On May 1, the Portland District closed a public comment period on a draft Environmental Assessment (EA) for the federal navigation channel dredging and dredge material placement.
The Corps is proposing operations and maintenance dredging and dredge material placement for four federal navigation channels at Skamokawa Creek, Westport Slough, Old Mouth Cowlitz River, and Upstream Entrance to Oregon Slough. The dredged material would be placed in the Corps’ existing network of upland placement sites and in the Columbia River from River Mile 3 through 145 where water is 20 feet deep or greater.
The Corps preferred plan, identified in the EA, would perform maintenance dredging with mechanical and hydraulic dredges.
The typical volume of dredged sediment removed per year and authorized channel depth for each project site includes: Skamokawa Creek, 6.5 feet, 15,000 cubic yards; Westport Slough, 20 feet, 30,000 cubic yards; Old Mouth Cowlitz, 8 feet, 40,000 cubic yards; Upstream Entrance to Oregon Slough, 10 feet, 75,000 cubic yards.
The Skamokawa Creek area has not been dredged by the Corps since 1992. In 1993, local interests dredged approximately 5,000 cubic yards from the channel. Based on recent river depth surveys, approximately 60,000 cubic yards of material has shoaled in the authorized channel to the advanced maintenance depth of 8 feet.
Based on input received from river users, the Corps is proposing to dredge Westport Slough to a channel depth of 20 feet below CRD (Columbia River Datum) to allow ocean-going barge access. Westport Slough is authorized to a depth of 28 feet below CRD and a width of 200 feet, but has recently been maintained to a depth of 9 feet below CRD for the Wahkiakum Ferry. The Corps anticipated that Westport Slough may be dredged two or three times every five years. Based on recent river depth surveys, approximately 195,000 cubic yards of material has shoaled in the authorized channel to the advanced maintenance depth of 22 feet.
The Old Mouth Cowlitz River is at river mile 67 on the north side of the main-stem Columbia River in Longview, Washington. The site is no longer an active component of the Cowlitz River drainage and serves as an access channel for chip barges at the local WestRock paper mill (formerly Kapstone Kraft Paper Corporation and Longview Fibre) as well as the heavy civil marine construction company J.E. McAmis facilities.
The Old Mouth Cowlitz River channel is authorized to a depth of 8 feet below CRD and 150 feet wide. A private contractor for Longview Fibre dredged the area three times from 2009 to 2014. In 2018, a private contractor for the Kapstone Kraft Paper Corporation partially dredged to a depth of -10 ft CRD plus 2 feet of over dredge.
The Corps is proposing to dredge this area two or three times every five years. Based on recent river-depth surveys, approximately 80,000 cubic yards of material has shoaled in the authorized channel to the advanced maintenance depth of 10 feet.
The Oregon Slough area was last dredged in 2001 when a clamshell dredge removed 55,799 cubic yards of material. The current project would like to revisit the area one every five years. The authorized depth is 10 feet below CRD.
Based on recent river-depth surveys, approximately 130,000 cubic yards of material has shoaled in the authorized channel to the advanced maintenance depth of 12 feet.