Coos Bay, the estuary of Coos River, is located on the southern coast of Oregon. Although Coos River is one of the smaller rivers draining the Oregon Coast Range, its estuary is one of the largest along the Oregon Coast. The estuary occupies a structural depression and has been formed by drowning of the river mouth following rise of sea-level in late Pleistocene time. The outer part of the bay has been developed by the southward prolongation of North Spit by longshore drift. This estuary opens onto a narrow continental shelf, which, in this vicinity, ranges in width between 10 and 15 miles.

Texture and mineralogy of sediment samples taken along the 12-mile length of Coos Bay suggest derivation of the estuarian sediments from two sources: (1) sediment in the inner bay is chiefly fine sand and is similar to the detritus carried by Coos River; (2) sediment in the outer bay is mostly medium-grained sand, with an increase in grain size toward the bay mouth. This sand is most closely related mineralogically to sands along the coast north of Coos Bay. Thus, it appears that although sediment in the inner bay has been brought to it by Coos River, sediment in the outer bay has been derived chiefly from the influx of beach and dune sands carried over North Spit by the prevailing westerly winds and into the mouth of the estuary by tidal currents.

Sediment samples from the continental shelf and adjacent to Coos Bay, between the mouths of the Umpqua and Coquille Rivers, range in median diameter from medium sand to silt, and from very well sorted to moderately sorted. They display a general decrease in grain size and degree of sorting in a seaward direction. However, a band of fine, well sorted, glauconitic sand lies along the outer margin of the continental shelf. The location and mineralogy of this sand suggest that it is relict sediment (Emery, 1952) related to a lower sealevel during Pleistocene time that has not yet been covered by modern sediment.

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