Forces That Create Estuaries in Oregon

The estuaries of the Oregon coast are a unique result of the interplay of geologic forces, ocean conditions, and weather. These forces vary so that no two estuaries are alike, although many are similar.

Geologic forces

The Oregon coast is part of the geologically active margin of the North American continental plate. This plate is moving slowly westward. As it does, it is overriding the last fragments of the oceanic Juan de Fuca plate, which are moving eastward away from the sub-sea volcanic origins along the Gorda and Juan de Fuca Ridges one hundred miles or so to the west. Forces from this inexorable collision have forced the oceanic plates downward and uplifted and crumpled the entire western edge of North America. This process uplifted the Rocky Mountains far inland and, more recently, the Cascade and Coast Range mountains along the coast. The process continues today; the Oregon coastline continues to slowly emerge from the sea.

Rising sea level

During the last great ice age, much of the water from the world's oceans was locked in ice. Ten thousand years ago, sea level was far lower than today. Then, the Pacific Ocean lapped at the edge of a wide plain some ten to forty miles to the west of the present coastline. As the great glaciers melted, water returned to the oceans, and sea level rose to cover that plain, which is known today as the continental shelf. This rising ocean gnawed at the edges of the coast range mountains and flooded into the canyons of rivers leading down from the mountains. These steep canyons eventually filled with sediments, the surfaces of which are now the broad tide flats of today's estuaries.

Seasonal rainfall

In the summer, a high pressure system typically builds over the entire Pacific northwest and pushes storm systems far to the north. Oregon receives very little rainfall. Because the coastal mountains build no snowpack in winter, have steep, small drainage basins, and have relatively thin soil cover, there is no groundwater reserve to sustain river levels during the summer drought. Coastal streams therefore dwindle. Summer freshwater input to the estuary is very low.

Summer winds

Along the beach, this same high pressure system sets up strong winds which blow from the north/northwest and generate a fast-moving southward flowing ocean current near the shore. These "longshore currents" can carry great volumes of sediment and move the sand into long spits parallel to the ocean front. Sand spits divide and protect the estuarine environment from the dynamic influence of the ocean. In summer, this large volume of moving sand, coupled with low estuarine outflow, allows the sand spit to move into the mouths of estuaries and perhaps, if no jetties have been built, across the channel altogether.

Winter storms

In winter, low pressure systems move back in over the northwest coast, bringing storms which blow onshore from the south or southwest. A strong northward flowing current, the Davidson Current, moves great quantities of sand northward along the coast. These storms drop tremendous amounts rain onto the coastal mountains that discharge intoand throughestuaries. Combined with high tides and storm-generated high sea levels, the vigorous streamflow removes some of the sand spit built during the summer. Prior to the construction of jetties at the mouths of the rivers, high river runoff would often cause the river to breach the spit at an unpredicted location and create a new outlet to the sea. Winter also brings the highest tides flooding into the estuaries, removing plant material from even the highest marshes and distributing this organic debris throughout the estuary.

Tides

Stratification
Freshwater streamflows and intruding seawater form two wedges of water going in opposite directions. The freshwater flows on top of the heavier saltwater. These wedges create surface-to-bottom differences in salinity that significantly influence life and conditions in the estuary.
This layering, known as stratification, is strongest where the two wedges meet and when river flows are high. When stratification is strong, there is little mixing between surface and bottom waters. Stratification is weakest at the sources of the wedgesthe river and the oceanand when river flow is low. Weak stratification results in greater vertical mixing.
Turbidity is highest at the upstream end of the saltwater wedge, the zone of maximum resuspension of bottom sediments.

Year around, the ocean force with the greatest effect on estuaries is the daily tidal cycle. In Oregon, there is a dual high and low tide pattern with the high and low approximately six hours apart. These tides are seldom equal. On a daily basis, there is a "higher high" tide followed by a "higher low" tide, then a "lower high" tide, and finally a "lower low" tide. The elevations of these four tides vary as the moon moves through its phases. The highest tides of the year are in winter, when the Earth is closest to the sun and the moon is aligned with the sun in the "new moon" position. The lowest tides of the year come in the early summer.

The pull of the sun and moon create a "tidal bulge" on the ocean which affects the Oregon coast from south to north; high tide at Coos Bay is 20 to 30 minutes earlier than it is at the Columbia River. This regular ebb and flow of the tides brings saline, nutrient-rich ocean waters into the estuary to meet the sediment-laden fresh water. This interaction drives the sedimentation process that builds the broad tide flats and creates a wide variety of saline conditions that provide a diversity of habitat for plants and animals.

Tidal Rhythms on the Oregon Coast
A monthly progression of high tides and low tides at Coos Bay illustrates daily and monthly fluctuations in tide heights. The Earth rotates daily beneath tidal bulges, but the tilt of the Earth's axis results in a higher high tide at (A), a lower low tide at (B), and lower high tide at (C), and a higher low tide (hidden) before returning to (A). The Moon's orbit around the Earth brings it in and out of line with the sun. (from the Oregon Ocean Book)