Coos Bay
Coos Bay is an estuary where the Coos River enters the Pacific Ocean, the estuary is approximately 12 miles long and up to two miles wide. It is the largest estuary completely within Oregon state lines. The Coos Bay watershed covers an area of about 600 square miles and is located in northern Coos County, Oregon, in the United States. The Coos River, which begins in the Oregon Coast Range, enters the bay from the east. From Coos River, the bay forms a sharp loop northward before arching back to the south and out to the Pacific Ocean. Haynes Inlet enters the top of this loop. South Slough branches off from the bay directly before its entrance into the Pacific Ocean. The bay was formed when sea levels rose over 20,000 years ago at the end of the Last Glacial Maximum, flooding the mouth of the Coos River. Coos Bay is Oregon's most important coastal industrial center and international shipping port, with close ties to San Francisco, the Columbia River, Puget Sound and other major ports of the Pacific rim.
The city of Coos Bay is located on the peninsula formed by the inside of the loop of the bay. Charleston is located near the entrance to South Slough. Many of the commercial fishing and recreational fishing boats that call Coos Bay home are docked in Charleston. North Bend, located at the apex of the peninsula, is home to the Southwest Oregon Regional Airport.
The estuary has been altered over 150 years of modern anthropogenic use. Dredging, deepening, river diversion and spoil disposal has led to physical, biological, and chemical changes to the system over time.
Geology
The bedrock of the greater Coos Bay region was formed from the Mesozoic through the Pliocene eras out of volcanic rock, sedimentary rock, and igneous rock intrusions. The area's geologic history is highlighted by tectonic interaction between oceanic and continental plates. Subduction and abduction of the oceanic plate with the North American Plate has led to a thicker crust in Southwestern Oregon. The Empire Formation, on which Coos Bay lies, is mainly composed of sedimentary rock created by marine sediments that were deposited offshore before being pushed onshore over millions of years. This process results in the oldest rock being furthest east. East of the bay, and in the Coos River watershed, the bedrock is the oldest, formed during the Eocene. The center of Coos Bay, where the towns of North Bend and Coos Bay sit, was formed during the late Pliocene or early Pleistocene. The North Spit, the most western area bordering the ocean, is composed of sand dunes. These formed much more recently as sand was deposited along the coast after eroding from other areas. By studying the transition from peaty to muddy intertidal sediment and the associated microfossils in the estuary, researchers were able to determine 10 instances of sudden rises in sea level as a potential proxy for earthquakes, including two dating 1,700 and 2,300 years ago when sea level rose by at least half a meter. This same study also found evidence of a large earthquake 300 years ago along the great plate boundary. In addition, sea level rise 20,000 years ago drowned the prior river mouth, creating Coos Bay estuary as we know today.Physical geography
Tidal flats
Tidal flats are areas where sediments that are deposited from rivers and tides accumulate. Coos Bay is primarily made up of mud and sand flats that form in areas of low tidal activity. The estuary covers 10,973 acres at mean high water and 5,810 acres at mean low water, and about 48% of the watershed is tideland. Tidal effects can extend up the tributaries 27 miles from the ocean. Of the tidal flats in Coos Bay estuary, organic content is highest in the mudflats compared to the sandflats. These areas tend to lack visible vegetation, but do support benthic diatoms, mats of green and yellow green algae, and eelgrass beds. Tidal flats in the estuary have a network of shallow channels that allow water to drain when the flats are above water, and when submerged, allow water and sediment to pass through the flats. In this way, the flats act as a barrier that slows the water passing through, reduces how much water can flow through, and encourages the deposition, re-suspension, and transport of particles with the tides as the flats are constantly experiencing deposition and erosion.The mudflats are primarily made of a mix of medium and fine-grained sands, silts, and clays, while the sandflats are primarily made of medium-sized sand grains derived from the erosion of the nearby cliffs. The higher organic content of mudflats is composed of plant and animal tissues and wastes, diatoms, bacteria, and flocculants, chemicals that pull suspended particles out of the water to form into sediment. The environment a few centimeters below the sediment doesn't have enough oxygen, so sulfate-reducing bacteria live there, breaking down organic matter and producing hydrogen sulfide, giving the mudflats the classic "rotten egg" smell.
