and records since 1932 at Yankton Municipal Airport, 10 miles northwest. NOAA, through RCC-ACIS.
