Southwest - ValleyFloor_TellurideStreamRestoration_Final Report

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210 • Z.RUBIN ETAL. Process-based restoration relies upon understanding what et al., 1993). By acknowledging a range of historic forms and the natural forms and functions of a watershed were and can processes,we gain insight into the potential for passive resto- be, given imposed climatological, ecological, geological, and ration,and when active restoration can be applied effectively. land-use constraints. Failure to appropriately assess HRV of a This research investigated the HRVof a subalpine valley by system, or to appropriately define contemporary physical analyzing stratigraphic records to quantify aggradation rates dynamics,has resulted in several well documented restoration and processes of sediment delivery before and after European failures such as Uvas Creek in California, USA,where restora- settlement of the region in the mid-19th century.To investigate tion activities attempted to impose a meandering planform in HRVover a time scale of 102 to 103 years,we conducted anal- a location where watershed history suggested a system that ysis of aerial photography, ground penetrating radar surveys, was historically braided in response to disturbance (Kondolf, radiocarbon dating, and sediment descriptions via trenching 2006).Innumerable other restoration projects have not achieved and coring.In May 2003,a breach in the Grand Ditch initiated any broad ecological benefit because restoration attempted to a debris flow in Rocky Mountain'National Park that transported impose forms that were inappropriate or unsustainable given --36,000 M3 of sediment into the Upper Colorado River and present climate, land-use, and watershed conditions (Wohl deposited up to 1 m of sediment in the Lulu City wetland. et al., 2005). Process-based restoration is particularly impor- Material deposited in the wetland was derived from both the tant for watersheds such as the one discussed in this paper, hillslope and from eroded valley-bottom sediments.The National which is subject to relatively frequent intense disturbances Park Service plans to restore channels, riparian areas, and in the form of debris flows. As in the Uvas Creek example, wetlands impacted by the debris flow. Understanding HRV attempts to 'permanently' impose a form that represents only of disturbance regimes provides context for the 2003 debris one variant of a continually changing process-form continuum flow and restoration of the Lulu City wetland. Results were are unlikely to succeed. used to assess the usefulness of active restoration. HRV in river systems can be(1)extrapolated from theoretical or empirical models (either conceptual or quantitative) of process—form interactions in general, (2) based on historical Study Area records from the study site,or(3)extrapolated from nearby,less impacted, reference sites. Because no single method compre- The study area is the headwaters of the Colorado River on the hensively describes historical conditions, a combination of west side of Rocky Mountain National Park,with particular fo- approaches should be used when possible. Aerial photos, cus on the Lulu City wetland and the —29 km2 upstream discharge and sediment gage records, specific gage analysis, (Figure 1).The elevation of the watershed ranges from —2830m surveys, and dating modern deposits with dendrochronology at the Lulu City wetland, to 3944m at Mount Richthofen. The are the most common methods to provide a historical context watershed is bounded by the Never Summer Mountains on the for restoration (Soar and Thorne, 2000;Watson et al., 2007). west, La Poudre Pass on the Continental Divide to the north, Typically,the aforementioned methods are capable of catego- and the Front Range of the Rocky Mountains to the east. The rizing impacts to channel form and process for a time-scale on Never Summer Mountains consist primarily of Oligocene granite, the order of 10'years. Given the short time-periods covered and the Front Range is primarily Proterozoic biotite schist with by 'historical' investigations, restoration targets commonly some Oligocene rhyolitic flows and tuff(Braddock and Cole, seek to emulate pre-disturbance or reference forms.However, 1990).Debris flows occur in the area as a result of steep slopes periods that are long enough to include several major dis- along the main valley and are particularly prevalent in weath- turbance events,typically 102 to 103 years,are probably nec- ered zones of hydrothermally altered bedrock (Sanford, 2010). essary to categorize the dynamic nature of landscapes(Turner A debris fan from the eastern valley wall forms a constriction rr _ Failure i Grand Ditch Lulu City Legend Wetland Roads �!RMNP Border Rocky Mountain Colorado National Park River 40.45°N,105.83°W N 2005 0 0.3 0.6 km I I i Figure 1. The project area is the upper Colorado River in Northern Colorado on the west side of Rocky Mountain National Park,USA.White arrows indicate direction of water flow.Tributaries that flow into the Colorado River are diverted out of the basin to the north by the Grand Ditch(northward pointing white arrow). i Copyright©2011 John Wiley&Sons,Ltd. Earth Surf.Process.Landforms,Vol.37,209-222(2012)