Predicting Spread of Invasive Exotic Plants into 

Dewatered Reservoirs After Dam Removal on the 

Elwha River, Olympic National Park, Washington

Open-File Report 2011–1048

U.S. Department of the Interior

U.S. Geological Survey
Cover: Scot’s broom (Cytisus scoparius) beside Elwha River, Olympic National Park 

Washington. Photograph taken by Joshua Chenoweth in 2008. 
Predicting Spread of Invasive Exotic Plants into 

Dewatered Reservoirs After Dam Removal on the 

Elwha River, Olympic National Park, Washington

By Andrea Woodward and Christian Torgersen, U.S. Geological Survey, and Joshua Chenoweth, 

Katherine Beirne, and Steve Acker, National Park Service

Open-File Report 2011-1048

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011 

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Suggested citation:

Woodward, Andrea, Torgersen, Christian, Chenoweth, Joshua, Beirne, Katherine, and Acker, Steve, 2011, Predicting 

spread of invasive exotic plants into de-watered reservoirs following dam removal on the Elwha River, Olympic 

National Park, Washington: U.S. Geological Survey Open-File Report 2011-1048, 64 p.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 1

Background................................................................................................................................................................ 1

Geomorphic Template for Invasion ............................................................................................................................ 2

Invasion Dynamics..................................................................................................................................................... 2

Objectives .................................................................................................................................................................. 3

Methods......................................................................................................................................................................... 3

Surveys of Invasive Species ...................................................................................................................................... 3

Identification of Focal Species ................................................................................................................................... 4

Relations between Species and Environment............................................................................................................ 4

Invasive Communities................................................................................................................................................ 5

Wind Dispersal........................................................................................................................................................... 6

Long-Distance Dispersal Mechanisms of Focal Species ............................................................................................... 6

Cirsium arvense ......................................................................................................................................................... 6

Cytisus scoparius....................................................................................................................................................... 6

Geranium robertianum............................................................................................................................................... 7

Hypericum perforatum................................................................................................................................................ 7

Lathyrus sylvestris...................................................................................................................................................... 7

Phalaris arundinacea ................................................................................................................................................. 7

Rubus armeniacus and Rubus lacinatus.................................................................................................................... 8

Distribution of Invasives................................................................................................................................................. 8

Spatial Distribution ..................................................................................................................................................... 8

Factors Predicting Species Distribution...................................................................................................................... 9

Associations among Invasive Species ..................................................................................................................... 10

Wind Vector................................................................................................................................................................. 10

Wind Direction and Speed during Seed Dispersal ................................................................................................... 11

Effect of Topography on Wind Direction................................................................................................................... 11

Water Vector................................................................................................................................................................ 12

Mammal Vectors.......................................................................................................................................................... 13

Bird Vectors ................................................................................................................................................................. 14

Road- and Trail-Associated Vectors ............................................................................................................................ 14

Priority Areas ............................................................................................................................................................... 15

Pre-Dam Removal Treatments................................................................................................................................. 15

Setting Priorities for Treatments and Species .......................................................................................................... 16

Species Locations Outside of Olympic National Park .............................................................................................. 16

Post-Dam Removal Vulnerability.............................................................................................................................. 16

Conclusions ................................................................................................................................................................. 17

Acknowledgments........................................................................................................................................................ 18

References Cited......................................................................................................................................................... 18
iv

Figures

Figure 1. Locations of the Elwha and Glines Canyon Dams and respective reservoirs on the Elwha River, Olympic 

National Park, Washington. ......................................................................................................................................... 23

Figure 2. Project area, Elwha River, Olympic National Park, Washington. ................................................................. 24

Figure 3. Subareas used to determine predictive variables for occurrence of invasive plant species, Elwha River, 

Olympic National Park, Washington. ........................................................................................................................... 25

Figure 4. Kernel density representation of ‘hot spots’ of 30 invasive species (table 1; high to low density 

represented by hot to cool colors), isolated observations (green points), and area surveyed during 2001 and 2008 

(lilac), Elwha River, Olympic National Park, Washington............................................................................................. 26

Figure 5. Kernel density representation of ‘hot spots’of focal invasive species (high to low density represented by 

hot to cool colors), isolated observations (green points), and area surveyed during 2001 and 2008 (lilac), Elwha 

River, Olympic National Park, Washington. ................................................................................................................. 27

Figure 6. Cluster analysis of Bray-Curtis resemblances among survey sites.............................................................. 30

Figure 7. Non-metric multidimensional scaling ordination of sites............................................................................... 31

Figure 8. Locations of eleven ordination groups ......................................................................................................... 32

Figure 9. Summary of distribution of wind speed and direction (source) from RAWS station at Hurricane Ridge....... 33

Figure 10. Effect of topography on wind measured at 15 m/s from the north at the Hurricane Ridge RAWS station 

predicted by WindNinja................................................................................................................................................ 34

Figure 11. Effect of topography on wind measured at 12 m/s from the east at the Hurricane Ridge RAWS station 

predicted by WindNinja................................................................................................................................................ 35

Figure 12. Effect of topography on wind measured at 18 m/s from the south at the Hurricane Ridge RAWS station 

predicted by WindNinja................................................................................................................................................ 36

Figure 13. Vegetation height surrounding Lake Mills and Lake Aldwell reservoirs based on LiDAR data................... 37

Figure 14. Species with distribution patterns consistent with water dispersal ............................................................. 38

Figure 15. Locations of one radio collared elk tracked by the Lower Elwha Klallam Tribe from February 2009 to 

November 2009 ........................................................................................................................................................... 43

Figure 16. Locations of two radio-collared elk tracked by Olympic National Park staff, one from September 2008 to 

June 2009, and one from September 2009 to February 2010 ..................................................................................... 44

Figure 17. Locations of observed and likely beaver sign, Elwha River, Olympic National Park, Washington, 2007.... 45

Figure 18. Distribution of most likely bird-dispersed invasive plant species compared with highest densities of most 

likely avian dispersers (red ovals), Elwha River, Olympic National Park, Washington................................................. 46

Figure 19. Schematic description of the approach to identifying areas and environments within areas that should 

be treated for invasive plant species prior to dam removal and that will be vulnerable to invasion after dam removal.47

Figure 20. Potential source areas of vectors of invasive species that could impact the de-watered reservoirs. 

Source areas are designated for wind (yellow), birds (orange), and mammals (blue) ................................................. 48

Figure 21. Environments (green within black lines) where C. arvense is most likely to occur north of Lake Aldwell... 49

Figure 22. Environments that are potential sources of invasive species (green areas within black lines) in the area 

south of Lake Aldwell to the Elwha Campground......................................................................................................... 50

Figure 23. Environments that are potential sources of invasive plants (green areas within black lines) between 

Elwha Campground and Lake Mills ............................................................................................................................. 51

Figure 24. Environments that are potential sources of invasive plants (green areas within black lines) along Lake 

Mills up to Geyser Valley ............................................................................................................................................. 52

Figure 25. Environments that are potential sources of invasive plants (green areas within black lines) in Geyser 

Valley. Elk and trail users are the most likely vectors to spread invasive plants to the de-watered reservoirs............. 53

Figure 26. Digitized records of invasive plants on the Olympic National Forest, west of the Elwha reservoirs. .......... 54
v

Figure 27. Probability of invasive plant establishment in areas of the de-watered Lake Aldwell reservoir determined 

using logistic regression equations based on observations collected between the Glines Canyon Dam and Highway 

101 for (A) all invasive species observations, and (B) focal species observations....................................................... 55

Figure 28. Probability of invasive plant establishment in areas of the de-watered Lake Mills reservoir determined 

using logistic regression equations based on observations collected between the Glines Canyon Dam and Highway 

101 for (A) all invasive species observations, and(B) focal species observations. ...................................................... 56

Figure 29. Predicted areas between Glines Canyon Dam and Highway 101vulnerable to invasive plants using 

logistic regression equations based on observations collected in this area for (A) all invasive species observations, 

and (B) focal species observations compared with those observations. ...................................................................... 57

Tables

Table 1. Invasive species surveyed in the Elwha River valley, Olympic National Park, Washington.......................... 58

Table 2. Total number of observations, number of 50×50 m grid cells containing focal species, and percentage of 

grid cells by subarea with species present, Elwha River, Olympic National Park, Washington. .................................. 59

Table 3. Maximum likelihood odds ratios for predictor variables and cross-validation results for logistic regression 

models predicting the occurrence of invasive species in sites across the entire study area and by subarea, Elwha 

River, Olympic National Park, Washington. ................................................................................................................. 60

Table 4. Falling velocities of focal species, other species of concern and related species reported by Tackenberg 

(2001). ......................................................................................................................................................................... 61

Table 5. Summary of wind speed and direction detected at Hurricane Ridge RAWS station for July 15–September 

30, 1999–2008, Olympic National Park, Washington................................................................................................... 61

Table 6. Comparison of wind speed and direction predicted by WindNinja with actual conditions at Glines Canyon 

Dam, Elwha River, Olympic National Park, Washington.............................................................................................. 62

Table 7. Potential destinations of Cirsium arvense seeds from present populations as a result of wind dispersal 

from the most common wind patterns based on output from the WindNinja model, Elwha River, Washington. .......... 62

Table 8. High priority potential sources of exotic species as a result of wind dispersal from the most common wind 

patterns based on output from the WindNinja model, Elwha River, Washington. ........................................................ 62

Table 9. Summary of species traits for focal species and species of high concern that can be used to prioritize 

species threat, and the resulting priority. ..................................................................................................................... 63

Table 10. Summary of areas, species of concern that are already present (searched areas), or potentially present 

(areas not searched), and potential management actions, Elwha River, Washington. ................................................ 64
vi

Conversion Factors and Datum

SI to Inch/Pound 

Multiply By To obtain

hectare (ha) 2.471 acre

meter (m) 1.094 yard (yd)

square meter (m2) 10.76 square foot (ft2)

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

millimeter (mm) 0.03937 inch (in.)

kilometer per hour (km/h) 0.6214 mile per hour (mi/h)

square kilometer (km2) 247.1 acre

milligram (mg) 0.00003527 ounce, avoirdupois (oz)

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD83).
1

Predicting Spread of Invasive Exotic Plants into 

Dewatered Reservoirs After Dam Removal on the 

Elwha River, Olympic National Park, Washington

By Andrea Woodward and Christian Torgersen, U.S. Geological Survey, and Joshua Chenoweth, 

Katherine Beirne, and Steve Acker, National Park Service

Abstract

The National Park Service is planning to start the restoration of the Elwha River ecosystem in 

Olympic National Park by removing two high head dams beginning in 2011. The potential for dispersal 

of exotic plants into dewatered reservoirs following dam removal, which would inhibit restoration of 

native vegetation, is of great concern. We focused on predicting long-distance dispersal of invasive 

exotic plants rather than diffusive spread because local sources of invasive species have been surveyed.

We included the long-distance dispersal vectors: wind, water, birds, beavers, ungulates, and users of 

roads and trails. Using information about the current distribution of invasive species from two surveys, 

various geographic information system techniques and models, and statistical methods, we identified 

high-priority areas for Park staff to treat prior to dam removal, and areas of the dewatered reservoirs at 

risk after dam removal. 

