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. . TaJlldfislC.OIl CWCB PrcsclIt.ttioJl, 1-25-2006 SUlluual'Y Tamarb.:k InvcntOl'Y fOl' the Colorado, Arkansas, and Purgatoire Ri\'ers Hackgl"ounu -11w Tamarisk Coalition received a Sso,ooo grant from the Colorado \Vater Conservation Board to perform an inventory of tamllrisk infestations on the Colorado and Arkansas Rivers within the state. This report sumlllJri:t.es the findings from this effort. The purpose of this work was to estahlish and implement an inventory protocol th;H is economical to perform and would pro\~de a clear understanding of the extent of the tamarisk problem. The Colorado River and Arkans<.ls River watersheds are identified bec;Juse they are heavily infested and represent two distinct topogrJphic conditions - J western slope canyon setting that is predominantly Federal lands and a front range prairie .setting that is predominantly private agricultural lands. A survey of the Purgatoire River in southeastern Colorado was completed with local funding and this information is included within this summary report. The "lampa River and its tribulJncs, and the headwaters of the Dolores to McPhee Reservoir were also surveyed; however, this information is currC'ntly being analyzed and is not yet available. Inventory - IIwentoI}' and mapping were performed during the summer and fall of 2005 and coordinated with the U.S. Geological Survey's (USGS) efforts at establishing a national on-line database which would conform to the weed mapping ."tandards developed by the North AmeIican Weed Management Association. The basic approach was to use existing aerial photography and satellite image!)' and local knowledge available from counties, river districts, soil and water conservation districts, state agencies, Army Corps of Engineers, National Resources Conservation Service, USGS, CSU, and The Nature Conservancy. This information was then "ground-tmthed" by a 2-man team to confinn infestation density, maturity, terrain, presence of native species, and several other site characteristics. GPS data and digital photo records \\'crc taken and shape tiles were developed utilizing GIS capabilities at Mesa State College. Over 800 miles on the Color<lJo, Arkansas, and Purgatoire rivers and their major tributaries were surveyed using this approach. This information, in the fonn of shape files and characteristics data, has been transfonned into a digital GIS database which soon will be aVJilable on the USGS invasive species website, ww\v.niiss.org. Finding - The inventory summJI)' for the three river systems is presented in Tables 1 and 2. TributalY infoJ1l1ation is presented in T<Jbles 3 and 4. Tables 1 and 3 present the general mapping data; whereas, Tables 2 and 4 provide information on estimated current water and future water losses associated with the tamarisk infestations and the estimated costs for tamarisk control and revegetation. These water losses and cost estimates are based on the most recent research and statistical analysis available through the USDA, NOAA, USGS, CSU, National Invasive Species Council, T;:lInarisk Coalition, and others. The following represents our findings: I. The Arkansas River is the most heavily infested \\-1.th 29,280 tot]l acres, has the highest average density of 57%, and the greatest average width at 1,226 feet.