Event‐scale floodplain accretion revealed through tree‐ring analysis of buried plains cottonwoods, Powder River, MT, USA

Abstract

Six plains cottonwoods along the axis of a meander were excavated to determine if dendrochronology could identify the year and location of germination and date past overbank sedimentation events. Samples from all excavated trees showed clear anatomical changes associated with burial, including increased vessel size, decreased definition of annual ring boundaries, and decreased ring widths. Some of these burial signatures were created by deposition of only a few centimeters of sediment, and most burial events were detected by multiple samples from the same tree. Four of the trees germinated at or near the upper surfaces of bar deposits, while two germinated within thin overbank deposits draped over bar deposits, indicating that germination is closely associated with bars. Dates and inferred thicknesses of overbank sedimentation events are consistent with repeated topographic surveys and data obtained from cesium‐137 (137Cs) analyses. However, the record of overbank sedimentation extracted from the trees does not entirely reflect the history of past peak discharges documented by stream gaging, largely because individual trees are progressively less likely to be flooded through time as the river migrates farther away. Germination dates and locations closely track past positions of the river channel. Germination elevations and the elevations of the tops of point bars appear to be decreasing with time as the bend migrates, implying vertical incision by Powder River at a rate of 7.1 ± 4.3 mm/yr. The rate of floodplain growth determined by elevation changes decreases progressively through time, ultimately reaching an apparent plateau after 0.8–1.3 m of vertical accretion. While similar patterns of vertical accretion have previously been interpreted as resulting from decreasing flood probability with increasing floodplain elevation, distance from the channel is also a first‐order control on vertical floodplain growth. © 2019 John Wiley & Sons, Ltd.

Floodplain Sediment Storage Timescales of the Laterally Confined Meandering Powder River, USA

“…Eight of these cross‐sections are located within the present study reach (Figure 1). The cross‐sections and related data have supported studies along Powder River of floodplain development (Metzger et al., 2020; Moody & Troutman, 2000; Moody et al., 1999; Pizzuto et al., 2008), meander cutoffs (Gay et al., 1998), sedimentation and erosion by varying discharges (Moody, 2019; Moody & Meade, 2008; Pizzuto, 1994; Schook et al., 2017), and point bar development (Ghinassi et al., 2018; Moody & Meade, 2014).…”

“…Terraces identified along Powder River are not necessarily isolated from ongoing overbank deposition. At many locations along the river, the Lightning and Moorcroft terraces were fully inundated by flood waters in 1923 and 1978 (Moody & Meade, 2008), and likely also in 1890 (Metzger et al., 2020). During the 1978 flood, sediment deposited on the Lightning and Moorcroft terraces between Moorhead and Broadus represented 93% and 16% of the total suspended sediment transported during the flood at the Moorcroft stream gage (Moody & Meade, 2008; note that the suspended sediment brought into the reach at Moorhead was supplemented by erosion between Moorhead and Broadus, so the amount deposited can exceed the amount supplied by suspended sediment transport at Moorhead).…”

Ecogeomorphic interactions of Russian olives (Elaeagnus angustifolia) and point‐bar morphology along Powder River, Montana, USA

“…This thinning process is accompanied by an assumed decrease in root density and thus soil strength. The vertical aggradation rate is slower further from the river (Metzger et al, 2020). This part of the point bar slowly becomes floodplain and then, with time, is eventually eroded by channel migration from upstream.…”

“…Older cottonwood saplings anchor the ridges along with a variety of grasses (formerly identified as Elymus repens , Elymus trachycaulus , Bromus inermis , Elymus canadensis , Spartina pectinata and Spartina cynosuroides ; Hitchcock, 1935). As the sloping point bars level off at the distal end, older cottonwood bands prevail (Metzger et al, 2020; Schook et al, 2017), with Russian olives scattered beneath (Bywater‐Reyes et al, 2021).…”

An Integrative Conceptualization of Floodplain Storage

“…Sediment volume can be measured with (i) geophysical instrumentation such as ground penetrating radar and electrical resistivity ground imaging (e.g., Froese et al., 2005), (ii) boreholes (e.g., Alexander & Fielding, 2006), and (iii) extrapolation from floodplain thickness exposed in channel banks (e.g., Dearman & James, 2019). Changes in sediment volume through time can be inferred by estimating volumetric changes in sequential remote imagery (e.g., Bakker & Lane, 2017; Notebaert et al., 2009) and by examining changes in partly buried riparian trees (e.g., Metzger et al., 2020). Sediment inputs can also be estimated as fluxes of suspended sediment inputs based on suspended sediment concentration and water level (e.g., Park, 2020).…”