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~ ~ III IIIIIIIIIIIIIIII EFFECTS OF GRAVEL MINING ON NATURAL COTTONWOOD STANDS Michael L. Scott, Erik D. Eggleston, Gregor T. Aubie, Jonathan M. Friedman, and Lee S. Ischinger National Biological Survey Midcontinent Ecological Science Center 4512 McMurry Ave. Fort Collins, CO 80525-3400 ABSTRACT Alteration of groundwater hydrology has had widespread influence on western riparian vegetation; however, there have been few attempts to quantify the response of riparian vegetation to such changes. We describe structural and growth responses of mature cottonwoods to a gradient of alluvial groundwater decline resulting from sand mining in Coal Creek, an ephemeral stream in the Colorado plains. Twice-monthly, groundwater measuretnents and annual tree crown volume assessments were made within transects along mined and unmined reaches beginning prior to the onset of mining in 1992. Within two months following mining, the groundwater declined between 1.45 m at the channel edge to 0.94 m a.t the valley wall. There was a corresponding decrease in crown volume (estimated on a scale of 0-100) for mature cottonwoods ranging from 35 at the channel edge to 5 at the valley w;tll. By the following August, the groundwater decline in the mined reach tonged from approximately 2.3 m to 1.8 m. Similarly, tree mortality ranged from 15% to 75%. For surviving trees, crown volume decreases ranged from 15 to 45 and there was little net increase in stem basal area. By contrast, in the unmined reaches, tree mortality was <5%, and changes in crown volume ranged from decreases of 10 to increases of 20. In addition, there wa:; a net increase in stem basal area over the same time period. Such information can improve assessment of impacts from mining and water development activities on existing riparian communities. INTRODUCTION Western riparian ecosystems are unique landscape features that provide a wide range of important ecological goods and services. Because these ecosystems are associated with scarce surface water resources, riparian zones have long been influenced by human activities. As the west becomes increasingly urbanized, riparian resources will be subject to an increasing array of impacts. Among the most serious impacts are water impoundment and diversion, groundwater pumping from alluvial aquifers, livestock grazing, land clearing for agriculture or to increase water yield, mining, road development, heavy recreational demand, fire, the c;limination of native organisms (e.g. beaver) the introduction of non-native plant