Southwest - ValleyFloor_TellurideStreamRestoration_Final Report

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220 • Z.RUBIN ETAL. recent Grand Ditch-caused debris flows.Comparing the rate of overbank deposition on the west side of the valley,the recent recent deposition with the historic rate of preservation is of shift is not outside the HRV of aggradational processes. limited usefulness. Furthermore,as deposits to the head of the Although debris-flow deposits are spatially heterogeneous in wetland increase the valley gradient, the aggradation itself the wetland, making generalization difficult,the 2003 deposit should lead to greater transport capacity and a potential is not unusually large in either thickness or spatial extent when mechanism for reducing sediment preservation rates. As compared with other deposits of the past two centuries or mentioned previously, for more than 3000 years the eastern when compared with pre-settlement deposits of the past four portion of the wetland has aggraded almost exclusively from millennia. The southernmost extent of the 2003 deposit was peat production and overbank deposition without any fluvial near Pit 28 (Figure 2). Near-surface deposits at Long W @150 reworking or channel migration into the region,whereas the and SW Hole are –60 and –150m further south, respectively, west side of the wetland has filled primarily through debris and were therefore probably deposited by larger,pre-2003 events. flows and overbank deposition. The uniform, horizontal Similarly, coarser deposits and deposits equal to or thicker than reflections in the GPR sections XS2–XS5 (Figure 4) indicate the 2003 debris-flow deposit are visible in the majority of holes that the preserved deposits are from peat and overbank in the west side of the wetland.The increased rate of aggradation deposition, and the absence of fluvial reworking of sediments in the Lulu City wetland is therefore attributed to increased suggests that the aggradation has been purely depositional. debris-flow frequency not increased magnitude. Interestingly,although the east and west sides of the wetland Restoration projects are inherently driven by cultural pre- have been dominated by different processes, over the past ferences. What species we seek to protect, which locations 3000+ years aggradation has occurred at roughly the same merit restoration, and project designs are all guided by rates(Figure 8).This suggests that the wetland topography is cultural, political, and aesthetic values. These biases are in a state of dynamic adjustment. Aggradation in the west not inherently problematic, but we believe restoration is might disconnect the floodplain from the channel and thus most effective when guided by a clear picture of how land- overbank deposits would be confined to the east. Peat scapes undisturbed by human activities function. HRV high- production in the east may similarly elevate and disconnect the lights these processes by demonstrating the variability of eastern side of the valley bottom from the river, encouraging dynamic and changing landscapes. Furthermore, by recog- overbank deposition in the west.Similar equilibrium adjustments nizing the range of historic variability, we also become may also occur longitudinally.Deposition at the head of the wet- aware of future variability.A landscape manager is then able land increases the channel slope and facilitates reworking of those to interpret contemporary monitoring data within a broader deposits and/or deposition farther downstream.The dichotomy of narrative. In the case of the Lulu City wetland restoration, aggradational processes on the east and west sides of the valley we are left to ponder new questions such as: is a short-term suggests deposits will be preserved. Because east and west have increase over long-term aggradation rates problematic, and aggraded at similar rates historically and recent aggradation rates what restoration measures can be applied to recover funda- are of a similar magnitude throughout the radiocarbon-dated mental processes such as preventing future debris flows from profiles in the wetland,we expect the east side will continue to initiating along Grand Ditch? be purely depositional and that the recent debris-flow deposits will The key information gained from our characterization of this persist. Modern aggradation rates can thus be compared with study site is that(i)the Lulu City wetland has been aggrading for historic rates without introducing bias. several millennia, and the aggradation rate of the past 1-2 centuries is substantially higher than long-term pre-settlement averages, although short-term pre-settlement aggradation rates Conclusions of the past may have been similar to recent times, (ii) valley aggradation has occurred at similar rates on east and west sides Aggradation rates were variable through the late Holocene, of the Lulu City wetland, and (iii) depositional processes and perhaps as a reflection of climatically driven changes in sediment soil textures on both sides of the wetland are within HRV.We yield,valley-bottom depositional processes,and peat production interpret the similar rates of aggradation on east and west sides rates. The Grand Ditch and earlier land-use changes (mining, of the wetland, and the similarity of pre-settlement and post- beaver trapping,timber harvesting,human-ignited fires,etc.)have settlement depositional processes and sediments, to suggest altered the rate of deposition beyond long-term average rates that the wetland is capable of adapting to the recent debris-flow prior to Euro-American settlement of the region.Inferred rates of deposition. These findings suggest that intensive restoration aggradation range from 4-20 cm/100 yr from –4250 yr BP until measures and hard engineering structures are not needed and –200 yr BP. The 160years between 4089 and 4249 yr BP potential benefits of intensive restoration are likely outweighed aggraded at 20 cm/100 yr,or half the rate of the last 1-2 centuries, by the disturbance of heavy equipment. However, it is impor- suggesting that some pre-settlement periods were comparable tant to limit anthropogenic debris-flow sediment from entering with recent aggradation rates. the wetland to ensure its long-term function and maintenance. In general, rates are spatially consistent throughout the Lulu City wetland. Similarly, modern rates (-45cm/100yr) of the Acknowledgements—We thank Rocky Mountain National Park for past approximately two centuries are consistent throughout financial, logistical, and informational support. The final manuscript the wetland and approximately six times higher than historic benefited greatly from the thoughtful comments of Frederick J.Swanson, long-term average rates.The east side of the wetland has been Stuart Lane,and an anonymous reviewer. stable for 3000+years with peat accumulation and overbank deposition as the dominant processes of fill. Deposits are therefore expected to be persistent features of the landscape. Peat was present in the west(SW Hole and Long W @150 m), References the top of which was dated at roughly the time of human Baker GS,Jordan TE,Pardy J.2007.An introduction to ground penetrating impacts (-130yr BP). Land-use and Grand Ditch sediment radar(GPR). Geological Society of America Special Papers 432: inputs shifted the mechanisms of fill in the west side from peat 1_18, accumulation to processes of sedimentation. However, since Beres M Jr, Haeni FP. 1991. Application of ground-penetrating-radar long-term aggradation was primarily through debris flows and methods in hydrogeologic studies.Ground Water 29:375-386. Copyright 0 2011 John Wiley&Sons,Ltd. Earth Surf.Process.Landforms,Vol.37,209-222(2012)