Introduction

Background 

The National Park Service (NPS) is planning the restoration of the Elwha River ecosystem in 

Olympic National Park by removing two high head dams beginning in 2011, which have been in place

for more than 90 years. Several specific goals include restoring native anadromous fish runs and 

sediment transport to the coast, maintaining water quality, and re-vegetation of 276 ha of barren soil in 

the former reservoirs (Hoffman and Winter, 1996). These goals are inter-related such that restoring 

native vegetation provides necessary services to achieve the goals for fish and water quality (for 

example, sediment stabilization, shade). The potential for dispersal of exotic plants into dewatered

reservoirs following dam removal, which would inhibit restoration of native vegetation, is of great

concern (Lenhart, 2000; Shafroth and others, 2002; Zedler and Kercher, 2004; Orr and Stanley, 2006; 

Auble and others, 2007). Among the many potential ecological effects of invasive plants, a particular

threat is the inhibition of establishment of native trees, thereby preventing shade and large woody debris 

inputs adequate for fish habitat (Boersma and others, 2006; Harrington and Reichard, 2007). 

Consequently, preventing establishment of exotic plants is the highest priority of Olympic National Park 

for management of the former reservoirs (Chenoweth and others, 2011). This project will (1) identify 

the most threatening sources of invasive plants in time for park staff to manage them, and (2) predict 
2

which areas of the former reservoirs are most at risk for exotic invasion and therefore needing the 

greatest focus of limited resources following dam removal.

The dam-removal project is happening in a complex jurisdictional context. The Olympic 

National Park boundary encompasses the upper parts of the Elwha River, the Lake Mills Reservoir, and 

part of the middle river section between the Elwha and Glines Canyon Dams (fig. 1). These areas 

comprise more than 80 percent of the drainage basin. At this time, the NPS also owns the Lake Aldwell 

Reservoir with eventual dispensation of the area to be determined. The land at the lower section of the 

river downstream of the dams is a mixture of privately owned lands, as well as lands managed by the 

Olympic National Forest, the Lower Elwha Klallam Tribe (LEKT), and the Washington Department of 

Fish and Wildlife. Parts of the estuary and nearshore are managed by Clallam County, the City of Port 

Angeles, and LEKT. Restoration of the drainage basin will be a multi-agency project. 

Geomorphic Template for Invasion

The Elwha River flows from south to north, emptying into the Strait of Juan de Fuca (fig. 1). 

The Elwha Dam impounds Lake Aldwell at river km 7.9; the Glines Canyon Dam impounds Lake Mills 

at river km 13.7. This dam removal project will be the largest dam removal ever done in the United 

States (Gregory and others, 2002) and will expose deep layers of fine and coarse sediments to erosion. 

After dam removal, the former reservoirs will undergo dramatic morphologic and biologic succession. 

The free-flowing river is expected to carve a channel through the lakebed sediments leaving terraces and 

floodplains on its edges and removing 15–35 percent of the coarse sediments and 50 percent of the fine 

sediments (U.S. Department of Interior, 1996). The expected landforms will include upland slopes in 

one-half of the exposed area and terrace/riparian areas in the other one-half of the exposed area. The 

resultant spatial pattern of sediment deposition will reflect current distribution of fine sediments in deep 

and/or tranquil water and coarse sediments in shallow water and around points of land or any place 

experiencing wave action (Keddy, 1985). 

Invasion Dynamics

Early efforts to predict the invasive ability of organisms focused on traits of the invader and of 

the community being invaded (Myers and Bazely, 2003). Invasion dynamics generally have been 

modeled as diffusion events (Hastings and others, 2005) using various mathematical approaches [for 

example, reaction-diffusion models (Buchan and Padilla, 1999), and integrodifference equations (Kot 

and others, 1996)]. More recently, invasion has been recognized as having two phases of dispersal. 

Specifically, diffusive dispersal occurs near the parent plant, usually zero to a few tens of meters for all 

plant life forms (Howe and Smallwood, 1982). In contrast, long-distance dispersal is a result of unusual 

stochastic events that move plant propagules over long distances, creating new foci for diffusive 

dispersal (Myers and Bazely, 2003). Long-distance dispersal can be defined as a minimum distance (for 

example, 100 m as used by Cain and others, 2000) or as some portion of the tail of a dispersal curve. 

Because there are few datasets to describe dispersal curves, we define long-distance dispersal as being 

greater than 100 m. 

The reservoirs present a particular set of conditions for plant invasion. Specifically, 

characteristics of existing plant communities are not relevant because they do not exist, although 

competition with propagules of native plants will occur after the water is gone. Additionally, longdistance dispersal is of greater concern than diffusion because management staff would like to remove 

new foci before they begin to diffuse. Predicting the most likely sites for foci will increase the 

probability that foci will be located while they are still small. 
3

Objectives

Management staff of Olympic National Park requires three basic types of information in order to 

minimize the effect of exotic plant invasion that might hamper efforts to restore native vegetation in the 

former reservoirs of the Elwha River:

1. Identification of source areas to concentrate exotic control efforts before dam removal based on 

current distribution of exotics and flow of vectors. 

2. Identification of vulnerable sites to concentrate exotic plant control in reservoirs after dam removal. 

3. Reservoirs will be accessible only by foot during 16 months of dam removal during which native 

and exotic seed rain, and dispersal by foot-travelers, will be driving re-vegetation. Park management 

needs predictions of vulnerable locations to treat and possibly plant with native plants during dam 

removal. 

Park management will use this information (1) to prioritize areas for treatment by the Exotic Plant 

Management Team, and (2) to plan for effective planting of the former reservoirs. Meeting these needs 

constitutes the objectives of this project. Although this project addresses a specific circumstance, it also 

illustrates a methodology that can be applied to exotic plant datasets held by other land- management 

units to address other particular information needs. 

Methods

Surveys of Invasive Species

The current distribution of invasive species was determined by surveys done in 2001 (Olson and 

others, 2001) and 2008 (as part of this project). The 2001 survey was done by foot and boat, as 

appropriate. Areas surveyed included the immediate shoreline (that is, within 3–10 m) of reservoirs; 

reservoir deltas; the river corridor between the reservoirs to at least 50 m from water’s edge; the western

side of the Elwha River to Herrick Road; roads within the park boundary; West Elwha, West Lake Mills 

and Wolf Creek Trails; the power line corridor from the Glines Canyon Dam powerhouse to the park 

boundary; and administrative areas, such as the ranger station, campgrounds, stock corrals, etc. (fig. 2). 

Occurrences of 26 target species (table 1, Survey 2001 column) were mapped using GPS or topographic 

maps. These species were selected using professional judgment based on species known to occur in the 

Elwha drainage, species listed by the Washington State Noxious Weed Board, the ability of species to 

invade undisturbed vegetation or riparian areas, and the difficulty of control. Population occurrence was 

mapped by marking the center of the population or a nearby location (if surveying from a boat). The 

population size was recorded in four categories: (1) 1–10 plants, (2) 11–100 plants, (3) 101–1,000 

plants, or (4) greater than 1,000 plants. Extensive populations were marked with a GPS waypoint every 

50–100 m, and only federal land was surveyed. 

The 2001 survey was augmented in 2008 by a survey of Geyser Valley, the road north of the 

park boundary to U.S. Highway 101, the Hot Springs Road upstream of Glines Canyon Dam, and near 

Elwha Dam. Trails intersecting these areas were surveyed to 610 m elevation. This survey also was 

done on foot or by boat and included 4 species in addition to the 26 target species (table 1). In addition 

to location and population size, several environmental variables also were noted—distance to water, 

woody cover, habitation category (herbaceous, forest, shrubland, and lawn), the three most abundant 

associated species, and area infested. A total of 2,297 point locations were documented in the 588.2 ha 

searched across both surveys. 
4

Spatial distribution of invasive species is displayed using kernel density maps. These maps use a 

color ramp to describe the density of points within cells. They are a refinement of histograms or 

frequency plots based on a non-parametric estimation of a probability density function and often are 

easier to visually interpret than a plot of individual points. 

Identification of Focal Species

We identified seven focal taxa for detailed analysis—Canada thistle (Cirsium arvense), Scot’s 

broom (Cytisus scoparius), herb Robert (Geranium robertianum), common St. John’s wort (Hypericum 

perforatum), everlasting pea (Lathyrus sylvestris), reed canary grass (Phalaris arundinacea), and 

blackberry [Rubus spp.] (including Himalayan blackberry [Rubus armeniacus] and evergreen blackberry 

[Rubus laciniatus], table 1). We selected these species because they have great potential to cause severe 

environmental harm and because there is already a large enough sample size in the area to support 

analysis. Harmful environmental effects of these species include the ability to aggressively outcompete 

native vegetation (King County, 2008). In addition, dense areas of shrubby species or large grasses can 

prevent tree establishment and inhibit wildlife (Rubus spp., Cytisus scoparius) or fish (Phalaris 

arundinacea) movement (King County, 2008). Besides spreading rapidly and attaining 50–100 percent 

cover over large areas (Tisch, 1992), there are some data suggesting that Geranium robertianum inhibits 

other species through allelopathy (Barndt, 2008; Jones and Reichard, 2009). Lathyrus sylvestris is not 

included on state or county weed lists as a species of concern. Nevertheless, staff of the Olympic 

National Park have observed Lathyrus sylvestris to be extremely aggressive in the Elwha River valley, 

especially on riparian shorelines. Moreover, Lathyrus sylvestris appears to be recovering in areas where 

it was controlled, and it is among the most abundant exotic species (table 1). 

Another group (butterfly bush [Buddleja davidii], English ivy [Hedera helix], English holly [Ilex 

aquifolia], Japanese knotweed [Polygonum cuspidatum], giant knotweed [Polygonum sachalinense], 

and Bohemian knotweed [Polygonum x bohemicum]) are of high concern because of their potential 

harm, but are too rare for analysis and will be removed whenever they are found (table 1). Buddleja 

davidii is of concern because it is highly invasive in riparian areas and can impede forest regeneration 

(King County, 2008). When Hedera helix covers slopes, it can contribute to slope failure by channeling 

water, and when it covers tree trunks, it can contribute to tree mortality as a result of windthrow or by 

enabling disease and rot (King County, 2008). Ilex aquifolia can form dense thickets and outcompete 

native shrubs. The Polygonum species can clog waterways, increase bank erosion, and impair habitat for 

fish and wildlife (King County, 2008). 

Three other common invasive species were not considered because they are too ubiquitous to 

treat (orchard grass [Dactylis glomerata]) or are not perennial (common burdock [Arctium minus], bull 

thistle [Cirsium vulgare]). 

Relations between Species and Environment

Environmental variables including site characteristics and distance to dispersal-vector pathways

were used to predict the distribution of invasive species. Areas with invasive species were distinguished 

from those without by dividing the survey area into square grid cells (50×50 m). Environmental 

conditions and distance to dispersal-vector pathways was compared between cells with and without an 

invasive species. We used multiple logistic regression (Statgraphics Centurion XVI, version 16.0.08) to 

determine the probability of a cell having an invasive species, an approach that predicts presence or 

absence rather than population size (Hosmer and Lemeshow, 1989). 
5

Explanatory variables included GIS layers describing slope, aspect, elevation, precipitation 

(Daymet model, University of Montana, 2010, www.daymet.org; Thornton and others, 1997), heat load, 

solar radiation in June and January, distance to water, roads and trails, and height of vegetation. 

