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Geology and Hydrology
The southwestern coast of Oregon experiences two distinct climatic seasons during a typical year. The warm and relatively dry season extends from May through September, with an average rainfall of less than 4 inches (10 cm). A cooler wet season brings approximately 56 inches (142 cm) of rain to the area from October through April. Regional temperatures typically range between 40° and 75° F (4.5 – 24 °C).
Winds from the north and northwest averaging 17 mph (27 km hr) are typical of the high pressure systems that prevail in summer. In the winter, wind blows from the south and southwest at an averages of15 mph (24 km hr). Storms driven by the southwesterly winds move inland periodically and can deliver 7-10 cm of rainfall during a 24 hour period. Wind velocities during intense storm events can reach hurricane velocities (> 75 mph, 120 km hr). Winds are less intense between winter storm events and generally blow from the north and northwest.

Oregon Coast
The hills, flood plains, sand dunes, and headlands that characterize the vicinity of the Coos estuary are the result of a complex series of coastal geomorphic events. The shoreline, landforms, and soils of the area reflect the interactions of tectonic plates over the past 50 million years, changes in sea level, and local weakness in the earth’s crust, along with more human recent land use.

South Slough NERR lies along a geologic fold, or syncline, which bears its name. Due to this formation, the watershed’s eastern and western sides are of distinct geologic types, with different elevations and gradients. The eastern shore formation, which rarely exceeds 250 feet, is typical of the larger Coos estuary watershed. Its highly erodable Quaternary marine terraces of unconsolidated to semi-consolidated sand, silt, and clay are gently sloping, and worn down along creek beds to sandstone and siltstone overlain by loamy sand and sandy and silty loam. The western side’s Empire Formation, with scattered Quaternary terraces, is unique to the South Slough. Its hard, impermeable marine sandstone rises 370 feet above sea level, in a long, steeply sloping north-south ridge. These western slopes are mantled with sandy and silty loam and loamy sand.

Sediments in the South Slough watershed and estuarine tidal basin are derived from several sources including terrestrial runoff, oceanic deposition, and biotic origins. The predominant soil type throughout the upland forested areas is silt loam of the Templeton-Salander group with medium to high runoff and erodability. Sandy marine terraces of the Bullards-Bandon-Blacklock group are also present on the northeastern slopes of the watershed. Soils in the tidally flooded salt marshes are classified as rich organic histosols (Haagen, 1989), which typically consist of compacted clay, sand, and fine mud in alternating layers with mineral sand/silt and organic peat materials. Tidal flats in the South Slough estuary are composed of mud flats and sand flats. The organic content of the mud flats is relatively high and they occur in areas of the estuary that experience low tidal energy. Sand flats, in contrast, occur in areas of high tidal energy and have a much lower organic content.
Core sediment samples from several marsh locations in the estuary indicate a buried layer of coarse-grain sand that overlies organic material (Peterson and Darienzo, 1989; Nelson et al., 1998), providing evidence that South Slough was inundated by a tsunami about 300 years ago (Satake et al., 1996).

Mineral Resources
Due to its marine origins, the South Slough drainage area includes small deposits of black sands where ancient waves and currents concentrated heavy minerals (iron, chromium, minor amounts of gold, titanium, zirconium, platinum and garnet). Two to four thousand feet beneath the Coaledo
Formation a U-shaped seam of coal encircles the Slough.

The Coos estuary is a drowned river mouth basin with an estimated surface water area of 12,380 acres (5010 ha). The estuary is relatively shallow, with an average depth of 2 m below MLLW and broad expanses of tideflats and mud are exposed at low tide. This shallow depth allows for the thorough mixing of fresh and saltwater for most of the year. Due to the seasonally high volumes of fresh water, however, the estuary becomes partially stratified in the winter, especially where deeper channels have been dredged for shipping.
Tides in the Coos estuary are mixed and follow a semi-diurnal pattern with two high and two low tides per day. Tidal currents are substantial throughout the estuary, with average flows over 1 meter per second (Baptista, 1989). Mean tidal range is 2.3 meters at the mouth of the estuary; the highest tides measuring 3.3 meters above MLLW and the lowest tides occur at –0.9 m below MLLW extreme tidal range is 3.3 m). Freshwater flow into the Coos estuary averages 5500 cfs during winter rains (January to April) and drops to 90 cfs from May through December (ACOE, 1993). The major rivers emptying into the estuary are the Coos and the Millicoma, which supply 66% of the freshwater entering the system. Numerous smaller tributaries also enter the estuary, often through long, shallow inlets, called "sloughs." These sloughs typically receive fresh water slowly and in small amounts, usually from several streams which may have only intermittent flow.
The average water depth in the Slough is 1 meter, with the deepest point measuring 5.5 meters. A single narrow channel, approximately 2 m deep, meanders down the center of the South Slough. Circulation patterns within the South Slough tidal basin are strongly influenced by tidal oscillations within Coos Bay and along the outer coast, but are complex and poorly understood (Juza, 1995; Roegner and Shanks, 2001). The South Slough estuary is well mixed vertically (except during heavy rainfall events). Maximum tidal velocities at Valino Island are nearly 1 meter per second, with average current velocities around 0.4 meters per second. Flushing time for the estuary is estimated at 6-8 tidal cycles, or about 3 days.
Six perennial streams and over 30 intermittent creeks in the South Slough watershed provide a highly seasonal source of fresh water to the estuary. The largest drainage system, Winchester Creek, flows north through the watershed into the western (Winchester) arm of the estuary. Other significant freshwater contributions to the Slough flow from the more gently sloping eastern side of the watershed. Due to seasonal variations in freshwater flows, South Slough salinity profiles vary during the year.

Water Quality and Chemistry
The physical, chemical, and biotic characteristics of the water in South Slough vary throughout the estuary on a seasonal and tidal basis, and are influenced by conditions in the nearshore ocean, in the greater Coos estuary, and by freshwater inputs. The water column is generally well-mixed vertically, both near the mouth in the marine dominated region (Harris et al., 1979; Roegner and Shanks, 2001) and in the riverine portions of the estuary. Heavy rainfall events may cause some stratification of the water column, but the extensive exchange of tidal waters that occurs daily in the estuary result in mixing and breakdown of vertical stratification. Strong gradients exist along the marine to freshwater axis for several parameters, including salinity, density, specific conductivity, nutrients, and chlorophyll concentrations. Weaker gradients occur for temperature, dissolved oxygen, and pH.