Silverton is situated on the eastern edge of the Willamette Valley, a fertile and alluvial plain which stretches from the western foothills of the Cascade Range on the east, known as the Waldo Hills, to the eastern foothills of the Oregon Coast Range on the west. Silverton lies on either side of Silver Creek, a tributary of the Pudding River, which joins the Molalla River before emptying into the northward-flowing Willamette River. Abiqua Creek also empties into the Pudding River; it flows across the eastern valley north of Silverton, further draining the land around the city.
Silverton’s elevation is between 200 and 250 feet (61 and 76 m) above mean sea level with the steep-sided, heavily-wooded Waldo Hills to the south rising an additional 200 feet (61 m). The agricultural richness of the environs is due to massive and repeated floods from prehistoric Lake Missoula in western Montana. Beginning approximately 13,000 years before the present, repeated flooding from Lake Missoula scoured eastern Washington and Oregon, carved out the Columbia River Gorge, and periodically swept down the Columbia River; when floodwaters met ice jams in southwest Washington, the backed-up water spilled over and filled the entire Willamette Valley to a depth of 300 to 400 feet (91 to 122 m) above current sea level, creating a body of water known as Lake Allison. The gradual receding of Lake Allison’s waters left layered sedimentary volcanic and glacial soils to a height of about 180 to 200 feet (55 to 61 m) above current sea level throughout the Tualatin, Yamhill and Willamette Valleys.
Until the mid-19th century, the Silverton area was a broad, open grassland with small stands of Oregon white oak, ponderosa pine and Douglas fir. Stands of Oregon white oak, red alder, big leaf maple, and black cottonwood lined streams and river banks. While these tree species are extant today, widespread farming in the Willamette Valley between 1850 and 1870 altered the land through the discontinuation of widespread seasonal burning in the valley plains previously employed by the Kalapuya people. Large stands of Douglas fir and western red cedar, mixed with Oregon white oak, remain in the Silverton area, especially on eastern ridge tops and on the slopes of the Waldo Hills to the south. Due to decades of intensive timber extraction, mature second- and third-growth trees comprise existing evergreen stands.