Vegetation height was determined from light detection and ranging (LiDAR) elevation data by 

subtracting the elevation of the last return (ground level) from the first return (top of vegetation). 

LiDAR data were collected in April 2009 (Entrix, 2009). We grouped the heights into classes to 

describe physiognomic groups—bare (0 m), herbaceous (0–0.33 m), low shrubs (0.33–1.67 m), tall 

shrubs (1.67–3.33 m), deciduous trees and young conifers (3.33–13.33 m), and conifers (> 13.33 m). 

Grid cells were described by the percentage of LiDAR cells (1 m2

) in each height category.

Analyses were done for focal species (table 2) over the entire study area and by subareas. 

Subareas are meant to reflect regions with similar exposure to invasive species vectors and similar 

habitat. These areas include Lake Aldwell, Lake Aldwell Delta, the area between U.S. Highway 101 and 

Glines Canyon Dam plus the Wolf Creek Trail, Lake Mills, Lake Mills Delta, Geyser Valley and Hot 

Springs Road (fig. 3). The area labeled Hot Springs Road was not analyzed because results would not 

describe an environment relevant to conditions expected in the dewatered reservoirs.

Additionally, logistic regression was used to determine the probability of occurrence of (1) any 

focal exotic species, or (2) any exotic species surveyed (table 1) for the subarea from Glines Canyon 

Dam to U.S. Highway 101. This subarea is affected by all long-distance dispersal vectors and is 

physically similar to what is expected for the dewatered reservoirs. These equations were used to 

calculate the probability of invasive plant occurrence in 50×50 m grid cells drawn over the dewatered

reservoirs using current lake bathymetry to describe slope, aspect, and to estimate where the river course 

will form. 

Invasive Communities

Multiple invasive species were present at many survey sites. We used cluster analysis of survey 

sites having any surveyed species (table 1) that occurred in at least 5 percent of the sites to look for 

associations among invasive species. These species included Acrtium minus, Cirsium arvensis, Cirsium

vulgare, Cytisus scoparius, Dactylis glomerata, Geranium robertianum, Hypericum perforatum, 

Lathyrus sylvestris, Phalaris arundinacea, Rubus spp., curley dock (Rumex crispus), and peavine (Vicia

spp.). The analysis used rank Bray-Curtis resemblances among sites for species abundance codes

(PRIMER-E version 6, Clarke and Gorley, 2006) to assign plots to hierarchically nested groups. To see 

how these clusters corresponded to groups identified by the non-metric multidimensional scaling (MDS) 

ordination technique and geographic regions, we mapped clusters having rank 100,000 onto the 

ordination, and geographically in the Elwha River valley. The level 100,000 was selected to represent a 

desirable amount of discrimination among groups after examination of the dendrogram. Non-metric 

multi-dimensional scaling is a non-parametric ordination technique that is useful for discerning patterns 

in plant community structure relative to environmental gradients because it is robust to common 

violations of statistical assumptions required by other ordination techniques (Minchin, 1987). Nonmetric multi-dimensional scaling maximizes the rank-order correlation between distance measures 

(Bray-Curtis resemblance in this case) and distance in ordination space (represented as a two- or threedimensional graph), with “stress” as a measure of the mismatch between these distances (Clarke and 

Warwick, 2001). Stress values less than 0.1 indicate a reliably interpretable ordination; ordinations with 

stress values from 0.1 to 0.2 are less reliable, especially at the high end of the range, and conclusions 

should be cross-checked against alternative analyses such as cluster analysis (Clarke and Warwick, 

2001).
6

Wind Dispersal

Frequency of wind speed and direction during seed dispersal (July 15–September 30) were 

determined using 10 years of weather data from the Remote Automated Weather Stations (RAWS) site 

near Hurricane Ridge Visitors Center. Potential destinations and sources for wind-dispersed species 

were predicted using the wind model WindNinja (Forthofer, 2007; Forthofer and others, 2009). The 

model uses an approach based on fluid dynamics to model wind speed and direction at 6 m above 

vegetation height derived from a digital elevation model in a geographic information system (GIS). We 

used a 200-m grid cell resolution and modeled wind speed and direction expected throughout the study 

area if it were measured from the north, east, and south at Hurricane Ridge at the maximum wind speed

from each direction over the 10 years of data (that is, north, 15 m/s; east, 12 m/s; south, 18 m/s). Wind 

from the west was not modeled because it occurs infrequently during July through September. 

The accuracy of WindNinja model results for the three directions and wind speeds were 

evaluated by comparing the prediction for the Glines Canyon Dam with actual wind speeds measured at 

the dam. Wind speed and direction at Glines Canyon Dam were measured by an anemometer 

established February 12, 2009, which reports 15 min averages. We identified periods during the 

anemometer record when conditions at Hurricane Ridge (averaged over 10 min) were within 5 degrees 

and 1 m/s of the three modeled conditions. Then actual conditions at Glines Canyon Dam during these 

periods were compared with model predictions from the four points estimated by WindNinja that were 

closest to the dam.

Long-Distance Dispersal Mechanisms of Focal Species 

Cirsium arvense

Cirsium arvense seeds have a pappus loosely attached to the achene. When the pappus remains 

attached to the seed, it has excellent aerodynamics (Sheldon and Burrows, 1973) and is capable of 

traveling several kilometers (Wood and del Moral, 2000). In a controlled experiment, Sheldon and 

Burrows (1973) reported a maximum seed dispersal of 11.35 m in wind speeds of 16.41 km/h. The 

minimum dispersal in low wind speeds (5.47 km/h) was 3.79 m. Meadow thistle (Cirsium dissectum) 

seeds with a slightly higher falling velocity have been observed to travel up to 1 km with winds up to 

4.3 m/s (Soons and others, 2004) and 22.7 percent of seeds traveled more than 100 m when horizontal 

wind speed was 1.5 m/s with updrafts and the terrain was descending (Tackenberg, 2003). However, the 

pappus readily separates from the achene, which will prevent effective long-distance seed dispersal 

(Bostock and Benton, 1979; Jacobs and others, 2006). Water transport of seeds and plant fragments also

is an effective means of dispersal (Washington State, 2009), and seeds have periods of 36.5 and 54.5 h 

until 50 and 90 percent of seeds sink , respectively (Boedeltje and others, 2003). Seeds are dispersed 

mid-summer through autumn and a patch of Canada thistle (a single plant) is capable of producing as 

many as 250,000 seeds (Boersma and others, 2006). 

Cytisus scoparius

Regionally, Cytisus scoparius is spread by humans (Neubert and Parker, 2004). Locally, the 

primary mode of seed dispersal for Cytisus scoparius occurs when the seed pods ripen and ballistically 

launch the seed from the parent plant (Zouhar, 2005; Boersma and others, 2006). In one study, the 

average dispersal distance was 2.34 m with a maximum distance of 7.25 m; greater than 10 percent of 

the seeds traveled more than 5 m, whereas just less than 5 percent did not leave the pods (Malo, 2004). 

In another study, seeds dispersed 10 m from a Scot’s broom thicket, although the mode of dispersal was 
7

unclear (Zouhar, 2005). Ants, other animals, and flowing water (hydrochory) are known to disperse 

Scot’s broom seed (King County, 2008). However, although Cytisus scoparius seeds will float, it is only 

for a few hours, and vegetative propagation from transported plant pieces has not been observed in 

nature (Watterson and Jones, 2006). Nevertheless, floods and debris flows have been implicated in 

transporting Cytisus scoparius from roads to streambeds and then farther downstream at sites in western 

Oregon (Watterson and Jones, 2006). Seeds are dispersed in the summer and possibly into early autumn

as seedpods dry. Seeds can persist for many years in soil (Zouhar, 2005). A single shrub is capable of 

producing thousands of seeds in 1 year (Zouhar, 2005) but 80 percent of plants do not flower until 4 

years of age (Sheppard and others, 2002). 

Geranium robertianum

The seeds are forcibly ejected from the plant up to 6.5 m away (Tofts, 2004). Seeds are further 

dispersed by animals and machinery. Specifically, a sticky fiber at one end of the seed attaches readily 

to vegetation or animals (Boersma and others, 2006), providing an opportunity for long-distance 

dispersal. This form of dispersal is not specific for animal species. The sticky seeds frequently disperse 

on hikers’ shoes along trail corridors and along roads by cars, mowers, and weed-eaters. Although 

Geranium robertianum has not been described as experiencing hydrochory by some sources (Tofts, 

2004; King County, 2008) and was not collected from the water near terrestrial populations (Goodson 

and others, 2003), hydrochory may be a minor mechanism of dispersal (Grime and others, 1988; 

Goodson and others, 2003; Tofts, 2004). Herb Robert seeds can remain viable in the soil for up to 5 

years, but germination rates decreased significantly after the first year (Tofts, 2004). In Europe, plants 

typically produced 100–300 seeds (Tofts, 2004) and may flower more than once in a calendar year.

Hypericum perforatum

Hypericum perforatum primarily propagates by seed (Boersma and others, 2006). Although the 

small sticky seeds have no specific adaptations for wind dispersal, Tisdale and others (1959) found that 

seeds readily traveled 3, 6, and 9.1 m downwind of parent plants, although most of the seeds fell in the 

3-m zone, and seeds have a relatively low falling velocity (Tackenberg, 2001). However, the primary 

mode of long-distance dispersal is likely to be zoochorous; the seeds are borne in sticky capsules that 

readily adhere indiscriminantly to the bodies of birds and mammals (Zouhar, 2005). In the Elwha River 

valley, Hypericum perforatum populations are predominantly found on riparian landforms. Plants are 

capable of producing seed in the first year after germination and individual plants can produce from 

15,000 to 23,350 seeds (Tisdale and others, 1959), which disperse from late summer into autumn. 

Lathyrus sylvestris

The major reproductive strategy of Lathyrus sylvestris appears to be by rhizomes, but when 

seeds are produced, the primary dispersal mechanism appears to be ballistic. The seedpods twist and 

eject the seeds upon maturation (Godt and Hamrick, 1991). Although seeds are dispersed between 1 and 

20 m from the parent plants of perennial pea (Lathyrus latifolius), dispersal is not as far for Lathyrus 

sylvestris because of shorter pods and the position of the seeds in the pods (Hossaert and Valero, 1988). 

Phalaris arundinacea

Phalaris arundinacea primarily propagates vegetatively, but is capable of prolific seed 

production—up to 600 seeds per stem (Tu and others, 2004). However, germination and seedling 

establishment rates are low (Stannard and Crawder, 2001; Tu and others, 2004). Indeterminate 
8

maturation of seeds, which generally occurs in summer, prolongs the season of dispersal (Stannard and 

Crawder, 2001). Phalaris arundinacea seeds are small, dense, and naked (Stannard and Crawder, 2001), 

with no specific adaptations for wind or other dispersal mechanism (NatureServe, 2008). Nevertheless, 

seeds can be wind dispersed (Rhoads and Block, 2002). Seeds also can float for a few days and then 

germinate on drained soil (Piper, 1924; Coops and Van der Velde, 1995). Shoots also can arise from the 

nodes of water-dispersed culms (Marten and Heath, 1973). 

