Southwest - ValleyFloor_TellurideStreamRestoration_Final Report

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216 • Z.RUBIN ETAL. deposits on the surface,underlain by peat and overbank deposits the infrequent occurrence and spatial heterogeneity of debris- to the west of center,and coarse deposits to the east that may be flow deposition, some sites in the west side of the wetland had fan deposits from the east or coarse channel-lag deposits. not received sediment deposition for hundreds-to-thousands of In summary, the west side of the wetland is vertically and years prior to post-settlement deposits. However, debris flows laterally heterogeneous with down-valley fining,although coarse did occur during the centuries and millennia prior to human deposits in the southwest suggest direct colluvial inputs in the settlement. Radiocarbon dates suggest peat and organic-rich soil form of debris flows,landslides,or avalanches.Finer-grained soils development and accumulation ceased at auger core Long and peat were present below the surficial series of coarse depos- W@150 m in the central west portion of the wetland (Figure 2) its, although coarse deposits are identified below that, demon- concurrent with the human impacts of the 19th and 20th century. strating that high-energy fluvial and colluvial inputs occurred Long W@150 m (Figure 5) had been a peatland for-545 years prior to intensive human alteration.The east side of the wetland and accumulated 35 cm of peat until three deposits occurred in has been consistently dominated by peat and overbank depo- the past -189 years, accumulating 70cm of sediment (Table 1). sition throughout the entire imaged and sampled sequence. Long W@150 m was beyond the area of deposition in 2003, Debris-flow deposits are identified in the northeast and southeast suggesting that higher-energy debris flows,capable of deposition corners of the valley from tributaries that enter the valley from the in the southern wetland, had occurred in the past two centuries. east. Interpretation of GPR relied heavily upon verification from Similarly,the core at SW Hole(Figure 2) indicates it had been a auger and excavator holes. Interpretation of strata distant from peatland with no substantial sediment deposition for 2225 years or deeper than holes has a potential for greater error than deposits until 50 cm of sand was deposited in the past two centuries near augered or excavated holes.Nonetheless,the distribution of (Figure 6). Also, at auger core XS3 @158 m (Figure 2) in the augered and excavated holes provided good spatial coverage for northeastern part of the wetland,aggradation was dominated interpreting the aggradational history of the valley up to the GPR by gradual processes of peat production and pond filling and sampling depth limitations(-3.5 m). from 3389-679 yr BP (Figure 7). During this period, four sand-to-gravel sized deposits are preserved,with thicknesses ranging from 5 to 25 cm.Recent sand and gravel deposits are Timeline of aggradation substantially thicker, ranging from 15 to 65 cm. Recent changes in depositional regime at auger core XS4 @112.5m are not Recent aggradational processes on both sides of the wetland apparent, perhaps because the auger only penetrated through are within the range of pre-settlement processes. Owing to one deposit under the dated pond sediments and it is therefore Sediment descriptions from Long W @ 150 m Depth(m) Clay siltSand Gravel f m c - Fine surface soil Coarse sand and gravel (pre 2003 debris flow) 0.5 m Gray silt and sand(debris flow) Gray silt and organics(old floodplain surface) Age at 0.8rn=189 BP Dark brown peat 1.0 m Age at 1.1 m=734 BP --- — Gray silt,consolidated,wet 1.5 m Gray fine-medium sand/silt,well sorted _— Gray silt,fine black organics,wet Age at 1.75 m=2399 BP °Q.a: Angular gravel/gray sand RIM .o D:. a... . ' : . ... ri (debris flow) 2.0 m Gray sand/silt,consolidated Coarse sand and gravel,couldn't auger 2.5 m Figure 5. Long W @150 m is in the southwest portion of the wetland between XS4 and XS5.Ages obtained from radiocarbon dating are outlined for empha- sis.Dates from 1.15 m and 0.8 m depth indicate 35 cm of peat accumulation in the-545 years prior to 65 cm of sediment deposition in the past two centuries. Copyright @ 2011 John Wiley&Sons,Ltd. Earth Surf.Process.Landforms,Vol.37,209-222(2012)