Biodiversity Guide - The Intertwine

egional conservation strategybiodiversity guideAppendix I Watersheds5. Johnson Creek WatershedMatt Clark, Johnson Creek Watershed CouncilThe Johnson Creek Watershed is 54 square miles(34,000 acres) and includes parts of five cities—Damascus, Gresham, Happy Valley, Milwaukie,and Portland—and two counties: Multnomahand Clackamas. Johnson Creek originates inthe foothills of Mount Hood near Boring, flowsgenerally westward for approximately 24 miles,and enters the Willamette River just south of theCity of Portland border, 18.5 river miles above theWillamette’s confluence with the Columbia River.The upper watershed is predominantly ruralresidential and agricultural (largely tree nurseries),with less than 10 percent impervious surface.The lower watershed is heavily urbanized and isdominated by residential, commercial, and industrialareas, with generally more than 25 percentimpervious surface. Developed land representsabout 40 percent of the watershed. As of 2006 thewatershed had an estimated 175,000 residents,making it one of the most densely populatedwatersheds in Oregon. More than 90 percent ofthe Johnson Creek watershed is within the currentMetro Urban Growth Boundary, with 6,000acres added to the UGB in the last decade. Additionalareas near Highway 26 in both Clackamasand Multnomah counties were designated asurban reserves in 2010.The Johnson Creek watershed represents 3percent of the greater Portland-Vancouver region.Within the region, 91 percent of the JohnsonCreek watershed falls within the Metro UrbanGrowth Boundary.Key Facts: The Johnson Creek watershed withinthe greater Portland-Vancouver region:n Includes about 3,200 acres that are within theFEMA 100-year floodplain and just over 500acres of mapped wetlands.n Has 39 percent developed land, second onlyto the Willamette River-Frontal Columbia Riverwatershed. This level of development is a strongcontributor to flooding in the watershed.Land coverJohnson Creek Watershed60,113 acres% of WatershedAgriculture 6%Regen. forest 30 acres.**Cities in Portland area UGB.n Has significant tree cover (41 percent), particularlyconsidering the relatively high level ofdevelopment. The tree cover helps offset floodproblems that otherwise would be worse.n Has a relatively high proportion (13 percentcover) of low vegetation. This includes substantialamounts of backyard and landscaping habitat.The northern side of the watershed west ofGresham and the southern side west of I-205 arerelatively flat, with deep, permeable, sedimentarysoils. This contrasts with the steeper slopes andlow-permeability silt soils of the volcanic buttes(the East Buttes) in the southeastern portionof the watershed, which explains why most ofJohnson Creek’s major tributaries come from thesouth. The exception is ground-water fed CrystalSprings Creek, the lowest major tributary, whichenters from the north. Other major tributariesinclude Veterans, Kelley, Butler, Sunshine, andBadger creeks, with Kelley and Crystal Springscreeks contributing most of the stream volume.The summer base flow of Johnson Creek frequentlyfalls below minimum standards establishedby the Oregon Department of Fish andWildlife for salmonids, and winter floods arecommon. (A total of 39 flood events have beenrecorded since 1941.) Flooding causes erosionand bank scouring within the basin, as well asproperty damage.Historically, the Johnson Creek subbasin was70 percent coniferous forest, 11 percent oak, 15percent burned forest, and less than 1 percentprairie. By 2010, this basin had become the mostheavily urbanized basin in the region, with 69percent of the basin converted to urban uses(i.e., portions or all of Clackamas, Gresham,Lake Oswego, Milwaukie, Oak Grove, Portland,and Tualatin). Another 10 percent of the basinhas been converted to agriculture. Together, thetransition to urban and agricultural uses hasconsumed 67 percent of the oak habitat and 100percent of the prairie. Combined coniferous andmixed forest decreased about 55 percent. Agricultureconsumed about 5,000 acres of conifer forest,and urban consumed 25,000 acres of conifer forestand about 5,000 acres of oak.Early farmers (circa 1850) initially increasedthe meandering of the creek to expand the floodplainand increase nutrient deposition. By the1930s, the watershed had substantially urbanizedand flooding came to be viewed as a problem—aview that continues to this day. To address floodingconcerns, the Works Progress Administrationwidened and straightened much of the lower 15miles of Johnson Creek, lining the channel withrocks. Today it is recognized that historical floodprevention efforts were largely counter-productive,and significant public and private investmenthas been made to reconnect Johnson Creek to itshistorical floodplain. In addition to mitigatingnuisance flooding, reconnecting the historicalfloodplain provides critical off-channel rearingand refuge habitat for native fish, includingsalmon and steelhead. Historically Johnson Creekhad large salmon populations, which declineddramatically with urbanization and the WPAchannelization mentioned above.Today, most of the forest is on the volcanicEast Buttes. Upland forests on the East Buttesgenerally range from 40- to 100-year-old secondgrowth that typically is a mixed conifer-deciduousforest in mid-successional stage. Invasiveweed species are a problem throughout thewatershed, particularly in the herbaceous andshrub layers. (Invasive weeds are discussed inmore detail below.)Native fish—particularly those tolerant ofwarm water—represented 99.7 percent of thespecies sampled in the watershed by the OregonDepartment of Fish and Wildlife between April2008 and February 2009. Despite precipitousdeclines, cold-water species such as ESA-listedChinook and coho salmon and steelhead andcutthroat trout are still present. Steelhead andcutthroat trout are found along most of JohnsonCreek, while coho and Chinook salmon havebeen found primarily in the lower mainstem andin Crystal Springs Creek. Western brook lampreyand Pacific lamprey also are present in lowerJohnson Creek and its tributaries.Little data are available about fish distributionin the upper mainstem of Johnson Creek. However,in December 2010, four wild coho salmon(three spawned-out carcasses and one live fish)were sighted on mainstem Johnson Creek at RiverMile 15, near the eastern border of Gresham,much farther upstream than they have beenrecently documented.In 2010, several populations of westernpearlshell freshwater mussels were found inupper Johnson Creek. Although still relatively272273