Green Cove Creek Watershed

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Greencovecreek.jpg

Green Cove Watershed is the largest watershed on Cooper Point, and straddles the western Boundary of the City of Olympia. The headwaters of Green Cove Creek are a large intact wetland complex with portions protected by conservation easements, acquisitions, and a City of Olympia . The riparian corridor is comprised of mixed conifer and deciduous forest with only a few sites warranting improvement. Because it is relatively intact, Green Cove Creek is Olympia’s priority for salmon habitat protection measures.

The Green Cove Creek Comprehensive Drainage Basin Plan (1998) recommended minimum canopy cover of 60% within the watershed to prevent excessive stormwater impacts. The City of Olympia has downzoned the upper Basin and instituted more stringent development standards to maintain the forest cover and protect the creek. Although the City has instituted these standards, significant residential development pressures exist in this desirable area, especially new home construction. Tidal influence occurs in the estuary and at the mouth of the creek, allowing for a fresh and saltwater mixing despite the presence of some bulkheads. Green Cove has two fishways assessed by WDFW; one has retrofitted baffles which impact upstream adult and juvenile migration while the other is passing fish. (Thurston CD 2004

Planning

  • A Comprehensive drainage plan was adopted in 1998 by Thurston County - http://www.thurstoncountywa.gov/sw/Pages/basin-plan-greencove.aspx
  • Thurston CD 2004 describes salmon recovery strategies for the WRIA 13, including Green Cove Creek, and reports use by Chum and Coho Salmon, and Steelhead and Cutthroat trout with anadromous use over 3.4 miles of stream.
    • The plan proposes replacement of the culvert on Country Club Road, protection and enhancement of riparian forest, increasing large wood recruitment, and protection of hydrology form development impacts.
    • The plan proposes a riparian assessment, but that appears to have not occurred.

Hydrology

  • Cooke Environmental Services provided wetland assessments following hydrologic modelling in support of the 1998 Plan. The following observations are from the hydrologic summary.
  • Kaiser road serves as a weir separating Lake Lousie from the main stem creek and regulating lake discharge.
  • Some of the uppermost portions of Green Cove Creek, downstream of the Kaiser Road wetlands have been ditched.
  • Cooke speculates that increased runoff from development may increase flooding in some modified wetlands dominated by reed canarygrass.
  • Forested wetlands are most vulnerable to modified hydrology. Frog eggs are most vulnerable to modified hydrology in Grass Lakes.
  • The estimated surface storage (ESS) of the Lake Louise/Grass Lake complex is 509 acre feet. ESS of Kaiser Road wetlands are 294 acre feet. ESS of Evergreen Parkway wetlands are 185 acre feet.
  • The Lake Lousie basin is dominated by till soils, so soil saturation strongly affects discharge to the lake.
  • The storage of the Kaiser wetlands strongly mitigates stormwater flows, moreso than the Evergreen Parkway wetlands.
  • Roads discharge to the stream at Evergreen Parkway, 36th Street, and Green Cove Street. Several drains discharge to the creek from residential development west of the creek on Green Cove Street.
  • The lack of wetlands storage, steeper slopes, lower soil infiltration among compacted tills, and other landscape attributes results in flashier flow in the creek, north of Evergreen Parkway. In addition the seasonal stream that joins Green Cove Creek immediately before Evergreen Parkway (associated with the Olneys Garden Subdivision Proposal) has lower storage and may contribute flow.
  • Based on well logs, the actual extent of outwash soils may be less than mapped, reducing basin infiltration.
  • Hydrologic modelling of build-out indicate that peak flow at 36th Ave would increase by 30% resulting in stream instability.
  • The 1998 study does not model increased winter precipitation predicted by climate change.



Conservation

Development