Subsystems of Oregon Estuaries

Estuaries in Oregon are, in reality, complex systems made up of four major parts of subsystems. These parts blend from one another with no clear demarcation, but each has some distinct characteristics.

Marine

The Pacific Ocean greatly influences the water and the ecology of the estuary near its mouth. The degree of this influence is a product of two major factors linked to the seasons of the year: the amount of freshwater outflow pushing against the ocean's waters (which, in turn, depends upon the size and shape of the drainage basin and the amount of rainfall or snowmelt), and the strength of the tidal surge into the mouth of the estuary (which is influenced by the shape of the channel mouth, the height of the tide and, in winter, storm surge).

In this marine-dominated zone there is a steady mix of marine life into and out of the estuary. The main channel serves as the entrance and exit for many fish and larger invertebrates that take advantage of the food-rich estuarine environment during some part of their life cycle.

Four Major Subsystems of Estuaries on the Oregon Coast
This drawing shows the four major estuarine subsystems at low tide.
The riverine subsystem dominates where the river flows from the mountains into the estuary. This wide single channel meanders through marshlands, many of which have been diked for pasture.
A slough subsystem occurs where small tributary streams with very little flow make their way toward the main channel. Salt marshes fringe these drainage ways.
The bay is dominated by broad tidal flats of mud and sand. This area will be covered by water at high tide.
At the mouth of the estuary, the surging flood tide brings the marine environment into the estuary.

Although virtually all Oregon estuaries have some marine component to them, Sand Lake estuary and Netarts Bay are two where the marine component dominates because they lack major freshwater inflow.

Bay

The bay portion of the estuary is characterized by broad mud flats which are exposed to the air at low tide and flooded by a mix of salt and fresh waters at high tide. These flats are not just mud. Sand grains carried from the mountains by the river are deposited in the upper bay and along the edges of main channels, while finer particles of silt and clay drift farther to the edges of the flats near the fringing marshes. Marine sand carried along the ocean front in the "longshore current" is swept into the estuary on incoming tides and may be deposited as far as several miles upstream.

The catalyst for the tremendous productivity of the bay subsystem is the broad expanse of shallow, nutrient-rich water which covers these flats twice a day. This water provides the ideal medium for phytoplankton-microscopic free-floating plants-to capture sunlight and thereby continually add energy into biologic food webs of the estuary. Solar energy drives the collective metabolism of the estuary.

The majority of the larger estuaries on the Oregon coast have extensive bay components. Alsea Bay, Yaquina Bay, Siletz Bay, and Coos Bay, for example, have relatively large bays as part of their estuarine system.

Slough

Sloughs are the smaller tributaries to the main bay and river channels. They have little freshwater inflow. Tidal flushing may not be as complete as in parts of the estuary that are closer to the ocean or main channel. Generally, sloughs consist of meandering channels that wind through fringing marshes and across mud flats to the main bay. It is these small channels that bring the tide up into the marsh and to the edge of the forest.

Coos Bay, for instance, has a number of sloughs which are relatively large and navigable for several miles, including Isthmus Slough, North Slough, and Catching Slough. In turn, smaller sloughs are tributary to these. South Slough, one of the major tributaries at Coos Bay, does not fit this general description. Rather, it is a separate, miniature estuarine system which shares with Coos Bay a common mouth to the ocean. South Slough was designated the first National Estuarine Sanctuary under a program established by Congress in 1972.

Riverine

Rivers and streams are parts of almost all estuaries on the Oregon coast. Coastal rivers often reach sea-level many miles inland while still confined by mountains and narrow river valleys (the Siuslaw River at Mapleton or the Umpqua River at Scottsburg). It is here the tide begins to effect the flow of the river. However, it is not until much further downstream that tide flats begin to appear along the edges of the river and the bay subsystem characteristics prevail. On the Coquille River, for instance, this riverine portion extends to near Myrtle Point, over thirty river miles inland.

The Columbia River estuary is one major Oregon estuary dominated by the riverine component, although the dramatic influence of the river has been tempered by the many dams upstream. Historically, the late spring and summer were seasons of major freshwater discharge from snowmelt far inland. Now, the flow of freshwater is more moderate year round. This change in riverine influence has disturbed the equilibrium between fresh and salt water. The influence of the marine environment has crept slowly upstream. In general, however, the Columbia River continues to dominate its estuary.

Cross section of the Coos Bay ecosystem at Mid-Bay (view south)

Coos Bay Ecosystem 1 The shallow edge of the estuary is submerged for only a short time at high tide. Woody debris and recently eroded sandstone rocks provides habitat for algae, barnacles, worms, and amphipods. At high tide, crabs and sculpins (locally called bullheads) scavenge in the jumble of rocks and sticks. At low tide, large algae like Fucus (seaweeds) lie limp on the mud and rocks to be grazed by small invertebrates.

Above the water, marshes ring the edges of sloughs, bays and rivers where the soil is wet at least part of the year. Plants which have evolved a tolerance for saltwater advantage of the varying degrees of salinity nearer or farther from the marine-dominated waters. These salt marshes are particularly productive. The combination of sunlight and saline waters yields a rich crop of marsh grass that dies in the fall, is harvested by winter high tides and is distributed as nutrient debris to the estuarine food web.

Coos Bay Ecosystem 2 Across the broad tide flats, eelgrass meadows provide sheltered habitat and act as a nursery for a variety of fish, crabs, and other creatures. Its rhizomes are buried in the mud and so stabilize sediments and prevent erosion. Eelgrass grows rapidly in sunlight, fixes nutrients from mud and water, and generates detritus which releases nutrients to the food web as it decays. Eelgrass growth is adversely affected by turbidity.

Flats are the result of thousands of years of sedimentary deposit onto the bottom of the estuary. As rivers and streams reach seal level, they lose energy necessary to retain their load of sand, clay and organic debris. Logging and road building in the watershed during modern times hastened erosion, added to the sediment load, and contributed to rapid filling of estuaries over the last century.

Coos Bay Ecosystem 3 Continuously submerged, the deep channels of the estuary are conduits for many species of marine life to enter and leave the bay. Jellyfish float near the surface while marine fish move with the more saline waters of the bottom. In these channels, salmon and shad migrate downstream through the estuary to the ocean.

The dendritic pattern of channels covers every portion of the mud flats and extends into the fringing salt marshes. The meander of these channels is influenced by the energy of the flow in them. The lower the energy, the more the meander. These dynamic environments