Southwest - ValleyFloor_TellurideStreamRestoration_Final Report

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*RIC RANGE OF VARIABILITY ASA CONTEXT FOR RESTORRNON 211 along the Colorado River valley, and may be the origin of the Receding limb flows driven by late-season snowmelt at high Lulu City wetland(Figure 1). altitude areas are almost completely captured by the Grand Pleistocene glaciers in the Colorado River Valley extended Ditch,severely shortening and reducing the recession (Woods, south —24 km from the Continental Divide to Grand Lake 2000;Rubin,2010).The flow reduction changes sediment trans- (Meierding,1980)and thus completely covered the study area. port capacity and lowers the water table,which impact riparian Although little work has been done in the upper Colorado River plant communities(Woods and Cooper,2005),benthic inverte- valley to define the chronology of glacial retreat, Richard brate abundance (Clayton and Westbrook, 2008), and beaver Madole (US Geological Survey, pers. comm., 2009) suggests that are dependent on willow and other riparian vegetation. an estimated age of 14 000 yr BP as the end of valley glaciation The most visible effect of the Grand Ditch is the repeated throughout the Front Range in Northern Colorado. occurrence of breaches or failures,and subsequent debris flows Holocene climate records have been reconstructed in the that increase sediment loads down Lulu Creek and adjacent Rockies through correlations with pollen (primarily sagebrush, channels. Aerial photos dating back to 1937 show evidence grasses,birch,willow, spruce,fir, and pine). Post-glacial warm- of Grand Ditch-initiated debris flows and gully erosion as well ing occurred from—14 000-11 000 yr BP(Doerner, 2007).The as debris-flow deposits in the Lulu City wetland (Rathburn Younger Dryas cooling interrupted the warming from —11 and Rubin, 2009) which suggest an elevated sediment and 000-10 000 yr BP, causing glaciers to advance and tree line disturbance regime that has persisted for at least 70 years. In to shift downward by as much as 120 m(Short, 1985;Doerner, the Lulu City wetland, the Ditch-related sediment inputs 2007). The middle Holocene Altithermal was categorized by changed the meandering single thread channel (visible in warmer and wetter conditions —9000-4500yr BP (Elias, 1985; the 1937 air photo)to a bifurcated zone of deposition at the Fall, 1985, 1997). During the Neoglacial, cooler and drier head of the wetland. The most recent debris flow in 2003 conditions persisted from 4000 to 2000yr BP causing deposited up to --1 m of sand and gravel in the wetland. another small-scale advance of alpine glaciers (Fall, 1997). The investigation of HRV seeks to evaluate whether similar The last 2000 years have been generally similar to the sediment inputs and planform changes occurred prior to the modern climate (Vierling, 1998). The Medieval Optimum 19th century. (1050-750yr BP) is not identified in local climate recon- Human impacts to the upper Colorado River basin follow a structions, but has been documented in North America and pattern similar to many parts of the central Rocky Mountains globally as a warm period. including water diversion, mining, beaver trapping, and vari- The Colorado River at the Lulu City wetland is a 3rd-order able affects on fire regime such as ignition and suppression stream (Strahler, 1952). Channel slope in the wetland is (Wohl,2001).Development of the'Lulu City'mining town just —0.9%, while the Colorado River upstream has a slope of upstream of the wetland occurred during the 1880s.Lulu City, 2-3%and pool—riffle sequences.Tributaries such as Lulu Creek now a 1 hour walk from the nearest road,reached a population are typically step-pool channels with slopes as high as 20%. of several hundred people during its brief existence(Buchholtz, Snowmelt dominates the annual hydrograph, with peaks typi- 1983). In addition to disrupting valley-bottom sediments for cally occurring in late May and early June.Over 80%of annual placer mining, and associated downstream increases in sedi- runoff occurs during the melt period from May through July ment yield,inhabitants of mining areas started fires and created (Woods, 2000). Annual precipitation is estimated at 110cm localized deforestation that probably destabilized hillslopes (Capesius and Stephens, 2010), with —84% as snow at the and increased sediment yields(Wohl,2001). nearby Lake Irene SNOTEL station (CO05J10S, elevation When beaver are present along a river,their dams slow flood 3260 m)(Westbrook et al.,2006). peaks,increase the extent of overbank flooding and groundwater Vegetation in wide portions of the valley bottom along the recharge(Westbrook et al.,2006)and retain sediment(Butler and upper Colorado River is a mix of riparian shrublands dominated Malanson, 2005;Westbrook et al.,2010). Beaver present in the by willow(Salix monticola,S.geyeriana,S. drummondiana),dry vicinity of Lulu City were probably removed within the decades meadows with Deschampsia cespitosa and Calamagrostis immediately preceding the existence of Lulu City(Wohl,2001); canadensis, and peat-accumulating fens dominated by any beaver colonizing the area following initial trapping in the willow(Salix planifolia)and Carex aquatilis. Hillslope vege- 1820s were probably decimated during the brief occupancy of tation is dominated by Engelmann spruce (Picea engelmannil), Lulu City. Beaver populations subsequently began to recover, lodgepole pine (Pinus contorta), and subalpine fir (Abies but during the late 20th century a drastic decline in beaver lasiocarpa) (Woods, 2000; Westbrook et al., 2006). Willow populations throughout Rocky Mountain National Park reduced communities throughout Rocky Mountain National Park an estimated Colorado River Valley population from 600 beavers have been in drastic decline in recent decades, probably a in 1940 to 5%of that amount(0 in the Lulu City wetland or result of multiple stressors including browsing pressure anywhere upstream of the Colorado River Trailhead) by from increased moose and elk populations (Hess, 1993; 1999(Packard,1947;Mitchell et al.,1999).Some,if not most, Kaczynski et al., 2010). of this decline may be associated with increasing elk and moose populations and heavy grazing pressure (Hess, 1993) that reduce or eliminate woody riparian plants on which History of human impacts beaver,elk and moose feed,and which influence stream bank stability,overbank roughness,and sediment transport in channels The Grand Ditch water diversion (Figure 1)was constructed in and overbank environments. Beaver populations prior to stages from 1890-1936(Woods,2000)to divert tributary runoff European settlement and trapping were probably even from the Colorado basin over the Continental Divide to the higher than the 1940 levels, although specific estimates for Cache la Poudre watershed.The diversion affects the Colorado the Colorado River are not available (Seton, 1953; Wohl, River by reducing flow volumes, shortening the duration of 2001;Westbrook et al., 2006). In aerial photos of Lulu City elevated flows,and increasing fluctuations in discharge(Woods, wetland no new beaver-dammed ponds appear after 1969, 2000; Rubin,2010).Although the Grand Ditch captures runoff although the resolution is poor in the earlier photos and the from approximately the upper 50%of the watershed,peak runoff ponds that are present may be several decades older than exceeds Grand Ditch capacity in most years,and peak flows on 1969. The combined effect from two centuries of alteration the Colorado River are thus reduced by less than 50%. has probably been to increase sediment yields to the valley Copyright©2011 John Wiley&Sons,Ltd. Earth Surf.Process.Landforms,Vol.37,209-222(2012)