BOARD01713

• Board Meeting Date: 7/28/2003
• Description: CF Section - Tamarisk Control and Eradication Update
• Board Meetings - Doc Type: Memo

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Impact of Tamarisk Infestation on the
Water Resources of Colorado
May 30, 2003

state's rivers and streams. Other plants, notably Russian olive (Elaeagnus angustifolia), co-habit
with tamarisk and also deserve attention. Therefore, within the context of this study, whenever
the tenn "tamarisk" is used it also includes Russian-olive as the other principal invasive plant
that may be important to control within riparian areas.

Report Outline

This report is intended to provide some fundamental answers on the tamarisk problem in
Colorado that may be important for water managers within the state to understand; i.e.,

~ How many acres of tamarisk exist in Colorado?
~ How much water is being used by tamarisk versus native plant species? and
~ What is the cost of tamarisk control and reve~tation?

2.0 Tamarisk Inventory in Colorado

An inventory of infestation by tamarisk and the other principal non- native phreatophyte
identified by the Colorado Department of Agriculture, Russian-olive, is a crucial element in tre
development of a control plan at the state-wide scale. The inventory becomes the basis for
project planning (e.g., cost estimates, resource allocation, scheduling) and tracking the long-tenn
success of control efforts.

The Colorado Department of Agriculture has assembled maps of weed infestations within the
state based on input from county weed managers. A tabulation, by county, of tamarisk and
Russian-olive infestation is presented in Table I and depicted in Figures I and 2, respectively for
each plant species. Based on this infonnation, there are over 28,000 acres of tamarisk and over
10,000 acres of Russian-olive. However, there is a problem with the infonnation. Many
counties did not report any infestations. For example Bent County in southeastern Colorado is
located on the lower Arkansas River and has the large John Martin Reservoir within its political
boundary. This area is known for its abundance of tamarisk with counties on all sides having
significant infestations. The Corps of Engineers, which operates the John Martin Reservoir site,
has reported that the lower Arkansas River main stem has an estimated 12,000 acres of tamarisk
as compared to 6,200 acres reported by the Department of Agriculture for this area; or a 100%
difference. In Yuma County, the county weed manager has found 500 acres surrounding Bonny
Reservoir but the state weed inventory has only 73 acres listed. EI Paso County reports no
tamarisk or Russian-olive, yet the invasive species coordinator for Fort Carson reports significant
infestation along Fountain Creek. Larimer County shows no infestations along the South Platte
River, yet Morgan and Adams counties on either side have some degree of infestation. Other
counties, from personal observation, such as La Plata, Archuleta, and Conejos counties have
infestations but have not reported any.

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