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Dredging the Clatskanie River

Clatskanie, Oregon is a community of approximately 1,900 people on the Clatskanie River, about halfway between Portland and the Pacific Ocean. The river flows through the downtown area before joining the Columbia, and the entire river system is tidally influenced in this area.

The community was developed as a timber town, and in the early 1900’s, a channel was federally authorized to allow barges to ship lumber products out of the town. The river was last dredged by the Corps of Engineers in 1968, and the decrease in logging across the Northwest over the past decade did not spare Clatskanie. The town still has some economic basis in lumber, but not to its historical extent. Without the economic justification to maintain commerce, the Corps is not required to dredge the federally authorized channel.

Since the last dredging project, the city has lacked the resources and the serious need to dredge the channel. Over time, normal sedimentation, combined with ash from the Mount St. Helens eruption in 1980, filled in parts of the river, which created a problem when a 100-year flood occurred in February, 1996.

During the flood, the river flowed three to four feet deep through the downtown area and caused about $5 million in damages to small businesses and public property. The flood permanently closed the Beaver Lumber Company and some other businesses. The loss of Beaver Lumber eliminated 50 jobs in the community.

Following the flood, the river had shoaled enough in the downtown area to allow people to walk to the middle during low tides in water that reached only to their ankles. Since the 1996 flood, a rising river would cause some of the townspeople to start sandbagging their doorways. The community wanted to explore options to reduce its risk from flooding and try to do something to protect itself from future floods.


Ogden Beeman & Associates, Inc. (OBAI) was hired to perform hydraulic modeling of flood reduction options for the Clatskanie River. The model results indicated that re-establishing the federally authorized channel by dredging would provide reduction in flood elevations of half a foot or greater, depending upon the backwater level of the Columbia River. For floods on the Clatskanie River during low water on the Columbia, the predicted drop in flood elevations within town is one foot or more. The flood reduction from dredging decreases as the Columbia River level increases. While a few inches might not seem significant, during the 1996 flood when the Clatskanie and Columbia Rivers were high, observers estimated that one levee might not have breached if the water had been four inches lower.

The city initiated the permitting process with assistance from OBAI, while the Oregon Economic Development Department (OEDD) worked to secure funding for the project.EnviroScience, Inc. conducted the biological sampling and developed the mitigation and monitoring plan, and Robert Lofgren inspected the dredging.

The Corps issued the public notice for dredging on August 29, 1997. The Division of State Lands received between 20 and 35 comments from local residents, all of which were in support of the dredging.

However, during the public notice review, the US Fish and Wildlife Service raised concerns regarding the project’s impact to fish habitat. An inter-agency meeting was held in February 1998 to develop a dredging plan that would satisfy the agencies’ concerns, with additional inter-agency meetings held in May and June.

During the inter-agency discussions, flood reduction alternatives to dredging were proposed and included the following concepts:

• The Oregon Department of Fish and Wildlife initially requested denial of the permit due to the impact of dredging on shallow water habitat. They preferred a Coordinated Resource Management Plan that focuses on a watershed approach.

• The USFWS wanted flood easement programs or ecosystem restoration projects that involved non-structural, less-impacting methods of achieving flood control.” One example included breaching the existing dikes to allow floodwaters to reenter the floodplain and temporarily flood pastures, with interior dikes constructed to protect structures.

• The Corps of Engineers suggested a full levee system to protect the community from future flooding.

From the city’s perspective, dredging provided a better solution than any of the proposed alternatives. The watershed approach and flood easement programs would not offer any reduction in flood elevations, and a full levee system would require four miles of levees, in addition to raising Highway 30 and two local bridges. The cost of constructing and maintaining such a levee system is well beyond the financial ability of a community of this size.

An additional issue that required resolution before the permit could be issued included a cultural resource survey of the proposed upland disposal sites. Though the State Historic Preservation Office (SHPO) signed off on the disposal sites, the Corps of Engineers’ archaeologist felt that any upland disposal site in the area should be surveyed for artifacts, since tribes had been known to exist in the general area. It did not matter that prior to diking, both sites were historically low-lying wet marshes that were not suitable for campsites. We were required to hire a professionally qualified archaeologist to review the history of the area and conduct a field survey of both sites. No artifacts were found at either site.


The downstream disposal site is located near Palm Island and the mouth of the Clatskanie River, and covers seven acres. During the construction of Highway 30, part of the nearby hillside was blasted to make room for the road, and the marshland was filled with several feet of boulders and rocks. Besides construction debris, the only other surface soil deposit found on the site was sand from a previous dredging episode.

The upstream site is approximately three acres and is located in downtown Clatskanie. Following construction of the levees, the marsh became suitable for agricultural use and also served as a disposal area during dredging in 1968.