Rubus armeniacus and Rubus lacinatus 

The fleshy fruit is a food source for birds and mammals, including non-native species of rats and 

birds, such as European starlings (Sturnus vulgaris) (Boersma and others, 2006). Because the fruits are 

highly desirable, they do not remain on the plant for very long (Tirmenstein, 1989). Seeds are dispersed 

late summer through early autumn. 

Distribution of Invasives

Spatial Distribution

Invasive species are densely distributed in the Elwha River valley near the river, in the reservoir 

deltas, and along roads (fig. 4). The area south of U.S. Highway 101 and west of the Elwha River was 

not surveyed because the land is privately owned. 

Locations of 30 invasive species were documented in 2,297 records (table 1). Among these site

records, the focal species Cirsium arvense, Geranium robertianum, Phalaris arundinacea, and Lathyrus 

sylvestris were the most common (table 2). Other common species included Dactylis glomerata (348 

records), Cirsium vulgare (361 records), and Arctium minus (136 records) (table 1). The focal species 

Hypericum perforatum, Cytisus scoparius, and Rubus spp. were less abundant (table 2). The areas with 

the highest frequency of observations of invasive species include the Lake Aldwell Delta and the area 

between Glines Canyon Dam and U.S. Highway 101 (table 2).

Distributions of seven focal species indicate that the species are all present between Glines 

Canyon Dam and U.S. Highway 101 (fig. 5). The greatest concentration of Cirsium arvense is in Geyser 

Valley (including Humes Ranch), upstream of roads. Likely vectors to this area could be wind, pack 

stock, other animals, or hikers. Elsewhere, Cirsium arvense is distributed along roads and in the 

reservoir deltas and appears to be spreading locally from sites of long-distance dispersal. Cytisus 

scoparius is found near the dams and in the Elwha River valley in a pattern that suggests human 

introduction followed by hydrologic spread. Geranium robertianum also occurs in Geyser Valley 

mostly associated with the Elwha River and Lake Aldwell Delta. A high concentration of Geranium 

robertianum is near the base of the trail that leads to private land to the west of the Elwha River and 

may be heavily used by elk (see fig. 4). The pattern suggests possible introduction by animals, vehicles, 

or machinery followed by hydrologic spread, or spread by anglers from infested campgrounds to fishing 

spots along the river. Hypericum perforatum has a similar pattern to Geranium robertianum, but is more 

closely associated with roads. Lathyrus sylvestris and Phalaris arundinacea occur along the river 

between Glines Canyon Dam and Lake Aldwell Delta with distribution patterns that suggest hydrologic 

spread. Rubus spp. is found along the road and near the base of the trail to Herrick Road. 
9

In general, the distribution patterns of invasive species in the Elwha River valley suggest that 

long-distance dispersal to the area primarily was by humans and possibly wind and animals. Once 

present, patterns of invasive species distribution are consistent with spread by local diffusion 

(vegetatively and by short distance seed dispersal) and by the river. Spread along the river may be 

hydrologic or may be a result of animals and people that travel along the river corridor. 

Factors Predicting Species Distribution

Environmental variables and vector proximity (defined in next paragraph) explained the 

occurrence of invasive species but the relative importance of each differed among species and locations 

(table 3). Interpretation of logistic regression results are complicated by a number of factors. First, not 

all potential sites are filled. Consequently, the model describes where species tend to first establish 

rather than where they are capable of invading. Moreover, the number of infested cells is small 

compared with the total number of cells analyzed (see table 2 and fig. 2 for numbers by area and 

species). Thus, the models are better at predicting uninfested cells than infested cells. Furthermore, a 

grid cell size of 50 m is quite large compared with the scale of microsite characteristics that likely are 

affecting the establishment success of invasive species. This analysis is designed to clarify the broadscale environmental factors affecting the patterns that are visually discernible in the distribution maps 

(fig. 5).

Results are presented as odds ratios where a value of 1.0 indicates that the variable has no 

predictive ability; values greater than 1.0 indicate a higher probability of species occurrence in 

association with the variable; values less than 1.0 indicate a lower probability of occurrence in 

association with the variable. For vector proximity, however, the variable is distance from the features 

(that is, road, trail, water). Consequently, values less than 1.0 indicate a low probability of occurrence 

with greater distance from the feature, which is a positive association with the feature. Odds ratios for 

these models are very close to 1.0, indicating that none of the vector proximity variables has a high 

predictive ability. This is partially a result of the many cells adjacent to roads that do not have exotic 

species even though the species may only occur close to roads. Odds ratios are most useful for 

indicating the relative importance of variables in the logistic model.

Among the long-distance dispersal vectors, water dispersal (hydrochory) was expected to be 

important for Phalaris arundinacea, but also appears to be important for predicting occurrence of all 

species except Rubus spp. and Hypericum perforatum. The predictive ability of the road was important 

for all species in the area between Glines Canyon Dam and U.S. Highway 101, except for Phalaris 

arundinacea (that is, the primary water-dispersed species) and important overall for Cytisus scoparius, 

Lathyrus sylvestris, and Rubus spp. Trails are significant predictors only for Lathyrus sylvestis and 

Rubus spp., with odds ratios very close to 1.0. 

Among environmental variables, elevation was inversely associated with invasive species 

occurrence (table 3). This reflects the greater abundance of invasive species in the valley bottom and 

downriver. Elevation was not predictive overall for Cirsium arvense and Geranium robertianum, which 

are distributed throughout the study area. Elevation is associated with Hypericum perforatum, which 

also occurs from Geyser Valley to Elwha Dam. Slope, aspect, heat load, and solar radiation were not 

good predictors of invasive species occurrence, and precipitation had inconsistent relationships. 

However, vegetation did have relatively high predictive ability. Specifically, bare ground was 

negatively associated with invasive species occurrence, whereas herbs, low shrubs, and low trees were 

positively associated with invasive species. The associations with bare ground (negative) and herbs 

(positive) may simply indicate the presence of the invasive species. The positive association with low 

shrubs and low trees (presumably deciduous) may indicate a protective environment that traps seeds but 
10

does not create too much shade. Although Geranium robertianum is shade tolerant (Tofts, 2004), it does 

not seem to have a stronger association with tall vegetation than other species, with the exception of a 

strong association with tall shrubs at Lake Aldwell Delta (odds ratio of 1.177 is the second strongest 

site-specific association). 

In the subarea between Glines Canyon Dam and U.S. Highway 101, the probability of 

occurrence of any invasive plant species or any focal invasive plant species was related to proximity to 

roads, elevation, low shrubs, and low trees (table 3). Occurrence of all invasive species also was 

predicted by herbs; all focal invasives were predicted by precipitation, slope, and solar radiation in July. 

Associations among Invasive Species

Most groups identified by the cluster analysis (fig. 6) had only one species or were dominated by 

that species. These groups tended to occupy different regions of the ordination (fig. 7), validating that 

these groups are distinct entities.

Because the groups of sites identified by cluster analysis and ordination largely indicate the 

presence of only one species, the map of groups (fig. 8) is similar to the maps of individual species (fig. 

5). Geographically, sites dominated by Cirsium arvense (group f) are found at Lake Mills Delta and 

Geyser Valley (fig. 8). Geranium robertianum-dominated communities (group k) primarily are in 

Geyser Valley, the deltas, and associated with roads. The groups associated with Dactylis glomerata and 

Phalaris arundinacea (groups j and l) are found near the lakes and the river. The least differentiated 

group (group m) has representatives throughout the survey area. 

Wind Vector

The importance of predominantly wind-dispersed species to populate barren areas was 

documented following the eruption of Mount St. Helens (that is, 34 seeds/0.1 m2/yr of mostly winddispersed species; Wood and del Moral, 2000). In the Elwha River valley, Cirsium arvense is the only 

one of the seven focal taxa that has seeds or fruits that are adapted for wind dispersal. Hypericum

perforatum primarily is dispersed by animals but seeds have a relatively low falling velocity 

(Tackenberg, 2001). Polygonum spp., which is a species of concern but not a focal species for analysis, 

propagates primarily by vegetative means, but has fruits that are morphologically adapted to aid air 

transport for long-distance dispersal (Beerling and others, 1994; Zhou and others, 2003). 

Mechanistic models have been developed to predict the long-distance dispersal of seeds by wind 

(Greene and Johnson, 1989; Okubo and Levin, 1989; Nathan and others, 2001; Tackenberg, 2003). 

These models are based on modeling the trajectory of seeds as a function of the falling velocity 

imparted by seed morphology and wind dynamics. Some of these models can be made spatially explicit 

because they have the ability to incorporate terrain. However, these models are quite complex and are 

computationally expensive. Moreover, many of the models are only effective at predicting shortdistance transport (Tackenberg, 2003). As these models have advanced, several lessons have been 

learned regarding factors affecting long-distance transport:

• Uplifting of seeds and fruits is necessary for long-distance dispersal (Tackenberg, Poschlod, and 

Kahmen, 2003), especially for low-growing species (Tackenberg, Poschold, and Bonn, 2003). In 

forests, updrafts are created by high horizontal wind velocity creating turbulence (Horn and others, 

2001; Nathan and others, 2002). In grasslands, high winds also are important (Soons and others, 

2004), but a more prominent process is high surface heating in the presence of low horizontal wind 

velocity, which can result in convective uplift (Tackenberg, 2003). 
11

• In addition to high winds, seed release height, height of surrounding vegetation, and seed or fruit

falling velocity determine dispersal distance (Soons and others, 2004). 

• Species with falling velocity less than 1.5 m/s have a high potential for long-distance dispersal 

(Tackenberg, 2003). Falling velocity of several invasive species present in the Elwha River valley or 

related species (table 4) confirm that Cirsium arvense is the most likely focal species to be wind 

dispersed followed by Hypericum perforatum. 

• Topography and forest gaps can create mechanical turbulence, which can affect seed or fruit 

distribution. Specifically, downdrafts forming at the upwind edge of gaps can deposit propagules 

whereas updrafts at the downwind edge can lead to long-distance transport (Nathan and others, 

2005). 

• Topography that descends in the direction of airflow can enhance dispersal of species with falling 

velocities greater than 0.25 m/s, but those with falling velocities of 1.6 m/s are too heavy for 

topography to matter (Tackenberg, 2003). 

• Strong vertical winds common in thunderstorms may lift even heavy seeds or fruits (Higgins and 

others, 2001).

Wind Direction and Speed during Seed Dispersal

All three potentially wind dispersed species produce seed from mid-summer through early 

autumn. During this time (July 15–September 30), winds averaged over a 10-yr period are primarily

from the north to north-northeast, east and south-southeast to south-southwest with a maximum speed of 

18 m/s (table 5). Although wind speeds in at least the 9–12 m/s range were recorded from nearly all 

directions, and up to 15–18 m/s from the south, winds were typically less than 6 m/s (fig. 9). These 

values are based on the average of wind measurements taken for 10 min at hourly intervals, 

consequently higher gusts may have occurred. 

