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Table of Contents:

OF 2004-1014

Disc Contents

Geologic
    Discussion

• Introduction

• Field Program

• Field Equipment

• Interpretation

• Discussion

• Summary

• Acknowledgments

Data/GIS

Metadata/Data
    Catalog

References Cited

Contacts

Interpretation of the Reservoir Floor

The sidescan-sonar imagery in concert with seismic-reflection profiles, bottom photographs, sediment samples, bathymetry and information on the geology surrounding the reservoir indicate that the geology of the reservoir floor is complex and highly variable. These data sets allowed dividing the floor of the reservoir into 10 distinct substrate types. These substrate types can be divided into three general categories: (1) outcrop of older material (Miocene basalt flows, Quaternary lacustrine and glaciofluvial deposits, and boulders deposited during late Quaternary to early Holocene floods), (2) deposits generated by fluvial processes (alluvial fans, bars, gravel beds, furrows and sediment waves), and (3) post-impoundment sediment (sand and mud). In addition to these geologic units, man-made features (roads and causeways) were also identified and mapped. A more in-depth description of each of these units and what criteria were used in their identification is given below. For the purposes of this report, a high-reflective substrate (high backscatter) is indicated by brighter image values (white), while a low-reflective substrate (low backscatter) is indicated by dark image values (black).

1. Miocene - Areas of alternating bands of high and low backscatter on the sidescan-sonar mosaic within sections of the river where the geologic maps of Oregon (Walker, 1977) and Washington (Weissenborn, 1969) show that it is bounded by Miocene age basaltic rocks. These areas occur in the western part of the reservoir. Morphologically, this part of the river is bounded by steep hills as indicated by the shaded relief image in Figure 3. No bottom photographs were taken in these areas, so there is no direct confirmation of the sidescan-sonar interpretation (Fig. 4).

   Figure 3.

   
   

   Figure 4.

   
   
2. Quaternary - Areas of uniform moderate to high backscatter or alternating high and low backscatter on the sidescan-sonar mosaic (Fig. 5)

   Figure 5.

   within sections where the geologic maps of Oregon (Walker, 1977) and Washington (Weissenborn, 1969) show that the river is bounded by Quaternary sedimentary deposits of dominantly eolian, lacustrine, and glacial outburst origin. The topography of these areas generally has gentle slopes, and aerial photographs show that parts of these areas had been farmed prior to impoundment.
   
   
   
3. Alluvial fan -

   Figure 6.

   Arcuate shaped areas of high backscatter intersected by sinuous to straight low-backscatter bands on the sidescan-sonar image (Fig. 6A). These features are all found off the mouths of tributaries that drain into this section of the Columbia River. The alluvial fans show on the pre-impoundment aerial photographs indicating that they formed prior to impoundment (Fig. 6B). Video transects across alluvial fan areas revealed mostly cobbles and gravel, which was commonly covered by a thin, but continuous veneer of fine sediment (Fig. 6C).
   
   
   
   
   
4. Bar -

   Figure 7.

   Elongate mounds that existed within the river channel prior to formation of the reservoir. The bars are 1-9 km long; have as much as 33-m relief, and their tops were subaerially exposed prior to filling of the reservoir. Pre-impoundment aerial photographs show that some of these bars had causeways to them and roads on them (Fig. 7A). Many of these manmade features are preserved on the sidescan-sonar mosaic (Fig. 7B). Bottom photographs show that the surface sediment on the bars is mostly cobbles or gravel with a thin, discontinuous veneer of fine-grained sediment (Fig. 8. photo 8).
   
   
   

   Figure 8.

   
   
   
5. Boulders - Areas of moderate to low backscatter with high-backscatter targets, which gives the sidescan-sonar image a speckled appearance (Fig. 8). Video transects show that the boulders are widely spaced, and that the intervening areas are covered by cobbles, gravel or fine-grained sediment (Fig. 8, photo 12). It is estimated that the boulders are larger than 50 cm in diameter.
6. Gravel - Areas of uniform moderate to high backscatter on the sidescan-sonar image (Fig. 8). Video transects show that surface sediment is mostly gravel with a discontinuous cover of fine-grained sediment (Fig. 8, photo 8). In some areas of this category, cobbles are interspersed with the gravel and in one case a localized patch of sand was sampled.
7. Sand -

   Figure 9.

   Areas of uniform moderate backscatter on the sidescan-sonar image. The acoustic appearance of this substrate is similar to gravelly areas, but the backscatter intensity is not as high (Fig. 9). Video transects show a smooth or rippled surface, and samples recovered sandy sediment (Fig. 9. photo 45).
   
   
   
   
   
8. Mud - Areas of low acoustic backscatter on the sidescan-sonar image (Fig. 8, 9). Video transects show a smooth surface with occasional pits or lineations that may have resulted from biological activity (Fig. 8, photo 11). Because of the fine-grained nature of the sediment and the fact that the camera system was towed close to the lake floor, bottom images from these areas frequently were obscured by sediment resuspended from the bottom. Samples from these areas recovered clayey silt and silty clay.
9. Furrows -

   Figure 10.

   Areas of alternating narrow bands of high and low backscatter on the sidescan-sonar image that cover parts of the original riverbed and trend parallel to its axis (Fig. 10A). This pattern is interpreted to represent shadows from the near sides and strong returns from the far sides of narrow, linear depressions. Some of the lineations converge in the upstream direction (towards the east). This geometry is similar to furrows found in estuarine, continental shelf, and deep-sea areas subjected to strong currents (Flood, 1983). A striking difference, however, is that the features in the John Day Reservoir occur in gravel beds, while in other settings furrows are cut into fine-grained sediment.
   
   
10. Sediment waves - An area of moderate backscatter interrupted by high-backscatter bands spaced 30-40 m apart that trend perpendicular to the original path of the river (Fig. 10B). No echo-sounder (bathymetry) profiles crossed this area to show the relief of these features. The one video transect through this area showed a gravel substrate (Fig. 10C).

Roads were identified on the sidescan-sonar imagery as narrow, continuous, linear high-backscatter bands (Fig. 7A), and they are included as a separate interpretive overlay. Roads show on the aerial photography that was collected prior to completion of the John Day Dam, and, for the most part, the roads on the sidescan-sonar imagery line up closely with those shown on the photographs.