The Yamhill Rivers Nature Reserve: a Proposal

By Neyssa Hays


     The South Yamhill and Yamhill Rivers combined run through 71 miles of Northwest Oregon farmland and small towns, including Sheridan, McMinnville, Dundee, and Dayton (from west to east). A tributary of the Willamette River, the bi-river system has become an important spawning ground for the threatened and evolutionarily significant unit (ESU) of Oregon Coastal Coho Salmon (Oncorhynchus kisutch) as well as a winter refuge for Chinook Salmon (O. tshawytscha) smolts and juveniles, and the historic grassland prairies surrounding the lower sections were home to the endangered Fender’s Blue Butterfly (Icaricia icarioides fenderi) as well as other enlisted species (Good et al. 2005, McIntire et al. 2007, Togstad 2011).

     Creating the Yamhill Rivers Reserve, a nature reserve that stretches the length of the South Yamhill and Yamhill River system at a width of at least 300 feet on each side of the river, would form a riparian zone of 5370 acres of contiguously protected habitat with minimal loss of agricultural land for any one farmer or business. In addition, adding a parcel of roughly 700 acres along the river that includes a small wetland area would provide land that could be restored to oak savannah and grassland prairie habitat.

     The source of the South Yamhill River is located at coordinates 45.110556, -123.727778, in the foothills on the east side of the Coast Range, at an elevation of roughly 551 feet (Google maps 2012). The river then flows east. By the time it joins with the North Yamhill River to become the Yamhill River, it has dropped to an elevation of 75 feet; the Yamhill River continues the descent to 59 feet at its confluence with the Willamette River. The Yamhill Basin watershed varies in elevation from a high of over 3400 feet at the peak of Trask Mountain to a low of roughly 59 feet at the confluence of the Yamhill River and the Willamette River (Bash & Ishii, eds. 2002). The watershed, including the Yamhill River system, was carved into rolling hills and an expansive flatland by the Missoula Floods, which also deposited granite, quartzite, and slate. The river floodplains are composed predominantly of deep alluvial deposits of sand, gravel and silt over sedimentary rock. Average annual rainfall is 50 inches or less, most of which falls between November and March; temperatures are mild with mean winter temperatures in the low 40’s (F) and high summer temperatures averaging in the low 80’s (F).

Conservation Aims

     Over the last twenty years, the population of Yamhill County has experienced a higher growth rate compared to the rest of the state and is predicted to grow from 101,000 to nearly 156,000 people over the next thirty years, with the highest concentrations in the towns near the South Yamhill River (Bash & Ishii, eds. 2002). As populations grow, pressure to subdivide and build on the land along the river will increase. Most of the land surrounding the Yamhill River system is currently farmed, and while some farmers allow a wide margin between plowed land and the river, many farmers work the land within a few feet of the embankment, a practice that leads to high soil erosion, muddied rivers, warm water temperatures, and higher run-off of farm chemicals (fertilizer, pesticides, and herbicides). Creating a wide, protected riparian zone would buffer the rivers from these effects and will ensure a healthier ecosystem for future generations.

     In addition to safeguarding habitat for Coho Salmon and Fender’s Blue Butterflies, the Yamhill Rivers Reserve would protect several other species, including plants, insects, birds,amphibians, mammals, and other fish (some threatened or endangered, others of concern; USFWS 2012). Threatened and endangered plant species that would potentially benefit from this reserve are Water Howelia (Howelia aquatilis), Willamette Daisy (Erigeron decumbens var. decumbens), Kincaid’s Lupine (Lupinus sulphureus ssp. kincaidii), and Nelson’s checkermallow (Sidalcea nelsoniana; CPC 2010). The last two plants are important food sources for the Fender’s Blue Butterfly. Other species of concern that may benefit from the reserve include the Streaked Horned Lark (Eremophila alpestris strigata), the Southern Torrent Salamander (Rhyacotriton variegates), the Long-eared Myotis Bat (Myotis evotis), Coastal Cutthroat Trout (Oncorhynchus clarki ssp) to name a few.

     The most obvious landscape features of interest for the Yamhill Rivers Reserve are the rivers themselves. Historically, the rivers wound through habitats such as mixed conifer-deciduous foothills and oak savanna (Bash and Ishii, eds. 2002). The latter of the two is perhaps the least obvious landscape feature of interest considered in this plan, though of high significance. Oak savanna and associated grassland prairie were once abundant habitats throughout much of North America and provide important food sources and refuge for a variety of organisms, including Fender’s Blue Butterfly, Streaked Horned Lark, Pygmy Rabbits, game birds, deer, and many others. Because the land on which they grow is prized for agriculture, today less than 0.5% of natural prairies and oak savanna remain and much of that has been severely affected by invasive species (McIntire et al. 2007).


