The Oregon Estuary Plan Book
Page 1: Introduction, Estuaries in Oregon
Estuaries in Oregon
Estuaries are special places where ocean and river mingle to create a dynamic, diverse, and highly productive environment. Plants and animals thrive in this unique environment driven by sunlight and the daily tides. Humans, too, are drawn to the estuary to harvest food, travel on its waters, and claim the flat lands for the purposes of civilization.
Twice each day, Oregon's estuaries are the stage for a slow, stately drama influenced by the moon, the sun, the wind, and the rain. Sinuous channels, branching and winding across the broad mud flats, are filled with incoming ocean waters. As the channels fill, the rising tide spreads slowly across the flat mud. The ever-deepening waters lift the eelgrass, fill the myriad burrows of little creatures, and creep into tiny channels that penetrate the fringing salt marshes. Finally, the waters surge upstream to the edge of the forest and gently lift trailing branches of rhododendron and cedar. The estuary is full.
For a moment, the drama pauses. Then as the earth turns, the ocean's push becomes a pull, and the waters of the estuary recede. Before long, logs at the edge of the salt marsh are grounded on the mud, the eelgrass lies limp and flat, and tiny creatures are stranded in isolated pools of water warming in the sun. Clam diggers move carefully across the muddy flats toward the edge of the winding channel. But in a short time, the cycle will begin again.
What is an estuary?
An estuary is defined as a semi-enclosed body of water, connected to the ocean, where salt water is measurably diluted with fresh water from the land. In reality, an estuary … or bay … is a whole lot more. It is a zone of transition between the marine-dominated systems of the ocean and the upland river systems, a zone where the mix of the two yields one of the most biologically productive areas on Earth.
Estuaries in Oregon
The large number of estuaries on the Oregon coast belies the fact that Oregon's total estuarine acreage is relatively small. Except for the Columbia River, all of Oregon's major and minor estuaries (approximate area of 53,000 acres) could fit inside of Grays Harbor estuary in Washington (approximately 58,000 acres). Most of the larger estuaries have been altered through dredging, filling or diking. Many of the smaller ones have escaped the impacts of civilization and remain in a natural state. In any case, all are important and are covered by Oregon's estuarine management program.
Distribution along the coast
The distribution of estuaries along the Oregon coast reflects the geology and topography of the mountains that meet the ocean. The Columbia River estuary overwhelms all the other estuaries on the coast. One of the major river systems in North America, the Columbia River has maintained its westward flow from the Rocky and Selkirk mountains across the rising Cascade and Coast Range mountains to empty into the Pacific. The present day estuary is a recent feature. Geologists now recognize that the Columbia once flowed across the Oregon country through long-eroded landscapes to the south of its present course, and may have once discharged its waters somewhere nearer Yaquina Bay.
From the Columbia River estuary south to Cascade Head, the mountains are a complex mix of more recent sedimentary and volcanic rocks. Except for the wide valley carved by the several rivers now feeding Tillamook Bay, and Nehalem Bay at the mouth of the winding Nehalem River, the estuaries on the north coast tend to be small, fed by streams which drain small watersheds, and enclosed in indentations between rugged headlands and sand spits. Netarts Bay, Sand Lake and Salmon River are such estuaries.
Between the Salmon River estuary at Cascade Head and the Coquille River far to the south are the estuaries of Siletz Bay, Yaquina Bay, Alsea Bay, the Siuslaw and Umpqua rivers, and Coos Bay. Along this portion of the coast, the mountains are mostly older marine sediments and sands, clays, and muds eroded from ancient mountains to the south and east. Deposited on the ocean floor in a great trough from the Klamath Mountains to Vancouver Island, these sediments were uplifted by the force of colliding continents and eroded once again to create relatively wide river mouths. Rising seas filled these river valleys with sediments and created the conditions for present-day estuaries.
South of Coquille River estuary at Bandon, there are few estuaries. Along this stretch of coastline, the hard, resistant cores of the ancient Klamath Mountains withstand erosion from rain, the river and the clawing surf. The gradient of the rivers and creeks are steep even at the ocean's edge. The Rogue, Elk, Sixes, Chetco and Winchuck Rivers have almost no tidelands. These rivers flow directly into the ocean.
Types of estuaries
There are several types of estuaries on the Oregon coast.
- River dominated: Some, like the Columbia River and Rogue River, are dominated by the freshwater flow of the river and have relatively small tideland areas.
- Drowned river mouth: The majority, like Coos Bay, Siletz Bay, and Yaquina Bay, are the drowned river mouth variety, where winter's floods discharge high volumes of sediments through the estuary. In summer, seawater inflow dominates the estuary because the streamflow is low.
- Bar-built: Others, like Sand Lake and Netarts Bay, are "bar-built," where a sand spit creates a separate estuarine environment which receives very little freshwater inflow. Sand Lake has a watershed of only 14 square miles.
- Blind: Some of the smaller estuaries, like Elk River and Sixes River in Curry County, are "blind" estuaries where low river flow in summer results in a sand bar completely closing off the mouth of the estuary.
Sponsored By: The Oregon Deptartment of Land Conservation and Development, Coastal-Ocean Management Program.