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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

Summary

During the 32-year history of the John Day Reservoir prior to the survey in 2000, sediment accumulated on its floor; but it is only a small amount in comparison to the 3,121 x 10⁶ m³ storage capacity of the reservoir (Dauble and others, 2003). The sidescan-sonar imagery shows that 12.2 x 10⁶ m² of the reservoir floor has a continuous cover of fine-grained sediment. Thickness of these deposits is less than the resolution of the seismic-reflection system. If it were assumed the sediment in these areas is 50 cm thick, the volume of post-impoundment sediment covering these areas would be 6.1 x 10⁶ m³ (~0.2% of the reservoir volume). Much of the remainder of the reservoir floor has a discontinuous dusting of post-impoundment sediment, which bottom photographs show to be about 1 cm thick. The volume of the thin veneer covering these areas is about 96,000 m³. The amount of sediment that has accumulated in this reservoir is less than that found in other reservoirs (Twichell and others, 1999; Vorosmarty and others, 2003) presumably because there were several dams that already existed upstream of the John Day Reservoir prior to its construction, and they trapped much of the sediment.

Geophysical mapping enables comparisons of pre- and post-impoundment habitats of interest to aquatic biologists. Sediments and bedforms directly and indirectly influence the biota of a reservoir. Sediments serve as a substratum for plants and benthic invertebrates and as egg incubation sites for some fish species, while bedforms, or a lack thereof, provide microhabitats essential to the survival of various life stages of aquatic organisms. In addition, sediments are a reflection of the hydraulic and chemical processes present, which indirectly indicate the degree of stability (or instability) of aquatic habitats (Poddubny, 1976). Although the volume of fine-grained sediment that has accumulated in the John Day Reservoir since impoundment is small, its presence influences the type and quantity of biota produced from affected habitats. Even thin veneers of sediment can preclude the development of some macrobenthic organisms and prevent the attachment of adhesive fish eggs or smother those that incubate in interstitial spaces of gravels and cobbles.

Chapman and Witty (1993) and Dauble and others (2003) indicate that geologic composition of the substrate is one of several factors that affect spawning habitat for fall chinook salmon Oncorhynchus tshawytscha, a species which historically spawned in the mainstream Columbia River whose habitats have been substantially altered by dam construction. Prior to impoundment, gravel and cobble beds along the riverbanks and on islands and bars had been the primary fall chinook salmon spawning habitats (Fulton, 1968). Clearly other conditions including water depth and flow velocity also influence habitat, but the results of this study allow quantification of the amount of the reservoir floor of appropriate composition to be suitable spawning habitat. Dauble and others (2003) indicate that bedrock, boulders, and fine-grain sediments are not suitable substrates for spawning salmon. The sidescan-sonar imagery and other data show that 23% of the reservoir floor is exposed basalt, 5% is boulders, and 9% is fine-grained sediment (approximately 50 cm thick). A discontinuous veneer (<1 cm thick), of fine-grained sediment is found over an additional 53% of the area on bars, gravel beds, alluvial fans, and exposures of unconsolidated Quaternary deposits that may have historically provided salmon spawning habitat. Bottom video images show gravel beds that are completely free of fine sediment only in the easternmost 20-25 km of the reservoir floor (Fig. 10 C), representing 10% of the total reservoir floor. Thus, in large part due to sedimentation since impoundment, very little of the reservoir floor is presently suitable substrate for salmon habitat.

This work illustrates that geophysical mapping with sidescan sonar and subbottom profiling provides a valuable overview of the types and distribution of substrates under the John Day Reservoir when it is accompanied by the ground-truth observations. While the sidescan-sonar image obtained with the SIS-1000 system does identify the areas that have been completely covered by fine-grained sediment, it cannot differentiate between areas that are partially covered by fine sediment versus those areas that are completely free of fine sediment. It is only through the integration of these different data types that the picture becomes clear as to how this section of the Columbia River has been modified by emplacement of the John Day Dam.