Effect of Topography on Wind Direction

We validated the WindNinja model by comparing actual wind speed at Glines Canyon Dam with 

the model predictions based on wind speed measured at Hurricane Ridge. Results show that incidences 

of the same conditions at Hurricane Ridge can result in a wide range of conditions at Glines Canyon 

Dam (table 6). On average, the actual conditions at Glines Canyon Dam match fairly well with 

predictions for easterly and southerly winds at Hurricane Ridge. The complex topography of Glines 

Canyon to the north of the dam, including the dam itself and adjacent berm, may create turbulence that 

reduces the comparability of wind speed and direction measured at the dam with the WindNinja 

predictions. In fact, southerly readings at the anemometer have been observed during northerly 

prevailing winds (Roger Hoffman, Olympic National Park, oral commun., 2010) 

Output from WindNinja shows how the speed and direction of wind flowing primarily from the 

north, south, and east at Hurricane Ridge will be altered by the mountainous topography of the Elwha 

River valley (figs. 10–12). Areas upwind of the reservoirs are potential sources of exotic species as 

indicated in black. However, it is likely that seeds will be dropped in areas of light wind. Consequently, 

areas where the wind reaches the reservoirs without decreasing to less than 11 m/s are considered high 

priority areas to treat before and after dam removal and are marked in green.

Winds from the east and south are most likely to disperse Cirsium arvense from current 

concentrations to the dewatered reservoirs (table 7). If these populations cannot be eradicated before

dam removal and access can be obtained during dam removal, then removal of any seed heads that 

protrude above the rest of the vegetation will decrease seed production and dispersal (Soons and others, 
12

2004). This approach also can be used with any remaining Polygonum spp. populations to prevent rare 

wind dispersal events. 

The wind model output also identifies several areas that could be sources of invasive species but 

do not have recorded surveys (table 8). These are areas to target for eradication efforts prior to dam 

removal.

Spatial heterogeneity of vegetation height can create patterns of wind turbulence that may affect 

plant dispersal (Nathan and others, 2005), such that air currents associated with forest gaps may pull 

wind-dispersed seeds from surrounding areas (Wenny, 2001). Specifically, uplift eddies can be 

produced between the center of a 250-m forest gap and the upwind forest edge, and downdrafts can 

occur at the downwind edge of the gap, potentially leading to disproportionate deposition of seeds. 

LiDAR-based images of the reservoirs show that tall vegetation forms a fairly abrupt edge around the 

Lake Mills Reservoir (fig. 13). Patterns of vegetation height are much more complex around the Lake 

Aldwell Reservoir, reflecting the higher diversity of land uses. The effects of patterns of vegetation 

height on airflow are not addressed in the WindNinja model. 

Water Vector

The expectation that Phalaris arundinacea primarily is a water-dispersed species is supported by 

logistic regression results (odds ratios <1, indicating positive association in table 3). Studies of 

propagules trapped from rivers indicate that vegetative diaspores outnumber seeds (Boedeltje and 

others, 2003, 2004). However, seeds are well adapted to hydrochory with 50 percent of a test group of 

seeds sinking after 19.9 or 41.0 h (T50) and 90 percent after 38.3 (Coops and van der Velde, 1995) or 

71.5 h (Boedeltje and others, 2003). 

Cytisus scoparius, Lathyrus sylvestris, Geranium robertianum, and Cirsium arvense may be 

carried by the river but also are associated with roads (fig. 14 and table 3). Cytisus scoparius may be 

transported in debris flows (Watterson and Jones, 2006) that may result from dam removal activities. 

Geranium robertianum may have some potential for hydrochory (Goodson and others, 2003) and 

Lathyrus sylvestris propagates primarily vegetatively, so it seems likely that pieces could be effectively 

moved by water. Cirsium arvense generally is found along the road and in fields and openings, but also 

is associated with the river (fig. 13) and can potentially be transported by water. Seeds of Cirsium

arvense will float (Boedeltje and others, 2003) and transport of plant fragments also is effective 

(Washington State, 2009). Other factors, such as people and animals that travel the river corridor, 

cannot be dismissed as potential vectors creating these patterns, although animals are not considered to 

be effective long-distance dispersers of at least Cytisus scoparius (Bossard, 1991). 

Making spatial predictions about the distribution of plant propagules resulting from hydrochory 

is hampered by not knowing the future course of the river through the dewatered reservoirs. However, 

we propose that certain areas should be carefully searched as the river develops a path. Based on release 

of seeds into an artificial flume (Merritt and Wohl, 2002) and rhizomes of water buttercup (Ranunculus 

lingua) into a river in Sweden (Johansson and Nilsson, 1993), several conclusions about likely sites of 

propagule deposition can be drawn. First, more propagules are deposited in areas of slow flow (for 

example, eddies, flow expansions, point bars, pool margins, slackwaters, near obstacles and in curved 

river stretches) than in areas of faster flow (for example, cut banks, flow constrictions, islands, straight 

margins). Second, propagules will be left at high water levels, and then will be left in bands that reflect 

temporary stations of the water’s edge as the hydrograph declines. 
13

Before dam removal, high-priority areas for species removal should include drainages that 

currently flow into the lakes from east and west in addition to removing potentially hydrochorous 

species upriver. No exotic species were captured flowing into Lake Mills and only one was captured 

flowing out in a recent trapping survey (Brown and Chenoweth, 2008). Additionally, Cytisus scoparius

and Polygonum spp. have the potential to be moved in mudslides and other earth movement that 

originate from dam removal activities at Glines Canyon Dam. Records exist for Cytisus scoparius at the 

dam (fig. 5) and for Polygonum spp. on the road leading to the dam. 

Mammal Vectors

Geranium robertianum and Hypericum perforatum are the two focal species with adaptations 

that facilitate dispersal by animals. However, other species may attach to (epizoochory) or be ingested 

by (endozoochory) herbivores, either intentionally or inadvertently. For example, Carey’s smartweed 

(Polygonum careyi) and Rubus spp. have been observed to germinate from feces of white-tailed deer

(Odocoileus virginianus) in the eastern United States (Myers and others, 2004). However, the 

Polygonum species of concern in the Elwha River valley typically are much taller than Polygonum 

careyi, placing seeds mostly out of the reach of deer.

Patterns of movement of several elk (Cervus elaphus roosevelti) herds are available for the 

Elwha River valley (figs. 15 and 16). The herd that frequents the Herrick Road area (fig. 15) does not 

travel to Lake Mills, nor does it cross U.S. Highway 101 to Lake Aldwell. This pattern may persist 

following dam removal if elk are not inclined to pass through Glines Canyon. This herd warrants

observation, however, because they have the potential to disperse the many invasive species occurring 

in the river valley between Glines Canyon and U.S. Highway 101 as well as the potential to disperse 

species occurring in the Herrick Road area on private land not surveyed. The herd near Lake Mills (fig. 

16) was observed near Dodger Point in summer, Geyser Valley in spring and autumn, and near Lake 

Mills from December to February. All points along the Whiskey Bend Road were recorded during 

January and February (Patti Happe, Olympic National Park, oral commun., 2010). A second collared elk 

from this herd showed a similar pattern but did not go near the lake during the milder winter of 2009–

10. The greatest danger of invasive spread associated with these elk is the potential to transfer species 

from Geyser Valley to the dewatered Lake Mills Reservoir when the elk move downriver from autumn 

to winter during harsh winters. If seeds are still present on invasive species occurring on the eastern side 

of Lake Mills in the Whiskey Bend area, they also could be transported to Lake Mills by elk during 

winter.

Black-tailed deer (Odocoileus hemionus) also are present in the Elwha River valley, but detailed 

information regarding distribution and movements is available only for the area between Whiskey Bend 

and the Lillian River (Jenkins and others, 1999). These animals were more abundant on south- and 

southwest-facing slopes adjoining bottomlands on slopes less than 20 degrees. They preferentially 

frequented alder flats and old fields but also spent considerable time in coniferous forests. Home ranges 

averaged 51 ha and individuals moved 85–470 m daily. Density was estimated at 4.64–6.58 deer/km2

for the whole area depending on the estimation method; density was estimated at 11.1 deer/km2 for the 

northern side of the river where favorable exposures occur. Several animals left the area in May and 

June and migrated south to higher elevations, returning by the end of October. If these numbers are 

consistent for the rest of the area affected by dam removal, then deer have a substantial potential to 

spread invasive species to all habitats.

Beavers (Castor canadensis) are active on the Elwha River, but sign primarily is limited to cut 

stems of shrubs and trees, rather than structures (Knapp, 2009). Sign is most abundant downstream of 

the Elwha Dam but also occurs on the western side of Lake Aldwell, between U.S. Highway 101 and 
14

Glines Canyon and on both reservoir deltas (fig. 17). These beavers are bank dwellers and prefer willow 

species. However, there are records of beaver cutting Cytisus scoparius (Knapp, 2009) and Polygonum

spp. (Cathy Lucero, Clallam County, oral commun., 2009). The greatest potential for long-distance 

dispersal comes from den failure as a result of flooding or other disturbance, which would distribute the 

den contents downstream. Because beavers tend to stay close to water for protection from predators and 

need food and construction resources to be nearby, beavers will not likely establish dens in the 

dewatered reservoirs until after they are vegetated. However, if the new river course should establish 

close to extant or planted willow, cottonwood, or alder stands, then beaver may be attracted. 

Bird Vectors

Rubus spp. is the only taxon among the focal taxa adapted for endozoochory, especially by birds 

but also by coyotes (Canis latrans), foxes (Vulpes fulva), squirrels (Tamiasciurus douglasii), and black 

bears (Ursus amicanus) (Hoshovsky, 2000). However, Hedera helix and Ilex aquifolia are of high 

concern even though they are relatively rare. Cytisus scoparius has been observed to be spread by 

grouse and quail in coastal areas of the Pacific Northwest (Bossard, 1991). Seeds of Geranium

robertianum and Hypericum perforatum are adapted for attachment to birds and mammals.

Distributions of bird species in the region of the Elwha River that will be affected by dam 

removal have been surveyed to predict distributions after dam removal (Gelarden, 2008). Of the 42 

species observed, the 4 most common species that inhabit riparian areas include the American robin 

(Turdis migratorius), chestnut-backed chickadee (Poecile rufescens), Swainson’s thrush (Catharus 

ustulatus), and warbling vireo (Vireo gilvus). Of the four species, Swainson’s thrush and warbling vireo 

have lower numbers and are migratory, leaving in August and September, and may miss seed dispersal 

for some invasive plant species. Based on surveys and habitat models, Gelarden (2008) estimated a high 

density of chestnut-backed chickadees throughout the area. The other three taxa share similar areas of 

high density, including Geyser Valley, Lake Mills Delta, the northeastern shore of Lake Mills, north of 

Glines Canyon, the area near the park boundary, and the Herrick Road area (fig. 18). These areas 

coincide with observations of all three bird-dispersed invasive species; hence, they constitute targets for 

invasive species treatment prior to dam removal. 

Perhaps the most important observation relative to restoration of the Elwha reservoirs is that 

seeds dispersed by birds tend to cluster near plantings (Robinson and Handel, 2000). Given the 

infeasibility of modeling movements of tens of birds species, perhaps the most important place to look 

for bird-dispersed invasive species is in areas with newly established woody vegetation and in planted 

areas.

