Southwest - ValleyFloor_TellurideStreamRestoration_Final Report

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RIC RANGE OF VARIABILITYAS A CONTEXT FOR RESTO9ON 213 l t r t �XS 31�'•�'w.4 �7 •,+• ` 1 [ 1._22 '!j! •°• • •ew Jong E at 275m at 150m30 a- XS4 55m d kLo 9 W a1150m *S5 at 240011? - f S•c#*• .4 4.5 at 175m • xssm f• ` 6, ! ong W •••�... xss�3m LongE Legend 0 37.5 75 150 Meters Nq I Radiocarbon Dated Cores • GPR Surveys - Augered Cores ■ Excavator Holes Figure 2. Location of GPR transects,cores, radiocarbon-dated cores,and excavator pits in Lulu City wetland. Cores were obtained using hand augers and reached maximum depths of 3.5 m;excavator holes were dug using a Bobcat excavator and reached maximum depths of 1.9 m.Radio- carbon samples were collected from augered cores and submitted for analysis.GPR cross-sectional and longitudinal transects were conducted with 100 MHz antenna and reflections were collected every 25 cm.Streamflow is from top of figure toward bottom. excavator pits were dug using a 46 cm bucket.Maximum depth modern ground surface. Dated materials were composite sam- was 1.9 m.Deposits were described based on grain size,sorting, ples:fine material of varied sources(gyttja),or accumulated in angularity, color, and organic content. Depositional processes situ peat.Ages therefore represent averages across the period of were inferred based on observations of contemporary deposits in accumulation,rather than event-based dates such as can be ac- Lulu City wetland.Debris-flow deposits are poorly to moderately quired through dating wood or charcoal fragments. Peat,how- sorted angular deposits of sand to gravel sized material.Overbank ever, has an advantage of being derived from short-lived deposits are well sorted,planar features of sand and finer material material that is not transported. deposited from suspension(Brookfield,2004).In-channel surficial Compaction of older sediments and peat skew comparisons deposits are rounded to sub-angular gravels. Peat is anaerobic of modern aggradation rates to older'rates because older depos- preservation of dead vegetation as intertwined fibrous mats its have had more time to compact and are deeper(more force generated in stable, groundwater-dominated systems such as applied). Because augering disrupted subsurface sediments, it bogs and fens that do not receive substantial sediment inputs was impossible to measure porosity or density and estimate and accumulate organic matter faster than it is decomposed compaction through comparison with surface sediments.Stud- (Cowardin et al., 1979). Organic-rich pond sediment(gyttja) ies of peat compaction under similar burial depths suggest is a facies of fine,black sediment and particulate organic matter compaction on the order of 30-60% (Van Asselen et al., (Hansen, 1959). In the contemporary wetland, such deposits 2009, 2010). Because the sampled peat was still spongy and were found in relic beaver ponds, and thus preserved gyttja is porous, the conservative 30% aggradation rate was assumed assumed to be beaver-pond fill(Butler and Malanson,1995). and applied to all peat below—1 m depth. Quantifying aggradation rates Results and Discussion Eleven samples (eight peat, three gyttja) from four vertical Aerial photo analysis profiles were dated using conventional radiocarbon analysis conducted by the University of Arizona Environmental Isotopes Imagery from 1937 shows a single-thread meandering channel Laboratory.Aggradation rates were calculated for each profile in the center of the wetland, minor deposition and braiding at assuming constant rates between dated samples and/or the the head of the wetland, and one beaver pond in the east Copyright©2011 John Wiley&Sons,Ltd. Earth Surf.Process.Landforms,Vol.37,209-222(2012)