     In 1996, the Oregon State University Extension Service surveyed Yamhill County residents and found that over 90% of respondents supported continuing strategic planning for water quality and watershed management (Bash and Ishii, eds. 2002). The South Yamhill River is listed under section 303 of the Federal Clean Water Act with concern over water quality issues including “[high] temperature, flow modification, and bacteria,” all having harmful effects to many stream organisms, including salmon. Riparian zones counter all of these negative effects, and the longer and wider the continuous zone is the more effective it becomes. Cool water temperatures are of utmost importance throughout the salmon life history and deep riparian zones with tall trees are ideal for shading and cooling rivers and streams. Recent studies in Ireland have shown that a mix of dense canopy and sporadically open areas create conditions beneficial to macroinvertebrates that are important food sources for salmon (McCormick and Harrison 2011); this condition can be created in the initial stages of the reserve through the planting of fast-growing, large native riparian species such as alder, willow, and cottonwood as well as smaller shrubs such as elderberry and spirea. As riparian zones age, debris from falling trees and other plants serve to modify the water flow and offer refuge to fish and other animals. The root systems of the plants and soil of riparian zones act as natural filter systems against bacteria and chemical pollutants.

     In addition to maximizing the filtering system described above, the 300-foot width of the Yamhill Rivers Reserve is necessary to encourage species diversity. It is currently standard practice (and law in many countries and states in the US) in the timber industry to leave a riparian buffer when cutting timber; such buffer strips vary in width from an average of 50 feet to a high of 165 feet (Whitaker and Montevecchi 1999 and Lee et al. 2003). While studies have shown that these widths are ample to moderate edge effects on trees along riverbanks, response of bird  populations varies with width of riparian zones (Whitaker and Montevecchi 1999 and Harper et al. 2007). Whitaker and Montevecchi (1999) found that in riparian buffer zones of any width populations of birds generally associated with river habitats resembled those of uncut river areas. Interior forest bird populations, however, increased somewhat with increasing width, and the scientists postulated that it was likely that wider riparian buffer zones would have a positive influence on these populations.

     Similarly, studies of Fender’s Blue Butterflies have shown that populations respond positively to larger patches of habitat (McIntire et al. 2007). Studies over a ten-year period followed by model simulations indicate that increasing butterfly refuges to at least 340 acres of connected patches have the potential to increase populations from the current 5,000 individuals to upwards of 65,000. Prairie acreage of this size would also be beneficial to the Streaked Horned Lark, which has been shown to require open areas of over 300 acres to support a healthy nesting population (FWS 2011). The remaining ~350 acres of the 700 acres planned would be restored to oak savanna, which would serve a variety of native animals that have lost most of their habitat to farming during the last two centuries.

Trophic Level Considerations

     Players in the trophic levels will depend to some respect on whether they are aquatic or terrestrial species, although there are certainly crosses as well. In the river, the top trophic level is most likely to be the salmon; with smaller fish and macroinvertebrates at the second level; and in the first level a combination of detritus (including salmon carcasses), higher level plants, and algae.

     The Yamhill River system was previously “cleaned” of woody debris used by all trophic levels as habitat, food, or substrate; subsequent winter flooding washed away gravel imperative to spawning (Bash and Ishii, eds. 2002). Management actions will include planting of fast growing, tall species of trees as well as slower growing trees to provide shade and eventual deadwood for all trophic levels. It may be necessary to include woody debris in the initial restoration projects as well as laying down appropriate gravel.

     Terrestrial primary producers along the riparian corridor will include black cottonwood, alder, willow, and understory plants such as elderberry and spirea in the initial stages, followed by such slow-growing species as big-leaf maple, bitter cherry, Douglas fir, and western red cedar. Some early producers and saplings of slow-growing species will require being planted while others will likely self-propagate once the land is no longer being cleared for farming. The oak savanna and grassland prairie will need to be extensively planted with native species and monitored for control of invasive species. Herbivores, the second trophic level, will include Pygmy Rabbits, Pocket Gophers, several species of birds, deer, and Fender’s Blue Butterflies; some of these species, such as deer, will arrive autonomously while the populations of others, such as Pygmy Rabbits and Fender’s Blues, will need to be transplanted after the plants are well established. Likely the top trophic level will be dominated by coyotes and foxes, but will also include birds such as osprey, hawks, eagles, and owls as well as minute predators such as Myotis Bats. It is not out of the question, however, that cougars would also use the riparian corridor, though this is likely to take several years.

