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O LSSO N <br />ASSOCIATES <br />Technical Memorandum <br />An in -depth and literature analysis of potential impacts to cottonwood trees <br />due to winter dewatering of the Dillon Ranch Pit <br />May 1, 2014 <br />Prepared for Gayle Lyman, Elam Construction, Inc. <br />By Jen (Stevenot) Sinclair, Olsson Associates <br />Olsson Associates ( Olsson) has performed a review of the information in the proposed plan from <br />Elam Construction, Inc., regarding dewatering activities and potential impacts to cottonwood trees <br />growing near the dewatering location of the Dillon Ranch Pit (project). This technical <br />memorandum summarizes the literature available, and full PDF documents of the referenced <br />articles are provided as an enclosure to this memorandum. <br />In search of scientific evidence that the project will not have an adverse effect on the cottonwood <br />trees adjacent to the project area, Olsson conducted a literature review of several published <br />articles regarding cottonwoods. According to the document, Responses of Riparian Cottonwoods <br />to Alluvial Water Table Declines (Scott et al. 1999), there is potential for adverse effects to <br />cottonwood trees in instances where the water table is permanently reduced. However, this <br />project is proposing a temporary, wintertime (December 15 — February 15) reduction to the water <br />table on a yearly basis in a small confined area; therefore, no permanent impacts to the <br />cottonwood trees are anticipated because the permanent water table will not change and the <br />water table will not be impacted during the active growing season. In the document, Drought <br />stress and recovery of riparian cottonwoods due to water table alteration along Willow Creek, <br />Alberta (Amlin and Rood 2003), there is evidence to support that even though there is potential <br />vulnerability of riparian cottonwoods due to drought caused by water table depletion, the <br />physiological recovery of cottonwoods is rapid following restoration of water availability. This study <br />observed a period of dewatering that lasted three full years, which is significantly longer than the <br />seasonal dewatering periods of the proposed project. <br />Transpiration is the process of water movement through a tree including evaporation from leaves. <br />In the article, Controls on transpiration in a semiarid riparian cottonwood forest (Gaza) et al. 2006) <br />it is noted that cottonwood trees have high rates of transpiration due to the trees' access to <br />groundwater and the high evaporative demand of the atmosphere. Building on that information, <br />Seasonal estimates of riparian evapotranspiration using remote or in situ measurements <br />(Goodrich et al. 2000) makes the important point that the transpiration period for cottonwood trees <br />1 <br />