Columbia Slough - Geology

Geology

The Columbia Slough is part of the roughly 770-square-mile (2,000 km2) Portland Basin, which lies at the northern end of the Willamette Valley of Oregon and extends north into Clark County in the state of Washington. The region is underlain by solidified lavas of the Columbia River Basalt Group that are up to 16 million years old. Covered by later alluvial deposits, the basalts lie more than 1,000 feet (300 m) below the surface within the basin. About 10 million years ago, eruptions of Cascade Range volcanoes to the east sent flows of mud, ash, and eroded volcanic debris into the Columbia, which was powerful enough to carry the material downstream. Deposited above the basalt during the Miocene and early Pliocene, these loose sands and gravels formed part of what is known as the Troutdale Formation. Extending to the Tualatin River Valley to the south and into Clark County on the north, the formation is an aquifer that is the primary source of drinking water for Vancouver in Washington and an auxiliary source for Portland.

The Portland Basin is being pulled slowly apart between faults in the Tualatin Mountains (West Hills) on the west side of Portland, the East Bank fault along the east side of the Willamette River, and other fault systems near Gresham further east. About 3 million years ago, many small volcanoes and cinder cones erupted through the thin, stretched crust of the basin and in the Cascade foothills to the southeast. Ash, cinders, and debris from these Boring Lava Field volcanoes added another layer of sediment to the Troutdale formation.

About 15,000 years ago, cataclysmic ice age events known as the Missoula Floods or Bretz Floods originating in the Clark Fork region of northern Idaho inundated the Columbia River basin many times. These floods deposited huge amounts of debris and sediment. Water filled the entire Columbia Gorge to overflowing and turned the Willamette Valley into a lake 100 miles (160 km) long, 60 miles (100 km) wide and 300 feet (90 m) deep. The floodwaters ripped the face off Rocky Butte in Portland and deposited a 5-mile (8 km) gravel bar, Alameda Ridge, that runs parallel to and slightly south of the Columbia Slough.

Faults associated with the expanding Portland Basin are capable of producing significant earthquakes. More than a thousand earthquakes, many too small to be felt, have been recorded in the basin since 1841. The stronger ones reached about magnitude 5 on the Richter scale. In 1892, one estimated at magnitude 5 shook downtown Portland for about 30 seconds. In 1962, one centered about 7 miles (11 km) north of Portland was estimated at between magnitude 4.9 and 5.2.