Stakeholder concern

     Of greatest concern to stakeholders would be the loss of land used for farming, timber, or development. A square acre is 208 feet per side, and one mile is 5280 feet long; so for every mile of riparian zoning, a landowner would stand to lose roughly 38 acres on each side of the river. Additionally, much of the upper end of the river runs through land owned by the Confederated Tribes of Grand Ronde or its members. While they may be supportive of this  plan, many of their tribal members are farmers and would be hard-pressed to give up their farmland for a nature reserve. In the bottomland of the river about five miles west of McMinnville, Riverbend Landfill operates right up to the edge of the river and in the last few years management expanded their operation, putting in a state-of-the-art waste disposal system.

     Further down river, the South Yamhill flows directly through McMinnville and in several places the highway and other roads cross over. These areas cannot be protected or restored at present and it is not likely that they will be in the future either. However, local planners are already establishing urban growth boundaries (UGB’s), which could include riparian zoning (Bash and Ishii, eds. 2002). Throughout the watershed, water quality would benefit by replacing culverts with bridges.


     The first management actions would be to make the proposal to the community and listen to the concerns of the primary stakeholders. Management would need to educate the stakeholders on the benefits of riparian buffering and healthy water systems, such as reduced flooding and bank erosion, and decreased need for fertilizing because the riparian zone would support more native pollinators. Perhaps there could be incentives for stakeholders to support the plan as well. Likely, the 700-acre parcel for oak savanna and grassland prairie would need to be purchased. All involved parties then would try to come to an agreement.

     Once the area is established, subsequent management actions should be first to design and carry out initial water quality and wildlife studies. Once a baseline is established, management will engage in removal of invasive species, planting native species, and continued monitoring of the ecological health of the area. Additionally, stakeholders will be invited to regular informational meetings to discuss progress and concerns.

Future Predictions

     The 100-year predictions for Pacific salmon are dire, with most populations in the lower latitudes going extinct due to climate change. However, if river systems such as the Yamhill can be set-aside as salmon sanctuaries and the waters cooled enough, the salmon stand an increased chance of survival. Many of the tree species of older riparian corridors are long-lived species with varying growth rates. In 100 years a Douglas fir may have reached nearly its full 230 feet and stand another 800 years growing slowly in diameter, while the oaks in the savanna will have only reached half of their full 85 feet and may persist another 250 years. Several of the oaks in the savanna will have lost limbs and become hosts for cavity dwellers, including Wood Ducks, Acorn Woodpeckers, and bats. Along the river, Black Cottonwoods would likely out-compete the Red Alder and dominate the embankment. Logs and debris from fallen trees as well as water-loving willows will have created a complex river scene. McIntire et al. (2007) predicted populations of Fender’s Blues in a 300-acre system would stabilize after 25 years to between 50,000 and 65,000 individuals. Other populations of short-lived species such as songbirds, bats, and rodents will likely have reached their carrying capacity as well and will have settled into relatively stable populations. Longer-lived species such as deer and coyotes will likely still be increasing in population.

     In 1000 years the area will have experienced some climax communities and some of the Douglas firs would be nurse logs for species such as Western Hemlock as well as under-story plants such as huckleberry, salal, and snowberry. Pileated woodpeckers will likely be heard searching for food and making homes in snags and diseased trees. The oak savanna as well will have seen replacements and successional changes. Historically oak savanna and grassland prairies were maintained through fire, both controlled and natural. It is conceivable that future management practices would also include controlled fire; this would have the desirable effects of removing many invasive species and ridding the area of tree-damaging fungus and disease.

Potential Impacts

     Because Coho Salmon are not native to the Yamhill River system, it is possible that  their increased presence would have a negative effect on other species in the river system. The Coho currently spawning in the Yamhill Rivers are naturally returning descendents of released fish from a far-off hatchery, a practice that was discontinued in 1997 after nearly fifty years (Togstad 2011). However, on a whole, wild Pacific salmon numbers are dwindling for many reasons, including climate change, over-fishing, and competition with hatchery-reared salmon; supporting populations that are now naturally spawning has the potential for preserving a species that is struggling in its historic rivers. Other impacts of the riparian zone include higher biodiversity, decreased run-off, and cleaner, cooler waters.

     The Yamhill Rivers Reserve would improve water quality in the South Yamhill and Yamhill Rivers, and increase Coho salmon and Fender’s Blue butterfly populations for many future generations. Moreover, it could become a model for riparian management systems throughout the Pacific Northwest and beyond.



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