Southwest - ValleyFloor_TellurideStreamRestoration_Final Report

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*RIC RANGE OF VARIABILITY AS A CONTEXT FOR RESTORATION 217 difficult to infer the aggradational history there. Interpretation of old,though only 5c of that layer is preserved,suggesting only the GPR image suggests previous coarse sediment deposition. a brief period of gradual sedimentation in a regime dominated The most recent sample of organic-rich sediments is—299years by episodic debris flows and overbank deposits. Six of the deepest auger and excavator pits were selected from across the wetland to estimate the relative contributions Table I. Radiocarbon sample descriptions of different aggradational processes (Table II). XS5 @240m, Pit 17 on XS3, and Pit 11 on XS1 were selected to represent Age(years the western portion of the wetland (Figure 2). XS5 @50m, before present, XS3 @158 m, and Long E @429 m on XS2 were selected for Location' Depth(cm) BP(2009)) Material the eastern portion of the wetland.The east side of the wetland aggraded primarily through peat(-50%)and overbank deposi- Long W @ 150m 115 734±40 Peat Long W @ 150m 80 189±40 Peat tion (-30%), and the west side through debris flows (-50%) o Long W @ 150m 175 2399±65 Gyttja and overbank deposition (-35%). Differences in process from SW hole 5o 109±33 2 Peat west to east are probably influenced by several factors includ- SW hole 144 2334±45 Peat ing the tributary fan at the head of the wetland that forces the SW hole 313 4089±50 Peat channel to the west,groundwater that supports peat production SW hole 337 4249±50 Peat in the east,differing lithologies(and therefore debris-flow sus- XS3 @ 158 m 110-125 679±45 Gyttja ceptibility)and,more recently,localized deposition at the head XS3 @ 158m 155-170 1459±50 Peat of the wetland that pushed the active channel to the west. XS3 @ 158 in 320-330 3389±65 Peat Although recent deposits to the west side of the valley differ XS4@ 112.5 in 125 299±45 Gyttja and from the immediately preceding deposits, several pre-impact 1 Figure 2 wood pieces debris-flow deposits of the past 4000 yr show similar textures. 2<120(99.4±0.4 pMC) The increased frequency of recent events, however, distin- guishes them from older deposits. In all, less than 10%of the wetland fill can be attributed to beaver-pond fill. Similarly, investigations of sediment dynamics and beaver on the Sediment descriptions from SW Hole Colorado River suggest dams in higher order streams function primarily by encouraging overbank deposition. In-channel deposition is less likely to be preserved because of frequent Depth(m) clay silt Sand Gravel breaching of dams and removal of in-channel sediment f m c (Westbrook et aL,2010). Dark brown sand-silt with organics large wood chunk and charcoal at 0.24 m `. 0.5 m Dark reddish brownsand with vegetation Comparison of recent and pre-disturbance Age at 0.5 m=109 BP aggradation rates Radiocarbon dating of peat and gyttja was used to identify Dark brown peat dry in places the rate of aggradation and the timing of depositional events .Om (Table I).Aggradation rates in Lulu City wetland range from 4 to 20 cm/100 yr for most of the 4000 yr sampled for this study,but then increase to over 40 cm/100 yr during the past 100-200 years Age at 1.4m=2334 BP (Figure 8).Similar rates of—40 cm/100 yr for the past one to two 1.5 m centuries occur at three dated profiles:Long W@150 m,SW Hole, and XS4 @112.5 m(Figure 2).The fourth dated profile is at XS3 @158 m where the uppermost(youngest)date is—620 yr BP and 1.1-1.25 m below the ground surface. It is likely that the rate of Gray sand and silt,well sorted,no organics aggradation at XS3 @158m was substantially higher during the 2.0 m most recent one to two centuries, but the temporal resolution of aggradation rates is limited by the samples collected; the assumption of a constant rate of aggradation between known dates is probably invalid. It is possible that aggradation rates 2.5m would be spatially heterogeneous throughout the wetland, although given the strong correlation of other dated samples, Gray medium<oarsesand,wet XS3 @158 m is expected to have followed a similar pattern of rapid aggradation during the past two centuries(Figure 9). 3.0m Variation in pre-disturbance aggradation rates is also appar- MPeat, eat�3.1m=4089 BP ent. Aggradation occurred faster during the period from j 4200-2500yr BP than from 2500-200yr BP (Figure 8). The -- Sand,no organics higher aggradation from 4200-2500yr BP corresponds to a eat' m 424 BP period recognized as cooler and drier than present (Fall, 3.5m • :" `% '•" Gravels,could not auger 1997), while the last 2000yr were generally similar to the Figure 6. SW Hole is in the southwest portion of the wetland.This modern climate (Vierling, 1998), although cooler and more was dug with shovels to —2 m, then augered to 3.5 m. Radiocarbon severe temperatures were present during the Little Ice Age from dates from 0.5 and 1.4m depth indicate 90 cm of peat accumulation 700 to 100 yr BP(Fall, 1985;Doerner,2007).Peat aggradation in —2200years prior to —50cm of sediment deposition in the past rates are positively correlated with precipitation and tempera- two centuries. ture, although local controls also influence growth (Ovenden, Copyright @ 2011 John Wiley&Sons,Ltd. Earth Surf.Process.Landforms,Vol.37,209-222(2012)