Depositional Structures and Processes in the Mouths of Small Coastal Streams, Southwestern Oregon

The mouths of coastal streams form a unique depositional environment, in which sedimentation is controlled by a combination of waves, tides, and river discharge. Examination of processes and deposits in the mouths of small streams along the coast of southern Oregon provides considerable data on stream-mouth sedimentation on a coast characterized by high wave energy and moderate tidal range.

Depositional structures and processes in such a setting depend primarily on discharge and may be classified accordingly. The very smallest streams (discharge on the order of 0.001 m³ sec^-1 or less) modify the beach topography but slightly. Such streams flow in small intricately braided channels; the internal structure of the streambed material consists mostly of highly lenticular bedding, commonly defined by concentrations of heavy minerals. Larger streams (discharge on the order of 0.01 m³ sec^-1) are more deeply incised into the beach. On the beds of such streams, sand ripples predominate, except in main channels that are occupied by standing waves and antidunes. Fluctuation between planar and antidune bedforms generates a pulsing flow in the lower reaches of such streams. During rising tide or periods of high waves, swash processes will either bury of destroy the stream deposit. If preserved, the deposit will consist of thin sheets of ripple or other lenticular bedding interlayered with parallel swash lamination.

Streams with larger discharge are deeply incised into the beach and have a bedform predominantly of megaripples. These structures may reverse their direction with incoming tide. Within the actual mouth of the stream, where wave action predominates, the bedform is generally planar.

The largest streams entering the coast of southern Oregon, such as the Rogue and Coquille Rivers, form complex tidal estuaries, the discussion of which is 116beyond the scope of this paper.

Seasonal variation in discharge may cause different parts of some streams to evolve through several of the stages described above — an evolution that appears, over several years, to be relatively systematic. The resulting deposit will therefore show corresponding systematic transitions between depositional facies.

Ancient examples of stream-mouth deposits are rare, but can be recognized owing to their character and association.