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1'!/VJ V1 'lHU JtJ: t.9 t•'A1 97U LGO tl4a1 <br />• <br />s <br />q° r <br />,~ <br />f--I "~' <br />5 km ,1` f <br />M <br />IQ 003 <br />349 <br />104 3T 33' <br />Study <br />~d'• Area <br />39° 37'33' <br />~~ ~ <br />~~ <br />I <br /> <br />Flgure 1. General location of the Coal <br />Creek Study area, including its position <br />within the.vatershed, and a detail of the <br />study reach with locations of transacts, <br />gauges. cottonwood stands, and mining <br />activity. <br />MESC; SP,ES <br />Cottonwood Respons8'fo Water Table Declines <br />Coal Creek drains approximacely 124 kms of short-grass <br />prairie 80 km southeast of Denver, Colorado, USA, acrd <br />flows north from the base of the Palmer Divide, which <br />separates the South Platte and Arkansas River basins. <br />Coal Crack is an ungauged ephemeral stream that <br />typically flows for a few days in sprang or for brief periods <br />after locally heavy rain. Most of the water in the basin <br />moves down valley as shallow alluvial groundwarer. <br />East of the Pront Range of the Rocky Mountains, the <br />South Platte River drainage is located within the Denver <br />Basin, a lazge, arymmetrical syncline. Strata overlying <br />the svuctural formation of the Denver Basin are primar- <br />ily sedimentary and range in age from Paleowic to <br />Recent. The prindpal bedroclt of the study azea is the <br />Dawson Arkose, which is composed of Arkosic sand- <br />stone, shale, mudstone, conglomerate, and local coal <br />beds of upper Cretaceous and Paleocene age (Duke <br />Tx =Transactksca9ons <br />® = Sudace scrape (1891) <br />® =Deep minhtg (1992) <br />® = Rlpadan cottonwoods <br />O =Crest stage gage <br />r 1 bra <br />and Longenbaugh 1966). Because strata of the Deaver <br />Basin are relatively impermeable, the alluvial aquifer at <br />Coal Creek is isolated from deeper iegional aquifers <br />(Bjorklund and Brown 1957). Logs from ten test bor- <br />ings at the study site indicate a medium hard claystone <br />to very hard claystone/sandstone bedrock at an average <br />depth of 4.95 m (so ~ 1.8 m) (A G. Wassenaar, Inc. <br />unpublished report). The alluvium consists of poorly <br />graded, slightly silty medium sands (0.297-1.19 mm <br />diameter) with some clay [o clayey sand lenses. <br />A mature cottonwood Forest dominated by Populiu <br />dekaidessubsp. moni4fera occurs along Coal Creek where <br />the alluvial water table occur within -1-3 m of the <br />ground surface. The riparian zone of Coal Creek is <br />dominated by mature cottonwoods with three diameter <br />claues centered around 80 cm, 50 rm, and l cat. The <br />trees range in age from a few to apprordmacely 150 <br />