? deforestation in the watershed
? the Tillamook Burns of 1933, 1939, 1945, and 1951
? land use practices in the watershed, including road construction, agriculture, and urbanization
The greatest impacts of these sources of sedimentation probably occurred prior to the 1957 bathymetric survey. In addition, the bay has been altered in a variety of other ways that have undoubtedly affected its bathymetry. These include the drainage of wetlands and construction of dikes and levees. The inlet from the ocean was modified by constructing jetties, and dredging of navigation channels has occurred throughout this century (Percy et al. 1974). The most catastrophic change occurred in 1952 when the spit was breached just north of the current location of Cape Meares Lake. This may have been due to erosion of Bayocean spit in response to construction of the north jetty. The breach remained open until a dike was constructed in 1956 to close it off. Bathymetric points collected in the 1867 survey were obtained from the U.S. Coast Survey Office. Data were derived from a paper map prepared by the U.S. Army Corps of Engineers. The coverage has 3750 points situated in an irregular network distributed throughout the bay and spaced approximately 200 to 300 m apart (Figure 1a). We noted that the x and y positional accuracy of the 1867 coverage appeared to have a shift when the outline of the bay was compared with the current bay outline (Figure 2). This was corrected using an affine coordinate transformation on twenty points located at locations presumed to be relatively stable on the shoreline of the bay. This transformation accounted for variability in scale, skewness, rotation and translation. The Root Mean Square Error of the transformation was 152 meters. The RMS error indicates how well the transformation matched the bay outline for all 20 data points across both data sets. It is a least square solution for the entire coverage. Several of the selected control points (i.e., Crab Harbor and one near McCoys Cove) may not have been as stable as we would like for this transformation. Excluding these points would only reduce the RMS error slightly. The transformed data set was used in the subsequent comparative analyses. The 1867 coverage also illustrated many landscape features which were not present in the 1957 and 1995 coverages due to revetment and other changes to the bay shoreline. Cape Meares Lake, Garibaldi, McCoys Cove, Larson Cove and the Jetty changed in the time period between 1867 and the more recent surveys due to construction activities and the spit breach. These areas were digitized and a new bay boundary coverage was prepared.
We developed bathymetric maps for each of the data sets in a consistent fashion. This allowed direct comparison between surveys to permit quantification of the changes that occurred in the depth of the bay during the intervening periods. Several studies of sedimentation and sediment composition have been conducted in Tillamook Bay (e.g., Avolio 1973, Glenn 1978, Komar 1997). The latter report provides an overview of what is known about sedimentation of the bay, which is an important environmental concern because of its influence on habitat quality and navigation. Bathymetric surveys were conducted in 1867, 1957, and 1994/1995. E&S performed a comparative analysis for TBNEP of the available historical bathymetric data sets for the bay. The objective of this analysis was to quantify, to the extent possible, the changes in bathymetry that occurred between the times of the available surveys. FOR MORE DETAILED INFORMATION SEE TILLBATH.TXT
BATHGRID67.VAT
COLUMN ITEM NAME WIDTH OUTPUT TYPE N.DEC
1 VALUE 4 10 B -
5 COUNT 4 10 B -