Road- and Trail-Associated Vectors

All focal species except Phalaris arundinacea are associated with roads (fig. 4, table 2), and 

roads may have been the initial pathway for most of the invasives in the Elwha River valley. C. 

scoparius appears to have been introduced at the maintenance yard due west of the Elwha Ranger 

Station and may now be dispersing down the river. Rubus spp. and Lathyrus sylvestris also are 

associated with trails according to logistic regression results (table 2), although visually Geranium 

robertianum has a stronger relation than Rubus spp. (fig. 18). To the extent that trail dispersal is a result 

of people and livestock, it will not be a concern during dam removal because trails will be closed. In 

addition, a vehicle and equipment de-contamination protocol has been developed for all construction 

and other vehicles that will access the area during dam removal (Chenoweth and others, 2011).
15

Priority Areas

In summary, the ultimate goal of this project is to identify high priority areas where invasive 

plants should be removed before dam removal, and to predict areas of the dewatered reservoirs that are 

likely to be infested before dam removal. To accomplish these goals, we analyzed several spatial 

datasets to identify source areas for vectors and to describe the relationships among species locations 

and environmental conditions, including relationships to vectors (fig. 19). The outcomes are maps 

showing areas and environments within those areas to treat before dam removal, and vulnerable areas in 

the reservoirs after dam removal.

Pre-Dam Removal Treatments 

Areas that constitute sources for each individual vector of invasive plants are described in 

sections on vectors above. When the source areas are combined on one map (fig. 20), the area between 

U.S. Highway 101 and Glines Canyon Dam-Lake Mills is the potential source for the greatest number 

of vectors. This area also has the greatest density and diversity of invasive species (figs. 4 and 5). 

Environments within these areas where invasive species are most likely to occur can be mapped 

based on the results of logistic regression (table 3). Specifically, the significant (α≤ 0.05) variables are 

proximity to water, roads, trails and cover of bare ground, herbs, low shrubs, and low trees. Tall shrubs 

also are significant (α≤ 0.05) predictors for Geranium robertianum. High resolution maps of the source 

areas (figs. 21–25) show where these conditions exist. 

Transport of Cirsium arvense by wind is the greatest threat from the area north of Lake Aldwell. 

Because this area is largely privately owned, the populations of Cirsium arvense are significant, and the 

search area is extensive, the most effective way to lessen this threat may be with a public campaign to 

enlist support for keeping the area mowed, especially during flowering. 

In the area between Lake Aldwell and the Elwha Campground, the river, and road corridors, the 

Herrick Road area and the hillslope to the south are potential sources of invasive plants. Because this 

area experiences all vectors (that is, wind, mammals, birds, roads, and trails) and there are many known 

locations of invasive plants, this is a significant potential source, primarily for the Lake Aldwell 

Reservoir. Because the Herrick Road area is privately owned, this is another place where encouraging 

mowing could be an effective strategy for minimizing the sources for exotic plants. However, mowing 

must be timed when Geranium robertianum is not in seed and mower blades should be high enough to 

avoid picking up Geranium robertianum seeds.

Between Elwha Campground and Lake Mills, the green areas associated with the river, roads, 

and trails are most likely to have invasive species. Many of these areas already have been searched and 

populations mapped, but perhaps more attention should be paid to the areas between the roads just north 

of the lake, and between the road and the lake on the western side of the lake, especially for birddispersed species.

Large populations of invasive plants have been mapped on the delta at the southern end of Lake 

Mills and the lake edge. Other high priority areas are the slope and drainages on western edge of the 

southern part of the lake. 

Geyser Valley has many instances of Cirsium arvense, Geranium robertianum, and Hypericum

perforatum. Cirsium arvense is abundant, and although it is not likely to spread to the reservoirs by 

wind, it may be carried by water. Consequently, all three species should be considered threats. 
16

Setting Priorities for Treatments and Species

It will not be possible to treat all areas and invasive plant species before dam removal. 

Consequently, it is necessary to establish priorities. Priorities for species can be set be considering the 

vector for each, whether each species is likely to use the expected habitat, and the consequences of 

establishment in terms of time to produce seeds, difficulty of removal, and rate of spread once 

established (table 9). Regarding vectors, it can be argued that water is the most important because of its 

proximity, followed by wind because of potential distances affected. Elk currently are not very active 

near Lake Aldwell and elk near Lake Mills are mostly frequenting the interior of the park. Birds likely 

will not be important until trees have been planted. Regarding factors other than vectors, species that are 

quick to produce seed, are adapted to the environment, are difficult to remove, and spread quickly 

should have the highest priority for removal. Species with all these characteristics among the priority 

group already identified include Buddleja spp., Cirsium arvense, Phalaris arundinacea, and Polygonum

spp. (table 9). 

Priority areas for treatment can be identified from those described in section, “Pre-Dam Removal 

Treatment” by considering whether priority areas are inside or outside of the park, which vectors are 

active, and how many species are known to be present (table 10). For areas that have not been searched 

outside of the park, advocating mowing is appropriate for the Herrick Road area and north of Lake 

Aldwell. 

For areas that have been searched and are inside the park, the area between the reservoirs is 

known to have the highest density and diversity of invasive species. Given the size of the area, however, 

it may be best to focus on the known sites of the priority species. The areas around the dams also will 

experience significant disturbance and should be cleaned of exotics before dam removal begins. Finally, 

the deltas also are known hot spots of invasive species and especially should be cleared of those species 

that are spread by water.

For areas that are inside the park but have not been searched, the highest priority is the 

appropriate environments (fig. 24) within the plateau and drainages on the west side of the south end of 

Lake Mills. This area has many active vectors and the potential to have several high priority species in 

close proximity to the dewatered reservoir.

Species Locations Outside of Olympic National Park

Few geo-referenced data describing locations of exotic species have been collected by others 

outside of Olympic National Park. However, some records are available for locations of species that 

have been treated for invasive species on Olympic National Forest lands north of Olympic National 

Park (fig. 26). These records give some indication of what can be expected outside of the park. An 

additional source of invasive plants is the commercial gravel pit owned by Shaw and Lane, located just 

off of Olympic Hot Springs Road. This pit is a source of Polygonum spp. and Cytisus scoparius at least 

(Cathy Lucero, Clallam County, oral commun., 2009). 

Post-Dam Removal Vulnerability

Predicting areas of the dewatered reservoirs vulnerable to invasive species is hampered by a 

number of challenges. First, we do not know where the river course will establish and hence the location 

of an important dispersal vector. Second, we do not know what form the topography of the reservoirs 

will take. Finally, even if we had a detailed prediction of future reservoir conditions, we do not have 

high-resolution descriptions of site preference for invasive plants. Nevertheless, we can use current 

reservoir bathymetry and predictive equations from logistic regression to make some crude forecasts 
17

(figs. 27 and 28). Results vary somewhat depending on whether the predictions are based on 

occurrences of all invasive species or only on the focal species, with higher resolution coming from the 

focal species-based predictions. These predictions are based on the distribution of invasive plants 

between Glines Canyon Dam and U.S. Highway 101; the efficacy of this approach to show that pattern 

is shown in figure 29. 

 

Several stochastic factors that can only be observed during and after dam removal and that affect the 

vulnerability of areas to invasive plants are not captured in these maps: 

• Areas of slack water because of obstructions (for example, large woody debris), bends in river 

course, etc. can be observed only after the new river course develops. 

• The importance of short-distance dispersal and vicinity to current infestation of invasive plants is not 

incorporated. 

• The effects of vegetation, especially trees, on wind deposition are not included but likely will result 

in deposition on the western edge of Lake Aldwell and the eastern edge of Lake Mills during 

easterly winds.

• There is a greater chance of aerial long-distance dispersal during thunderstorms. 

• Elk are more likely to move to lower elevations where they have greater opportunity to spread 

invasive plants during cold, snowy winters than during mild winters. 

 Additionally, areas newly planted with trees and shrubs as part of revegetation efforts are 

especially vulnerable to bird- and worker-dispersed invasive species. 

Conclusions

Protecting the dewatered reservoirs from invasive species is an important part of restoring native 

vegetation; we have used observational data, knowledge of species biology, and various statistical 

analyses and models to focus these efforts. Specifically, we have selected species locations from 

multiple surveys and integrated them with environmental data and distribution of vectors using GIS to 

identify priority areas and environments within those areas for treatment of or search for invasive 

species. High priority areas to treat inside of the Olympic National Park include (1) the dam sites, (2)

the area between the lakes, and (3) the deltas. The high priority area to search for invasives that has not 

been searched is the plateau and drainages on the western side of the southern end of Lake Mills. High 

priority areas to search and treat should access be granted include (1) the Herrick Road area, (2) areas 

due north of Lake Aldwell, (3) the gravel pit near Olympic Hot Springs Road, and (4) the hillslope east 

of Lake Aldwell. Next, we used information regarding the high priority species to further prioritize to 

which species the limited resources for treatment should be applied. High priority species include 

Cirsum arvense, Buddleja spp., Phalaris arundinacea, and Polygonum spp. Finally, we used the current 

relationships between species distribution and environment to predict where species may establish in the 

reservoirs based on current bathymetry. Predicting what will happen in the reservoirs after dam removal 

is extremely speculative because we cannot predict where the new river channel will form. In this case, 

the most productive way to address invasive spread is to recognize that water is likely the most 

important vector soon after dam removal and that areas of slack water are the most likely places where 

invasive species will establish.
18

Acknowledgments

We are grateful to the field crew members who conducted the two surveys for invasive species: 

Mignonne Bivin, Phillip Blackcrow, Garth Hopie,Wendy McClure, Caitlin McNulty, Kathleen Mitchell, 

and Tolle Murphy. Roger Hoffman participated in discussions leading to the development of the project 

and helped interpret data from the anemometer at Glines Canyon Dam. Cathy Lucero provided 

information about Clallam County weed records. Laurie Kurth provided us with example output from 

WindNinja. Susan Piper provided digital records of invasive species found on Forest Service lands. We 

also would like to thank Tracy Fuentes, Diane Doss, and one anonymous reviewer for their thoughtful 

and thorough reviews of this report. 

