by Eric Fairlee, City of Boulder Open Space and Mountain Parks Department

 Eurasian watermilfoil
Photo 1. Eurasian watermilfoil stem and leaves.

Eurasian watermilfoil (Myriophyllum spicatum EWM) is an aggressive, nuisance aquatic plant introduced into the United States sometime between the late 1800’s and the 1940’s. Native to Eurasia, EWM has rapidly spread throughout most of the northern hemisphere and is documented in nearly all of the lower forty-eight states. EWM was likely introduced into North America as an aquarium plant that quickly spread from one water body to another by mass flow of water and by accidental introduction on boats and boat trailers.
EWM is a perennial, herbaceous, submersed plant which forms a dense canopy of branches at the water surface (Photo 1). EWM mats impede all forms of water-based recreation (boating, swimming, fishing, etc.), reduce the aesthetic appeal of water bodies, clog industrial, power, and irrigation intakes, lower dissolved oxygen concentrations, increase populations of permanent pool mosquitoes, and alter aquatic food webs and population dynamics.
Colorado is one of the most recent states afflicted by this weed, with documented populations dating back for at least ten years. Populations are found in the Denver-Boulder metro area, the Rio Grande River in Alamosa County, and Huerfano County. EWM is present in reservoirs, ponds, wetlands, rivers, creeks, and irrigation ditches.
In Colorado, the density and extent of EWM populations in creeks correlates to riparian vegetation structure and stream channel morphology (personal observations). Nutrients, light, and temperature are documented to determine EWM populations and growth habits in lake systems (Madsen et. al. 1988, Smith and Barko 1990, Madsen and Smith 1997, Madsen 1998). Riparian vegetation structure and stream channel morphology appear to affect environmental conditions that influence EWM growth. Surveys of both Boulder Creek and St. Vrain Creek in Boulder and Weld Counties, Colorado, indicate that EWM establishes higher population densities and occupies more riverine habitats along reaches with sparse riparian tree canopy, with modified channel structure associated with flood conveyance or gravel mining operations, and with a low pool to riffle ratio (areas dominated by riffles or glides).

 Reclaimed channel of Boulder Creek
Photo 2. Reclaimed channel of Boulder Creek dominated by EWM. This area receives minimal shade.

EWM favors moderate to high nutrient water bodies (mesotrophic to moderately eutrophic) (Smith and Barko 1990, Madsen 1998). Elevated nutrient levels from increased erosion and urban runoff, fertilizer, or sewage treatment inputs in impacted streams will likely lead to a rapid growth response.
Water transparency plays a significant, but not exclusive, role in EWM distribution in a water body by determining the amount of sunlight available for plant growth at varying depths of water (Smith and Barko 1990). Moderate to highly turbid water generally restricts EWM to shallow water where EWM forms an extensive canopy of horizontal stems that branch off at or near the water surface from the distal ends of a relatively few vertical stems rooted in the sediment. This characteristic growth form of EWM shades the substratum and suppresses the establishment of other aquatic species, limiting species diversity where it occurs. In relatively clear water bodies, EWM grows at considerably greater depths and may or may not reach the surface, yet attenuates light penetration to the substratum.
In Boulder and St. Vrain Creeks, turbidity possibly limits EWM occurrence in deeper pools. Likewise, along certain reaches, shading from riparian vegetation appears to reduce light intensity at the channel level to below the threshold required to sustain continued growth of EWM. Observations indicate that EWM is less vigorous and/or not present in creek reaches shaded by riparian trees, even in shallow water areas. One 1,900 foot contiguous stretch of Boulder Creek contains +/- 700 feet of shaded stream channel and +/- 1,200 feet of stream channel exposed to the sun (Photo 2). Both reaches have similar channel morphology, water depth, substrate, and flow. The shaded area, however, contains very scattered EWM plants, whereas the exposed reach contains between 50% and 100% cover of EWM across the entire channel. Within this stretch, EWM starts when the riparian trees stop.
Observations suggest that EWM is a sun-loving species that succumbs to shading from riparian and transitional species.
Temperature can play an important role in determining the growth pattern of EWM (Smith and Barko 1990, Madsen and Smith 1997). Plants photosynthesize and grow over a broad temperature range of 15°C to 35°C, with maximum growth rates between 30°C and 35°C. It is probable that reaches of Boulder and St. Vrain Creeks with degraded, shallow, slow flowing water receiving full sun have higher water temperatures than reaches with good riparian structure. EWM may start growing earlier in the season and grow faster in these reaches. In addition, once present, dense mats of EWM absorb heat and further increase water temperatures within and around the mats (Madsen 1998, Unmuth et. al. 2000). Thus, observations suggest that reaches with poor riparian structure and low channel diversity are likely more conducive to EWM growth.
Other factors, such as the presence of in-stream objects, flow patterns, and substrate also contribute to EWM establishment (personal observations). However, it appears that riparian structure (presence of trees and shrubs) and channel morphology play large roles in determining density and extent of EWM populations in creeks and small rivers in Colorado.
Further research is needed to determine environmental factors that contribute to the establishment and spread of EWM in Colorado streams. Determining relationships between rate of colonization and spread is necessary for long-term management and potential impacts to aquatic and riparian systems. Observations of Boulder and St. Vrain Creeks indicate that effective management practices to suppress EWM in moderate-sized streams should include riparian enhancement to shade the stream channel and modulate water temperature.
Madsen, J.D., 1998. Predicting invasion success of Eurasian watermilfoil. Journal of Aquatic Plant Management 36:28-32.
Madsen J.D., L.W. Eichler, and C.W. Boylen. 1988. Vegetative spread of Eurasian watermilfoil in Lake George, New York. Journal of Aquatic Plant Management26:47-50.
Madsen, J.D., and D.H. Smith. 1997. Vegetative spread of Eurasian watermilfoil colonies. Journal of Aquatic Plant Management 35:63-68.
Smith, C.S. and J.W. Barko. 1990. Ecology of Eurasian watermilfoil. Journal of Aquatic Plant Management 28:55-64.
Unmuth, J.M.L., R.A. Lillie, and D.S. Dreikosen. 2000. Influence of dense growth of Eurasian watermilfoil on lake water temperature and dissolved oxygen. Journal of Freshwater Ecology 15:497-503.

Colorado Riparian Association