Willamette River

The Willamette River is a major river in the state of Oregon.

It is a primary tributary of the Columbia River. The Willamette provides between 12 and 15 percent of the Columbia's total water flow. The river's main stem lies entirely within northwestern Oregon. It flows northward through a large basin called the Willamette Valley. This valley sits between the Coast Range to the west and the Cascade Range to the east. The river and its many tributaries support a massive drainage basin. This basin covers about 12 percent of the total area of Oregon.

The geography of the river was shaped by many geological processes. About 35 million years ago, plate tectonics began creating the basin. The Farallon Plate subducted beneath the North American Plate. This movement created a forearc basin. The oldest rocks in this area are the Siletz River Volcanics. Later, volcanic activity and erosion continued to change the landscape. About 15 million years ago, basalt from the Columbia River Basalt Group flowed across the northern basin. This lava covered parts of the Tualatin Mountains with up to 1,000 feet of material.

At the end of the last ice age, massive floods changed the valley again. These were the Missoula Floods, which occurred between 15,500 and 13,000 years ago. These floods originated from Glacial Lake Missoula in Montana. Each flood discharge exceeded the annual discharge of all modern rivers combined. The floods filled the Portland region with water up to 300 feet deep. This created a temporary body of water known as Lake Allison. These floods also carried large icebergs and rocks into the valley. These are called glacial erratics because they are not native to the area.

The Willamette Valley is a very fertile agricultural region. This fertility comes from rich sediments deposited by ancient flooding. The area also receives prolific rainfall from the western side of the Cascades. Because of this, many 19th-century pioneers traveled the Oregon Trail to reach the valley. Humans have lived in this watershed for over 10,000 years. Indigenous peoples once lived in many tribal villages along the lower river. Today, the valley contains two-thirds of Oregon's population. This includes the state capital, Salem, and the largest city, Portland.

Modern life along the river involves many different types of transportation. There are approximately 30 different bridges that cross the main stem. Some bridges carry motorized vehicles, while others are for bicycles and pedestrians. Several bridges are used exclusively for rail traffic. Ferries also provide crossings for cars, trucks, and motorcycles. For example, the Wheatland Ferry runs between Marion and Polk counties. The river is also used for navigation, especially near Portland's industrial areas. The main channel is deep and serves as a primary conduit for the harbor.

Since 1900, humans have built more than 15 large dams in the drainage basin. Thirteen of these dams are operated by the U.S. Army Corps of Engineers. These structures serve several important purposes. They produce hydroelectricity and maintain reservoirs for recreation. They also help to prevent flooding in the valley. However, the river and its tributaries face challenges from pollution and habitat changes. Despite this, the river supports 60 different fish species. This includes many important species of salmon and trout.

The river's path is not a straight line. It meanders generally northward toward the Columbia River. The river has two major deviations at Newberg. At one point, it turns sharply east before turning north again. Near the mouth of the river, it splits into two channels. One is the main navigational channel. The other is a smaller distributary called the Multnomah Channel.

This channel is about 10 miles long and 1,000 feet wide. The river's flow is also affected by the tides of the Pacific Ocean. Near Portland, the river can actually experience reverse flows during certain tides.