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Figure 1. Locations of the Elwha and Glines Canyon Dams and respective reservoirs on the Elwha River, Olympic 

National Park, Washington. 
24

 

Figure 2. Project area, Elwha River, Olympic National Park, Washington.
25

 

Figure 3. Subareas used to determine predictive variables for occurrence of invasive plant species, Elwha River, 

Olympic National Park, Washington.
26

Figure 4. Kernel density representation of ‘hot spots’ of 30 invasive species (table 1; high to low density represented 

by hot to cool colors), isolated observations (green points), and area surveyed during 2001 and 2008 (lilac), Elwha 

River, Olympic National Park, Washington. 
27

Figure 5. Kernel density representation of ‘hot spots’of focal invasive species (high to low density represented by hot to cool colors), isolated observations (green 

points), and area surveyed during 2001 and 2008 (lilac), Elwha River, Olympic National Park, Washington.
28

Figure 5. Kernel density representation of ‘hot spots’of focal invasive species (high to low density represented by hot to cool colors), isolated observations (green 

points), and area surveyed during 2001 and 2008 (lilac), Elwha River, Olympic National Park, Washington.—Continued
29

Figure 5. Kernel density representation of ‘hot spots’of focal invasive species (high to low density represented by hot to cool colors), isolated observations (green 

points), and area surveyed during 2001 and 2008 (lilac), Elwha River, Olympic National Park, Washington.—Continued
30

Figure 6. Cluster analysis of Bray-Curtis resemblances among survey sites. Groups that are indicated on ordination (fig. 7) are shown in circles. [Groups are 

dominated by: a, Rubus spp.; b, Vicia spp.; c, Cytisus scoparius; d, Hypericum perforatum; e, Lathyrus sylvestris; f, Cirsium arvense; g, Rumex crispus; h, Cirsium 

vulgare; i, Arctium minus; j, Phalarus arundinacea; k, Geranium robertianum; l, Dactylis glomerata; m, mixture of species]
31

Figure 7. Non-metric multidimensional scaling ordination of sites. Ten groups labeled in figure 6 are indicated. The correlation of species abundance with dimensions 

of the ordination space are shown in the inset. 
32

Figure 8. Locations of eleven ordination groups. Groups are defined in figure 6. Species dominating groups include Rubus

spp. (a), Vicia spp. (b), Cytisus scoparius (c), Hypericum perforatum (d), Lathyrus sylvestris (e), Cirsium arvense (f), Rumex 

crispus (g), Cirsium vulgare (h), Arctium minus (i), Phalaris arundinacea (j), Geranium robertianum (k), Dactylis glomeratt (l) 

and mixed species (m).
33

Figure 9. Summary of distribution of wind speed and direction (source) from RAWS station at Hurricane Ridge. The vertical axis represents percentage of time at 

each speed and direction accumulated over 10 years (1999–2008).
34

Figure 10. Effect of topography on wind measured at 15 m/s from the north at the Hurricane Ridge RAWS station predicted 

by WindNinja (Forthofer, 2007). All sources areas are outlined in black; high priority source areas are outlined in green.
35

Figure 11. Effect of topography on wind measured at 12 m/s from the east at the Hurricane Ridge RAWS station predicted 

by WindNinja (Forthofer, 2007). All sources areas are outlined in black.
36

Figure 12. Effect of topography on wind measured at 18 m/s from the south at the Hurricane Ridge RAWS station predicted 

by WindNinja (Forthofer, 2007). All sources areas are outlined in black; high priority source areas are outlined in green.
37

Figure 13. Vegetation height surrounding Lake Mills and Lake Aldwell reservoirs based on LiDAR data.
38

Figure 14. Species with distribution patterns consistent with water dispersal. Area shown is approximately between the park 

boundary and Glines Canyon Dam. 
39

Figure 14. Species with distribution patterns consistent with water dispersal.—Continued. Area shown is approximately 

between the park boundary and Glines Canyon Dam. 
40

Figure 14. Species with distribution patterns consistent with water dispersal.—Continued. Area shown is approximately 

between the park boundary and Glines Canyon Dam. 
41

Figure 14. Species with distribution patterns consistent with water dispersal.—Continued. Area shown is approximately 

between the park boundary and Glines Canyon Dam. 
42

Figure 14. Species with distribution patterns consistent with water dispersal.—Continued. Area shown is approximately 

between the park boundary and Glines Canyon Dam. 
43

Figure 15. Locations of one radio collared elk tracked by the Lower Elwha Klallam Tribe from February 2009 to November 

2009. Points represent six observations per day.
44

Figure 16. Locations of two radio-collared elk tracked by Olympic National Park staff, one from September 2008 to June 

2009, and one from September 2009 to February 2010. Points represent four observations per day. 
45

Figure 17. Locations of observed and likely beaver sign, Elwha River, Olympic National Park, Washington, 2007 (redrawn 

from Knapp, 2009). 
46

Figure 18. Distribution of most likely bird-dispersed invasive plant species compared with highest densities of most likely 

avian dispersers (red ovals), Elwha River, Olympic National Park, Washington. 
47

Spatial Distribution

of Species

Spatial Distribution of

Environmental

Variables

Spatial Distribution 

of Vectors

Future Physical

Configuration

of Reservoirs

Input Information

Species-specific

Environmental & Vector

Relationships

Source

Areas

Vector distribution 

& flow analysis -- Logistic regression --

Pre-Dam Removal

Treatment Areas 

Pre-Dam Removal Priority Areas

Environments within

Treatment Areas

Post-Dam Removal Vulnerable Areas 

In Reservoirs

Environments within

Vulnerable Areas

Post-Dam Removal Priority Areas

Figure 19. Schematic description of the approach to identifying areas and environments within areas that should be treated for invasive plant species prior to dam 

removal and that will be vulnerable to invasion after dam removal.
48

Figure 20. Potential source areas of vectors of invasive species that could impact the de-watered reservoirs. Source 

areas are designated for wind (yellow), birds (orange), and mammals (blue). Locations of roads and trails also are 

indicated.
49

Figure 21. Environments (green within black lines) where C. arvense is most likely to occur north of Lake Aldwell. 
50

Figure 22. Environments that are potential sources of invasive species (green areas within black lines) in the area south of 

Lake Aldwell to the Elwha Campground. Hedera helix and Ilex aquifolia located within the red ovals has a high potential to 

be spread by birds. 
51

Figure 23. Environments that are potential sources of invasive plants (green areas within black lines) between Elwha 

Campground and Lake Mills. Areas where birds (red ovals) and elk (blue lines) are active are also indicated. (Put blue and 

red lines in key.)
52

Figure 24. Environments that are potential sources of invasive plants (green areas within black lines) along Lake Mills up to 

Geyser Valley. Besides roads and trails, elk and wind are the most important vectors in this area.
53

Figure 25. Environments that are potential sources of invasive plants (green areas within black lines) in Geyser Valley. Elk 

and trail users are the most likely vectors to spread invasive plants to the de-watered reservoirs.
54

Figure 26. Digitized records of invasive plants on the Olympic National Forest, west of the Elwha reservoirs. 
55

Figure 27. Probability of invasive plant establishment in areas of the de-watered Lake Aldwell reservoir determined using logistic regression equations based on 

observations collected between the Glines Canyon Dam and Highway 101 for (A) all invasive species observations, and (B) focal species observations.
56

Figure 28. Probability of invasive plant establishment in areas of the de-watered Lake Mills reservoir determined using logistic regression equations based on observations 

collected between the Glines Canyon Dam and Highway 101 for (A) all invasive species observations, and (B) focal species observations.
57

Figure 29. Predicted areas between Glines Canyon Dam and Highway 101vulnerable to invasive plants using logistic regression equations based on observations 

collected in this area for (A) all invasive species observations, and (B) focal species observations compared with those observations. 
58

Table 1. Invasive species surveyed in the Elwha River valley, Olympic National Park, Washington. [Washington State Noxious Weed Control Board classification: A=Non-native species that have limited distribution and State law requires that they be 

eradicated; B=Non-native species that are either absent from or limited in distribution in some parts of the State but very abundant in other areas. Management 

goals are to contain the plants where they are already abundant and prevent their spread to new areas; C=Non-native plants that are already widespread in 

Washington State. Counties can choose to enforce control, or they can educate residents about control]

Scientific name Family Common name Survey

2001 

High

concern

Focal WA

class

Records

Arctium minus Bernh. Asteraceae Common burdock X 136

Bromus tectorum L. Poaceae Cheatgrass 1

Buddleja davidii Franch. Buddlejaceae Butterfly bush X B 1

Calystegia sepium (L.) R. Br.ssp. sepium Convolulaceae Wild morning glory X 4

Centaurea jacea L. Asteraceae Brown knapweed X B 24

Cirsium arvense (L.) Scop. Asteraceae Canada thistle X X X C 257

Cirsium vulgare (Savi) Ten. Asteraceae Bull thistle X C 361

Cytisus scoparius (L.) Link Fabaceae Scot’s broom X X X B 78

Dactylis glomerata L. Poaceae Orchard grass X 348

Digitalis purpurea Scrophulariaceae Purple foxglove 29

Erechtites minima (Poir.) DC. Asteraceae Toothed coast burnweed X 4

Geranium robertianum L. Geraniaceae Herb Robert X X X B 356

Hedera helix L. Aralaceae English ivy X X 8

Hypericum perforatum L. Clusiaceae Common St. John’s-wort X X X C 87

Ilex aquifolium L. Aquifoliaceae English holly X X 14

Lathyrus latifolius L. Fabaceae Everlasting peavine X 5

Lathyrus sylvestris L. Fabaceae Small everlasting peavine X X X 136

Phalaris arundinacea L. Poaceae Reed canarygrass X X X C 223

Polygonum cuspidatum Siebold & Zucc. Polygonaceae Japanese knotweed X X B 10

Polygonum sachalinense F. Schmidt ex Maxim. Polygonaceae Giant knotweed X X B 2

Polygonum x bohemicum (J. Chrtek & Chrtkovβ) 

Zika & Jacobson [cuspidatum x sachalinense]

Polygonaceae Bohemian knotweed B 1

Potentilla recta L. Rosaceae Sulfur cinquefoil X B 5

Prunus laurocerasus L. Rosaceae Laurel cherry X 1

Rubus armeniacus Focke Rosaceae Himalayan blackberry X X X C 43

Rubus laciniatus Willd. Rosaceae Evergreen blackberry X X X C 6

Rumex crispus L. Polygonaceae Curly dock X 53

Senecio jacobaea L. Asteraceae Tansy ragwort X 17

Vicia hirsute (L.) Gray Fabaceae Hairy vetch X B 40

Vicia sativa L. ssp. sativa Fabaceae Common vetch X 46

Vicia villosa Roth Fabaceae Wooly vetch X 1
59

Table 2. Total number of observations, number of 50×50 m grid cells containing focal species, and percentage of grid cells by subarea with species 

present, Elwha River, Olympic National Park, Washington. 

Subareas (percentage of grid cells with species)

Focal species Abbreviation Total 

records

Total 

grid 

cells

Total grid 

cells (% 

with 

species)

Lake 

Aldwell

Lake

Aldwell 

Delta

Glines

Canyon 

Dam to 

U.S. 

Highway 

101, Wolf 

Creek Trail

Lake Mills Lake

Mills Delta

Geyser 

Valley

Hot 

Springs 

Road

Cirsium 

arvense

CirArv 257 232 21.2 3.1 1.4 4.0 0.5 17.8 13.6 1.4

Cytisus 

scoparius

CytSco 78 75 6.8 1.3 3.7 3.5 0.5 0 0 3.2

Geranium 

robertianum

GerRob 356 324 29.6 0.4 26.4 15.9 0 1.5 1.9 6.8

Hypericum 

perforatum

HypPer 87 81 7.4 1.8 9.9 2.2 0.4 2.0 1.4 0.5

Lathyrus 

sylvestris

LatSyl 136 128 11.7 0.4 5.8 7.7 0 0 0 0.5

Phalaris 

arundinacea

PhaAru 223 214 19.5 5.7 35.1 6.9 0.4 0.5 0.1 0

Rubus spp. RubSpp 49 42 3.8 0.9 6.6 1.4 0.4 0 0 0

Total 1,186 1,096 100 13.6 88.9 41.6 2.2 21.8 17.0 12.4
60

Table 3. Maximum likelihood odds ratios for predictor variables and cross-validation results for logistic regression models predicting the occurrence 

of invasive species in sites across the entire study area and by subarea, Elwha River, Olympic National Park, Washington. 