Weather
The mild marine climate of Coos Bay is classified as Csb or a Temperate Mediterranean Climate zone and is heavily influenced by the Pacific Ocean and precipitation from the Coast Range. Regional weather consists of a cool, wet season in the winter with an average annual rainfall of 56 inches, and a mild, dry season in the summer with an average rainfall of less than 4 inches. Runoff follows this same pattern, with approximately one month of lag. Air temperatures range from 40° to 75 °F. Winter storms lead to accumulation of winter precipitation and which is a significant input of fresh water and sediments.Fluvial geomorphology
is the study of the way rivers move through and affect their surrounding landscape. For an estuary, this involves the tributaries and their flow rates, bottom topography, water discharge and the drainage basin, and sedimentation and deposition.Tributaries
Thirty tributaries enter the bay, including 13 freshwater sources, the Coos River being the largest freshwater source. About 1 m3/s of freshwater is released by the Coos River into the estuary in the summer and >300 m3/s in the winter. These waters travel through areas of heavy logging and limited agriculture, originating from the Coastal Range. The main channel of the estuary follows the north–south trending anticline, ending in the Isthmus Slough, Catching Slough, Haynes Inlet, and Coos River; while southward, the South Slough follows the syncline. The sediment accumulation rate is approximately 2.3–9 mm/yr in a 0.9-m deep flat in South Slough over a 300-year timescale. This is greater than the sea level is expected to rise locally, 1.10 ± 0.73 mm/yr.| Stream | Length | Drainage Area | Max Freshwater Annual Yield | Min Freshwater Annual Yield | Mean Freshwater Annual Yield |
| Coos River | 5.5 | 415 | 2,200,000 | 1,130,000 | 1,590,000 |
| South Fork Coos River | 31.3 | 254 | 1,280,000 | 660,000 | 930,000 |
| Millicoma River | 8.7 | 151 | 880,000 | 450,000 | 630,000 |
| East Fork Millicoma River | 23.9 | 79 | 460,000 | 230,000 | 330,000 |
| West Fork Millicoma River | 34.9 | 55 | - | - | - |
Bottom topography
Coos Bay is considered a , meaning it was a river valley that was flooded as sea levels rose. The shallow and narrow V-shaped topography of Coos Bay is more varied across the channel than it is longitudinally. It is deepest near the mouth of the estuary, where the bottom is uniform, and the slope leading to this decline is gentle. This topography, coupled with high tidal range and low runoff, allows for lateral and vertical mixing.Water discharge and drainage basin
Annually, Coos Bay is estimated to discharge 2.2 million acre-feet of fresh water, with a yearly maximum of 3,044,000 ac-ft and a minimum of 1,560,000 ac-ft, based on extrapolations of fresh water inflow measured at the West Fork of the Millicoma River mouth in 1974. According to a US Department of Commerce survey from 1954 to 1970, freshwater inflow enters the estuary at a rate of 90 cubic feet per second in the summer and 5500 cfs in the winter with an average of 2200 cfs. The drainage basin consists of forest, cropland, and rangelands. The South Slough watershed alone has a drainage area of about 7,932 ha.| Stream | Drainage area | High average monthly flow | Low average monthly flow | Mean average monthly flow |
| Coos River | 415 | 5,500 | 90 | 2,200 |
| South Fork Coos River | 254 | 3,300 | 50 | 1,300 |
| Millicoma River | 151 | 2,200 | 30 | 870 |
Sedimentation and deposition
As of 1975, the Coos Bay has experienced a greater influx of sediment than output. Such sediment includes silt from drainage basin erosion, and various sands due to erosion in the surrounding areas. Sediments are formed and moved via streams, littoral drift, or transport of sediments along a coastline parallel to the shore, processes due to wind activity, and erosion. Sediment grains increase in size with depth and decrease in size when moving further into the estuary, likely because tidal currents are less strong further in and can no longer carry larger particles. To keep the estuary fit as a navigation channel, an average of 1.65 million square yards of material was removed annually by the US Army Corps of Engineers prior to the start of the Deep-Draft Navigation Project. Sediments input averages 72,000 tons annually.Modeling of Coos Bay sediment movement compared to historical patterns reveals that suspended-sediment concentrations and sediment retention has increased in the estuary over time as dredging, river diversion, construction and spoils disposal continues. As a result of this development, sediment transport has been diverted into the central navigation channel, where more sediment is accumulating. Tidal flat sediment retention has also increased.