Northwest Fisheries Science Center

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Document Type: Contract Report
Center: NWFSC
Document ID: 7757
Title: Impact of flow-lane disposal at Dobelbower Bar
Author/Editor: Theodore H. Blahm, Robert J. McConnell
Publication Year: 1979
Publisher: National Marine Fisheries Service
Contracting Agency: U.S. Army Corps of Engineers. Portland, Oregon
Contract Number: DACW57-76-F-0918

Various methods of dredging are used by the U.S. Army Corps of Engineers to accomplish channel maintenance.  Hopper dredges are utilized at locations where the dredged material can be placed in deep areas of the river or taken out to sea.  Pipeline dredges are used in locations where material can be pumped to a site on land or to an in water site away from the channel.  Clamshell dredging is sometimes required; with the dredged material being placed on a barge for subsequent removal to a location for disposal or commercial use.

By whatever means, dredging involves the movement of a significant amount of dredged material even in low-volume flow years.  In the past, dredged material was used for the construction of islands and for beach nourishment along eroded areas.  During recent years, however, there has been a reduced need for this material along the Columbia River, and it has become increasingly difficult to find suitable disposal sites.

The concept of placing the material in water near the channel (flow-lane disposal) was developed with the idea that there would be less adverse impact on uplands and wetlands and on aquatic life forms and associated fisheries.  To test this concept, the Dobelbower Dredge Study was developed.  This study was designed to examine the extent of the turbidity plume, distribution of the dredged material, and the impact on the aquatic life forms resulting from inwater pipeline disposal.  Initial conclusions were:

  1. Turbidity in the Columbia River during the dredging effort did not exceed 30 JTUs; whereas natural turbidity in 1976 exceeded 70 JTUs.
  2. Turbidity levels during dredging and/or disposal apparently had little if any effect on finfish or shellfish.
  3. Nonfilterable residue increased with increasing turbidity.
  4. There were no long-term effects on those water quality parameters measured near the dredge and disposal operations.
  5. The dredging operation did not noticeably alter zooplankton abundance or diversity.
  6. The majority of benthic variations between sampling times (before, during, and after dredging) and sampling area (control, dredged, and disposal) can be attributed to seasonal variation and station location rather than a primary effect of the dredging operation.
  7. The entire Dobelbower Bar area is relatively low in benthic production when compared with the estuary and slough areas of the river.
  8. The material deposited in the flow-lane was dissipated within approximately 1 year.
Notes: DACW57-76-F-0918