With the support of Larry Potter of the Division of State Lands (DSL), the state and federal permits were issued on July 31, 1998, two weeks into the in-water work window which runs between July 15 and September 15. Until 4 p.m. that day, the terms of the permits were being negotiated, with the understanding that any further delay in issuing the permit would terminate the project. On an emergency basis, OEDD then contracted with the Port of Astoria to conduct the dredging, and the port started to mobilize their equipment. Construction of the disposal areas had started a few weeks earlier in anticipation of a permit. The downstream site was completed just before the dredge arrived.

The Port of Astoria performed the dredging using their 16-inch hydraulic cutterhead dredge. The dredge restored the dimensions of the federal channel, cutting a 40 foot wide channel to a depth of -7 feet CRD. The federal channel was authorized to -6 ft CRD and one foot of overdepth was included. The channel was restored from the mouth of the river to downtown Clatskanie, a distance of 8,000 feet.

Trees and a loaded dump truck worked as anchors on the south bank of the river, but the north bank presented a problem, since it consists of a dike that lacks trees or other structures. Assistance came from Michael Sykes of the Port of St. Helens, who contributed the use of his personal CAT to serve as an anchor.

The dredge completed its work on September 15, the last day of the fishery in-water work window, having removed 70,000 to 75,000 cubic yards of sand.

Since the entire dredging project impacted shallow water habitat, a 2:1 mitigation ratio was established during discussions with the agencies. Initially, the agencies had requested a 10:1 mitigation ratio.

The dredging impacted more than 12 acres, so the City of Clatskanie must provide around 25 acres of mitigation at an estimated cost of $90,000. It made no difference to the agencies that the final dredged depth remained well within the most stringent shallow water habitat definitions.


With the dredging finished, the town still has a great deal of work to do in order to complete the project. To satisfy the terms of the permit, the city must perform the various components of the mitigation plan as follows:

• Mitigation project: To be completed by September 15, 1999. The city’s mitigation plans include expanding existing side channels, restoring historical side channels that are currently backwater sloughs, and creating new connections between Clatskanie River channels to provide fish additional shallow water habitat. This plan is still in negotiation with the agencies.

• Pre-dredge and post-dredge benthic sampling: The city must sample the benthic invertebrates before and after dredging to determine the level of impact demonstrated by dredging.

• Monitoring: Annual monitoring of the shallow water benthic invertebrates is also required over the next five years to determine the level of impact and the recovery of benthic organisms. Beyond the first five years, the continuation of monitoring is contingent on DSL review.

• Post dredge assessment study: Due May 1, 1999. A report must be submitted to show that the dredging proved effective to reduce flood levels within the community, or the agencies will not allow future maintenance dredging.

• Local watershed council: The City of Clatskanie had already formed a watershed council prior to the permit issuance, but they need to implement a flood relief plan in the next year.

• Mitigation bond: In order for the agencies to grant the permits, they had to be assured that the mitigation would actually be performed. Consequently, Clatskanie had to pass a city council resolution that if they fail to perform the mitigation work within the next fishery work window, the city will pay the Division of State Lands $80,000.

The general costs associated with the project can be broken down as follows:

Dredging $230,000

Permit/Mitigation Plan $80,000

Mitigation Construction $90,000 (est.)

Total $400,000

This summary does not include the costs associated with the annual monitoring, post-dredge assessment study or the considerable time spent by City of Clatskanie personnel.

So far, Clatskanie has received $248,000 in grants. The community did not expect the amount of mitigation required by the permits, and consequently, the dredging costs that the city originally budgeted for are only half of the overall project cost. In order to cover the existing bills and perform the mitigation, they are pursuing additional sources of funding, knowing that failure to perform the mitigation will eliminate any chance of maintenance dredging in the future.


In the course of the project, Clatskanie realized the significant demands that the project would require in terms of finances and commitment. For the next several years, the community will be working to fulfill the terms of the permit and pay for the dredging and mitigation. One factor that greatly helped with the project was the active support from a state agency, which was OEDD in this case. OEDD kept the agencies at the table, continuing the negotiations, and served as a strong ally for the city.


Cynthia Lowe is a professionally registered engineer with more than four years experience with Ogden Beeman & Associates, Inc., Portland, Oregon. She received a BS in ocean engineering from Texas A&M University and an MS in civil engineering from Oregon State University. At Ogden Beeman & Associates, Inc., she works on dredging projects in addition to performing hydraulic modeling, conducting floodplain analyses, and developing port and marine facilities. This paper was originally presented at the 1998 meeting of the Western Dredging Association Pacific Chapter.

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