- Tillamook Bay is the second largest estuary on the Oregon coast, and concerns have been raised whether human induced impacts have been responsible for the perceived increase in sedimentation rates during the past century. Major land-use practices within the five watersheds of the Bay include logging, forest fires, the construction of forest roads, the placement of dikes along the channels of the main rivers and in the estuary, the removal of riparian vegetation, and the construction of jetties at the tidal inlet. Each of these practices has led to impacts on the entire ecosystem of the watersheds and the Bay, but this study focuses on the effects of human disturbances on the Bay's sediment accumulation. This study examines in detail the land-use practices that have occurred in the watersheds, on the beaches, and in the estuary, focusing on those that have had a direct impact on the sedimentation regime of the Bay. One goal of the study is to assess the relative roles of natural processes versus human impacts on the sedimentation. A general description of the physical characteristics of Tillamook Bay and its surroundings is included, and a brief discussion is provided about the tectonic setting of the Northwest Coast, including its history of subduction earthquakes and the associate sea-level changes. Also provided is a summary of the existing information concerning the arrival of Indians and their environmental impacts, followed by a more detailed account of the major impacts that have resulted from the settlement of the Euro-Americans in the Tillamook area, in the 1850's. The study then focuses on the description of the watersheds from a geomorphologic point of view, and the important land-use practices that may have affected sediment yields during the past century. Analyses of the hydrology of the Tillamook Bay watersheds are included, and the relations between annual water yields and total precipitation are examined in distinct time intervals, each corresponding to a different period with different amounts of land uses. The results of these hydrology analyses suggest that the Tillamook watershed gradually recovered from a period of major disturbances (from 1933 to 1955) to more normal conditions (from 1977 to 1998). In addition, this part of the study attempts to quantify the sediment transport regime of the rivers draining the watershed by using a hydraulic model that is based on the principle of stream power, and on considerations of availability of transported material. Application of this model during the 1933-1955 period for the major rivers suggests an average sediment yield on the order of 410,540 tons/year, but most important are the relative changes of the delivered
sediment through time. The results of the model suggest a 1.6-factor decrease of the amount of river sediments from the Heavily Impacted Period (1933-1955) of major disturbances to the Normal Period (1977- 1998). The spatial variations of beach and river derived sediments throughout the Bay are determined from textural and mineralogical analyses of surface sediment
samples, with the beach sands dominating the area close to the inlet and the river derived sands being mainly deposited at the southeast and northeast parts of the Bay. The relative contributions of these two major sources of sediment were found to be 60% for the marine beach and 40% for the river sands. Further attempt is made to distinguish between the sand transported into the Bay from the individual rivers, and to determine the main processes that are responsible for the dispersion of sediments within the Bay. The attempts to distinguish sands contributed by the individual rivers involved modal analyses of the frequency curves of the surface sediment samples, and the results mainly suggest a grain-size increase away from the mouth of the rivers as a result of sediment reworking by estuarine processes following its initial deposition during episodic river flooding. The main processes that control the dispersion of sediments and their deposition within the Bay were identified by using factor analysis, the results of which suggest that various estuarine processes are responsible for the observed dispersal patterns. A brief review is provided of the study undertaken by Dr. James McManus for the collection and analyses of core samples from Tillamook Bay. Down-core
geochemical analyses of major and minor elements indicate that there have been times of episodic input of marine sediment in the central and western portions of the Bay, which is a result of either periodic breaching or washover of Bayocean Spit, so that the beach sand source was more important in the past. This episodic input of marine sand as inferred from the down-core geochemical variations was related to the most recent subduction earthquake, which occurred on January 26th, 1700. Finally, a summary of the results and conclusions of different aspects of this study is presented, so that sedimentation in Tillamook Bay can be viewed as an
integrated process involving the watersheds, the estuary, and the ocean beaches.