Seasonal variation in sediment transport on the Russian River shelf, California

Near-bottom currents, light transmission and scattering, and bottom pressure were measured with GEOPROBE tripods and vector-averaging current meters during June 1979 to April 1980 on the central shelf 10 km west of the Russian River, California. The instruments were located on the mid-shelf mud belt composed of bimodal sandy clayey silts contributed principally by the Russian River. During the summer season of persistent northwesterly, upwelling-favorable winds, the average and maximum current speeds 5 m above the bottom were 11 and 31 cm s⁻¹, respectively. The mean (subtidal) flow at 5 m above bottom was poleward and slightly offshore at about 6 cm s⁻¹. The strongest wave-generated bottom currents were about 10 cm s⁻¹, but oscillatory velocities > 5 cm s⁻¹ were infrequent. Suspended-matter concentrations, derived from the optical data at 1.9 m above the bottom, ranged from 1 to 6 mg l⁻¹. The optical data show that the currents and waves were generally below threshold levels for sediment erosion through the summer. In contrast, during the autumn and, particularly, the winter months, the average and maximum concentrations of suspended matter increased substantially. The increases were primarily caused by larger waves from distant storms and short intervals of strong currents associated with local storms and, secondarily, by the large seasonal flow of the Russian River. Wind-driven and wave-generated bottom currents were as large as 37 and 45 cm s⁻¹, respectively, during local storms in December 1979 and February 1980. Suspended-matter concentrations averaged about 7 mg l⁻¹ during non-storm winter periods, but increased to nearly 150 mg l⁻¹ during a December storm. Estimates of suspended-matter flux near the bottom show that the local winter storms, which had a combined duration of about 12 days, could account for 30 to 50% of the total annual suspended-sediment transport at the mid-shelf site. Although intervals of large swell were at times superimposed on southward advective currents, the major sediment-transport events were caused by strong southerly winds that produced poleward bottom currents with a significant offshore component. The primary aspects of the distribution of modern sediments on this shelf are in good agreement with the observed poleward transport.