[Locations of subareas are shown in figure 2. Species: Abbreviations of species are defined in table 2. Odds ratios less than 1 indicate that the variable decreases 

the probability of species occurrence; odds ratios greater than 1 indicate that the variable increases the probability of species occurrence. Only statistically 

significant variables (α≤0.05) are presented. Elev., elevation; No. Obs., number of observations; Ppt., precipitation; SR-Jul, solar radiation on July 22] 

Odds ratios

Proximity to vector1 Environmental variables:

Species Location n Water Road Trail Elev. Precip Slope SR-Jul Bare Herbs Low 

shrubs

Tall 

shrubs

Low 

trees

Tall 

trees

Percent 

correctly 

predicted

CirArv All 232 0.996 0.987 1.028 1.040 1.055 293.6

Geyser Valley 85 1.001 0.931 0.963 0.988 86.4

Mills Delta 35 0.743 1.094 1.046 79.1

Glines-101 58 0.996 1.030 1.037 95.9

Aldwell Delta 33 0.922 1.043 85.6

CytSco All 75 0.997 0.984 1.035 1.048 97.9

Glines-101 50 0.985 0.995 1.037 96.5

GerRob All 328 0.995 0.831 0.948 0.968 1.032 90.9

Glines-101 228 0.996 0.998 0.968 1.046 0.985 1.027 84.0

Aldwell Delta 64 1.002 1.177 76.5

HypPer All 81 0.987 1.025 1.057 1.067 97.7

Glines-101 33 0.996 1.030 1.091 1.071 397.6

Aldwell Delta 24 1.070 90.1

LatSyl All 128 0.988 0.997 0.999 0.954 1.037 1.053 96.3

Glines-101 111 0.989 0.997 0.989 92.0

PhaAru All 214 0.989 0.971 0.985 1.027 1.035 294.1

Glines-101 99 0.989 0.953 93.0

Aldwell Delta 85 0.855 0.973 0.957 271.2

Lake Aldwell 26 0.975 1.220 1.062 294.2

RubSpp All 42 0.978 0.974 1.043 1.052 98.9

Glines-101 20 0.991 0.997 0.939 1.070 398.6

All Glines-101 343 0.998 0.984 1.009 1.020 1.033 73.5

All Focal Glines-101 298 0.998 0.976 1.011 1.037 1.001 1.027 1.035 77.4

1

Odds ratios <1 for proximity vectors indicate higher probability of species occurrence near to vector. 

2

P-value for chi-squared is less than 0.05, indicating poor relationship. 

3

Model has more than one variable per 10 observations.
61

Table 4. Falling velocities of focal species, other 

species of concern and related species reported by 

Tackenberg (2001).

[m/s, meters per second]

Species Falling velocity (m/s)

Cirsium arvense 0.2– 0.4

Centaurea jacea 3.4– 4.1

Geranium pratense 3.7

Geranium sylvaticum 2.8–3.0

Hieraciuim aurantiacum 0.3

Hypericum perforatum 1.3

Hypochaerus radicata 0.3– 0.4

Lathyrus pratensis 3.0

Potentilla erecta 1.7– 2.3

Prunus spinosa 5.5

Senecio jacobea 0.4 – 1.0

Table 5. Summary of wind speed and direction detected at Hurricane Ridge RAWS station for July 15–September 

30, 1999–2008, Olympic National Park, Washington. 

[Values indicate percentage of time wind came from each direction (average values greater than 3 percent are in bold). %, 

percent; m/s, meters per second] 

Year N NNE NE ENE E ESE SE SSE S SSW SW WSW W WNW NW NNW Calm

1999 11.0 0.8 1.4 2.5 2.1 1.3 6.9 7.0 3.2 1.3 1.0 0.8 3.1 1.7 0.8 4.2 50.9

2000 8.2 6.9 1.0 0.5 2.1 1.7 0.8 0.9 12.0 10.3 1.8 1.2 1.0 0.9 1.0 1.2 45.3

2001 8.9 6.4 2.8 2.8 4.2 3.0 3.4 3.0 3.3 3.0 2.1 2.4 2.1 2.5 2.6 4.2 43.2

2002 7.3 13.8 1.8 0.8 2.5 2.0 1.1 1.9 8.8 4.4 0.9 0.9 1.1 0.8 1.1 4.6 46.2

2003 4.6 5.8 0.6 0.5 1.7 3.7 1.1 1.8 14.0 6.1 1.4 0.6 0.6 0.3 0.8 2.1 54.3

2004 3.2 6.0 0.5 0.7 1.8 3.1 1.4 1.7 18.0 11.2 1.9 0.4 0.2 0.8 1.3 2.3 45.8

2005 0.4 6.7 3.3 1.2 7.8 2.6 0.5 0.4 0 0 0 0.1 0 0 0 0 77.0

2006 0.3 1.9 1.7 1.0 5.1 0.3 0.4 0.1 0 0 0 0 0 0 0 0 89.1

2007 0.1 0.9 1.9 1.2 18.0 0.3 0.5 0.3 0 0 0 0 0 0 0 0 76.6

2008 7.2 1.3 0.7 0.8 2.9 1.3 1.6 16.1 16.0 1.6 1.0 1.6 0.9 0.6 1.7 2.1 42.8

Avg 

% 

5.1 5.0 1.6 1.2 4.8 1.9 1.8 3.3 7.5 3.8 1.0 0.8 0.9 0.8 0.9 2.1 57.1

Max 

(m/s)

12-

15

9-

12

9-

12

9-

12

9-

12

9-

12

9-

12

9-

12

15-

18

9-12 6-

9 

9-12 9-

12

9-12 12-

15

9-12 <1.3
62

Table 6. Comparison of wind speed and direction predicted by WindNinja with actual conditions at Glines Canyon 

Dam, Elwha River, Olympic National Park, Washington. 

[m/s, meters per second; Dir, direction; o

, degrees] 

Hurricane Ridge conditions

From North, 15 m/s

n = 5

From east, 12 m/s

n = 5

From south, 18 m/s

n = 7

Dir (o) Speed (m/s) Dir (o) Speed (m/s) Dir (o) Speed (m/s)

WindNinja 

prediction

15-19 19-21 316-324 5-6 195-200 23-25

Glines Canyon Dam 

(average)

166 6 302 7 203 16

Glines Canyon Dam 

(range)

92-194 0-12 217-26 4-9 173-233 3-26

Table 7. Potential destinations of Cirsium arvense seeds from present populations as a result of wind dispersal 

from the most common wind patterns based on output from the WindNinja model, Elwha River, Washington. 

Wind Direction (Source)

Current Location North East South

Geyser Valley Up valley on western side of 

river

Down valley, possibly terrace 

SW of Rica Canyon

Downstream to Lake Mills 

Reservoir

Lake Mills delta Terrace southwest of Rica 

Canyon

Eastern shore Lake Mills Downstream to Lake Mills 

Reservoir

Lake Aldwell delta Herrick road area, river valley to 

south

Plateau northwest of U.S. 

Highway101–Elwha River 

intersection

Southern Lake Aldwell 

delta and upland area to 

west

Table 8. High priority potential sources of exotic species as a result of wind dispersal from the most 

common wind patterns based on output from the WindNinja model, Elwha River, Washington. 

Wind Direction (Source)

Reservoir North East South

Lake Aldwell –All areas due north including Elwha River delta None –Hillslope east of Lake Aldwell 

Lake Mills –Elwha River valley bottom to north

–Herrick Road plateau

–Hillslope west of Glines Canyon Dam

None –Plateau southwest of Lake Mills
63

Table 9. Summary of species traits for focal species and species of high concern that can be used to prioritize 

species threat, and the resulting priority. 

[Species data are from Washington State (2009). Habitat: Habitat with which species has been associated. Characteristics 

indicating a high priority for removal are indicated in bold. NA, not applicable; yr, year; veg, vegetation] 

Species Vectors Time to flower Habitat Removal 

difficulty

Spread 

rate

Priority

Buddleja spp. Wind

Water 

First yr River banks High –

seed bank

Rapid High

Cirsium arvense Water

Wind

NA (veg spread) Waterways Highherbicides

Rapid High

Cytisus scoparius Water > 3 yrs River banks High –

seed bank

Moderate Mid

Geranium robertianum Elk

Machinery

First yr Forests, meadows Moderate Rapid Mid

Hedera helix Birds 10 yrs Forests Easy Rapid Low

Hypericum perforatum Elk First yr or 2 to several yrs Bare High ? Mid

Ilex aquifolium Birds 2–10 yrs Forests Moderate Slow Low

Lathyrus sylvestris Water First yr Disturbance ? Moderate Mid

Phalaris arundinacea Water First yr River banks High Rapid High

Polygonum spp. Water

Wind

NA (veg spread) Flood zone High Rapid High

Rubus spp. Birds 2+ yrs River flats High Rapid Mid
64

Table 10. Summary of areas, species of concern that are already present (searched areas), or potentially present

(areas not searched), and potential management actions, Elwha River, Washington. 

[Priority vectors and species are indicated in bold. Abbreviations of species of concern are defined in table 2] 

History Area Vectors Species of 

concern

Management action

Not searched; outside of 

Park

North of Lake Aldwell Wind CirArv

BudSpp

Advocate mowing

Herrick Road Wind

Elk

Birds

CirArv

LatSyl

PhaAru

Searched Aldwell Delta Wind

Water 

CirArv

GerRob

LatSyl

PhaAru

Options:

Remove entire plants

Remove flowers or 

seeds

Lake Aldwell to northern end of 

Lake Mills

Wind

Water 

Birds

CirArv

CytSco

GerRob

HedHel

HypPer

IleAqu

LatSyl

PhaAru

RubSpp

Lake Mills Delta Wind

Water 

Elk

CirArv

GerRob

HypPer

PhaAru

Geyser Valley Water

Elk

CirArv

GerRob

HypPer 

PhaAru 

Glines Canyon Dam Machinery CytSco

GerRob

HypPer

Elwha Dam Machinery CirArv

CytSco

GerRob

HypPer

LatSpp

PhaAru

PolSpp

RubSpp

Not searched; inside of 

Park

Between roads, western side of Lake 

Mills

Birds CytSco

GerRob

HedHel

HypPer

IleAqu

RubSpp

Search and remove

Western side Lake Mills, southern

end, plateau and drainages

Wind

Water 

Elk

CirArv

GerRob

HypPer

LatSyl

PhaAru

PolSpp

RubSpp
Publishing support provided by the U.S. Geological Survey

 Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

 Director, Forest and Rangeland Ecosystem Science Center,

 U.S. Geological Survey, 777 NW 9th Street

 Corvallis, Oregon 97330

 http://fresc.usgs.gov
Woodward and others—Predicting Spread of Invasive Exotic Plants into Dewatered Reservoirs After Dam Removal on the Elwha River, Olympic National Park, Washington—Open-File Report 2011–1048