SEDIMENTARY GEOLOGY OF THE COLUMBIA RIVER ESTUARY

A multiyear study of the sedimentary geology of the Columbia River Estuary has provided valuable data regarding sediment distribution, bedform distribution, and suspended sediment distribution on spatial and temporal scales that permit delineation of sedimentary environments and insight into the sedimentary processes that have shaped the estuary. In comparison to other more-intensively studied estuaries in North America, the Columbia River estuary has relatively larger tidal range (maximum semidiurnal range of 3.6m) and large riverflow (6,700m3s-1). Variations in riverflow, sediment supply, and tidal flow occur over a range of time scales, making the study of modern processes, as they relate to long-term effects, particularly challenging. Analyses of more than 2000 bottom-sediment grab samples indicate that the bed material of the estuary varies in a relatively narrow range between 0 and 8 phi (1.0 and 0.0039mm) with an overall mean size of 2.5phi (0.177mm). Sediment size decreases generally in the downstream direction. Sediments from the upriver channels are coarse (1.5-2.0phi; 0.25-0.35mm) and moderately sorted; sediments in the central estuary show wider range and variation in grain size and sorting (1.75-6.0phi; 0.016-0.3mm). Sediment from the entrance region has a mean size of 2.75phi (0.149mm) and is well sorted. Seasonal changes in sediment size distributions occur and are best delineated by those samples containing more than 10% mud (silt plus clay). Sediments containing a significant fine fraction generally occur only in the peripheral bays and in channels isolated from strong currents. Thin deposits of fine sediments are occasionally found in main channels, and the ephermeral nature of these sediments suggest that they may erode and produce the silty rip-up clasts that appear intermittently in the same regions. The distribution of bedforms of various size and shape has been mapped with side-scan sonar during three seasons and at various tidal stages. The presence of bedforms with wavelengths of 6-8m and alternating slip faces about 40cm high indicates that the deeper portion of the entrance region is dominated by tidally reversing lower flow regime sediment transport. Bedforms in the upper reaches of the estuary are much larger, with heights of up to 3m and wavelengths of up to 100m. These bedforms, and the smaller, superimposed bedforms, imply downstream bedform transport under fluvial conditions. In the central estuary, bedforms in the deep portion of the main channels are oriented upriver while those on the shallow flanks of the channels are oriented seaward. The landward limit of upriver bedform transport varies seasonally in response to riverflow fluctuations. A complex array of sedimentary environments exists in the Columbia River estuary. Each environment is influenced by the relative importance of waves, fluvial currents, and tidal currents, as modified by the presence or absence of estuarine circulation, vegetation, or human activity. The importance of these environments to the ecosystem of the estuary is discussed in subsequent papers in this volume.