Mill Creek Side Channel (Leber 2014)

The final phase of the Mill Creek Side Channel (Leber) project first removed and disposed of 4,000 CY of arsenic and weed-contaminated soil from the site (this material was disposed of in Republic Services Subtitle D landfill in Klickitat County), and mixed another 4,000 CY of arsenic contaminated soil with clean soil and stockpiled this soil in two berms on-site above the 100-year floodplain. This work began in early May, 2016 and was completed by the end of June. Confirmation samples collected by AMEC Foster Wheeler consultants verified that all mixed and stockpiled soil was clean with arsenic and lead levels below the Ecology action levels of 20 and 250 milligrams per kilogram (mg/kg), respectively.

After the contaminated soil at the site was remediated, excavation of the floodplain wetland began. Nearly 20 of overburden was removed to get the channel bottom down to the level that would provide consistent refuge and rearing habitat for salmonids during the spring outmigration (January - June). By the middle of August, 2016, surveying revealed that 83,000 CY of soil had been removed, with 7,200 CY of fill placed in the two berms (see BEFORE and AFTER as-built figures loaded in the PRISM attachments). All told, approximately 1,125 LF of new side channel was created that is tributary to Mill Creek (Auburn) and the Green River, and 2.03 acres of new backwater area was created. A total of 51 acre-feet of new floodplain storage was created by this excavation that will reduce flooding in surrounding areas. At the end of excavation, 2 of compost (a total of 2.400 CY) was worked into all surface soils to ensure that adequate soil quality and organic matter was present for planting.

Next, large wood structures were added to the project site to help create a complex fish refuge and rearing habitat on the site. A total of 45 wood structures were installed adding 274 pieces of large wood (all were at least 15 long and 18 DBH with most 20 long, 24 DBH). This total large wood count included two extremely large cottonwoods that were found buried on-site (presumably from old lahar flows off of Mount Rainier) and were used as additional habitat logs.

Lastly, the entire disturbed area was hydroseeded, planted and mulched. A total of 6.0 acres of area was hydroseeded with grass seed mix to stabilize soils and reduce weed infestation in late September, 2016. Native riparian plants were planted on 4.7 acres between October and December -(totaled sf from planting plan = 205,953 sf and converted to acres = 4.72 acres) (not including the 5,824 plants that were planted in Phase I under grant 13-1098). All containerized plants were mulched with wood chip mulch in February and March, 2017. For further information about planting, see the Leber Phase II Planting As-Built loaded in the attachments in PRISM.

Since completion of planting and mulching, City of Kent Vegetation crews have maintained all plantings. This included weed whacking around plants to allow sunlight to reach all plants, and installing, maintaining and running an extensive irrigation system during the months of July, August and September to keep plants watered during an extremely dry and hot summer.

Monitoring of fish use occurred in February and March and revealed that three different species of salmonids were using the new habitat (Chinook, coho and chum). Inundation monitoring continues but has so far revealed that the new floodplain wetland was at least partially inundated consistently between January 1st and May 26th (146 days) during the Chinook outmigration period. Vegetation monitoring was conducted in late August and early September. Results will be compiled soon and a report prepared before the end of 2018.

The goal of the restoration project is to increase floodplain refuge habitat for Chinook and other salmonids, enhance riparian habitat, and increase floodplain storage. The side channel will be subject to backwater approximately 125 days per year when outmigrating salmon numbers are greatest.