The Issaquah 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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3.80 inches of rain fell in two days, the most for this week of the year since records began in 1945. The next wettest, 3.18 inches, fell in 2015.
Issaquah’s center stands about 110 ft above sea level. Within three miles the land runs from 20 ft to 2,320 ft, a difference of 2,300 ft: mountain country. The highest named summit within 15 miles is Mount Si, about 4,060 ft, 14 miles east.
Under the town center the state geologic map shows Alluvium (silt, sand), from the Holocene, less than 12,000 years old.
The section below crosses 8 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 6×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Fine-detrital, coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 9 mi of the line.
Andesite, basaltic-andesite. Age: Late Eocene, 34 to 38 million years old. About 3 mi of the line.
Silt, sand. Age: Holocene, less than 12,000 years old. About 3 mi of the line.
Andesite, basalt. Age: Eocene to Oligocene, 23 to 56 million years old. About 3 mi of the line.
Fine-detrital, coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 2 mi of the line.
Siltstone, sandstone. Age: Middle Eocene to Late Eocene, 34 to 48 million years old. About 2 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale. 2 smaller units 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 Issaquah reaches Puget Sound and the Pacific Ocean by way of Issaquah Creek and the Sammamish: about 40 river miles.
Lakes on the way: Lake Washington and Lake Sammamish.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Issaquah; river miles along the main stem, rounded.
Issaquah has a warm-summer Mediterranean climate. Summer afternoons average 78° in August; winter nights fall to about 37° in December. A normal year brings about 6 inches of snow; with the rain, that comes to about 39 inches of water.
On November 14 Issaquah normally reaches 52° and falls to 41°. The record high is 65°, in 1995; the record low 9°, in 1955. The most rain on this date, 2.61 in, fell in 2001.
Normals 1991–2020 and records since 1945 from the Seattle Tacoma Area threaded record (Seattle Tacoma Airport, 14 miles west-southwest, 260 ft higher). NOAA, through RCC-ACIS.
