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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

Discussion

The sidescan-sonar imagery, in concert with the seismic-reflection profiles, video, and bottom samples, has allowed a detailed mapping of the floor of John Day Reservoir. These data show that the surficial geology of the reservoir floor is highly variable, but that it can be divided into three general categories: 1) bedrock into which the original channel was cut, 2) fluvial deposits which were mostly Quaternary deposits reworked by fluvial processes prior to impoundment, and 3) post-impoundment sediment which has been deposited since 1968. The aerial extent of these three categories is summarized in Table 1.

Geologic maps show that Miocene rocks bound the western part of the John Day Reservoir while Quaternary deposits bound the eastern part (Weissenborn, 1969; Walker, 1977). The Miocene rocks are dark gray to black non-porphyritic and porphyritic flows of basalt with local occurrences of flow breccias and some interbeds of tuff. These basalts are part of the Columbia Plateau flood basalt province, which covers a large part of eastern Washington and Oregon (Dott and Batten, 1971). The Quaternary deposits are unconsolidated, poorly sorted silt, sand, gravel, and coarser material (Weissenborn, 1969; Walker, 1977). The silt is mostly loess that was deposited in front of the advancing glaciers. The coarser material is mostly deposits associated with the Missoula Floods. These floods resulted from the repeated rupturing of dams to glacial lakes in western Montana between 16,000 and 12,000 years ago (Waitt, 1985). While the basalts under the western part of the John Day Reservoir were resistant to erosion, the unconsolidated Quaternary deposits under the eastern part of the reservoir were eroded, transported, and reshaped subsequent to these floods by fluvial processes.

Figure 11.

The sidescan-sonar imagery shows that Miocene basalt is still exposed on large parts of the reservoir's floor west of Rock Creek (Fig. 11A). From Rock Creek eastward to approximately Arlington, OR, Miocene basalt is exposed along both banks of the reservoir and fluvial deposits cover the central channel floor. Miocene basalt appears to be exposed along much of the southern bank of the reservoir between Arlington and Boardman, OR while Quaternary deposits comprise much of the banks of the northern side of this stretch of the reservoir. East of Boardman, the banks on both sides of the reservoir are cut into Quaternary deposits. The Miocene basalt covers 31.6 x 10⁶ m² (22.5%) of the reservoir floor, and the Quaternary deposits cover 25.8 x 10⁶ m² (18.4%) (Table 1).

During the Missoula floods at the end of the Quaternary and onset of the Holocene, huge amounts of material were eroded and transported through this stretch of the Columbia River. Boulders along the banks of the Columbia River Gorge downstream of this reservoir attest to the vigor of these floods (Waitt, 1985). The boulders on the floor of the John Day Reservoir probably originated from these floods as well (Fig. 8). Boulders presently cover 7.7 x 10⁶ m² (5.4%) of the reservoir floor (Table 1).

Following the Missoula floods, the Quaternary deposits and flood deposits were further modified and reworked by fluvial processes. Alluvial fans (Fig. 6) formed at the mouths of tributaries feeding into the main river valley (Fig. 7). Bars and plane beds of gravel and cobbles formed under the main channel of the Columbia River (Fig. 7, 9). Many of these fluvial deposits are still preserved on the floor of the reservoir. Alluvial fans are apparent on the sidescan-sonar image off the mouths of several tributaries that enter the reservoir. These fans predate the reservoir, as they are present on the aerial photograph collected prior to completion of the dam (Fig. 6B). Interestingly, alluvial fans only occur in the western half of the reservoir, and only off tributaries along its northern side (Fig. 11B). Bars occur along this entire stretch of the river. They were formed by fluvial processes prior to creation of the reservoir as the tops of the bars show in the pre-dam aerial mosaic (Fig. 7A, 8). These bars are 1-9 km long, have as much as 33 m relief, and occur along the edges as well as in the middle of the pre-impoundment channel (Fig. 11B). Video images show that these bars now have a veneer of well-rounded gravel and cobbles (Fig. 8, photo 8). The presence of causeways to some of the bars and roads on bars in the sidescan-sonar imagery attests to the lack of major modification of these features since impoundment (Fig. 7B). Between the bars, much of the floor of the reservoir is gravel. Video observations show that the gravel often has a thin, discontinuous cover of fine-grained sediment. This sediment cover indicates that the gravel is not moving under the present hydraulic conditions. Even the sediment waves and furrows in the eastern part of the reservoir are in gravel (Fig. 10), and show no evidence of being actively reworked under present conditions. Deposits that were created by fluvial processes cover 63.1 x 10⁶ m² (45%) of the floor of the reservoir (Table 1).

Figure 12.

The sediment that has accumulated in the reservoir since impoundment is fine-grained and has a patchy distribution (Fig. 12). Bottom video imagery commonly shows a dusting of fine sand or mud on the tops of cobbles (Fig. 8, photo 8). Some of the observations of cobbles covered by fine-grained sediment are from sites that were subaerially exposed prior to impoundment indicating that these sediments accumulated since impoundment. For example, the location of station 7 in Figure 8 was subaerially exposed prior to impoundment and now is covered by fine sediment. In some places the fine sediment is thicker and obscures the gravel and cobbles (Fig. 8, photo 11). In the areas where the fine sediment is thick enough to cover the underlying cobbles the sidescan-sonar image shows a very low-backscatter (dark) signature. The areas of thick sediment cover account for 12.2 x 10⁶ m² (8.7%) of the reservoir floor (Table 1). The low-backscatter areas are limited in their extent, and tend to occur in the deeper parts of the reservoir beside or downstream of bars or shallower areas along the edges of the reservoir that are sheltered behind a bar or promontory (Fig. 12).

Figure 13.

High-resolution seismic profiles were used to measure the thickness of the fine-grained post-impoundment sediment (Fig. 13). In other reservoirs post-impoundment sediment is acoustically transparent in these profiles (Twichell and others, 1999), yet no acoustically transparent layers were seen on profiles from the John Day Reservoir. The resolution of the subbottom profiler that was used in this survey is about 25 cm. The fact that the post-impoundment sediment thickness was not resolved suggests that this sediment is less than 25-50 cm thick.