Flow reconstructions in the Upper Missouri River Basin using riparian tree rings

Abstract

River flow reconstructions are typically developed using tree rings from montane conifers that cannot reflect flow regulation or hydrologic inputs from the lower portions of a watershed. Incorporating lowland riparian trees may improve the accuracy of flow reconstructions when these trees are physically linked to the alluvial water table. We used riparian plains cottonwoods (Populus deltoides ssp. monilifera) to reconstruct discharge for three neighboring rivers in the Upper Missouri River Basin: the Yellowstone (n = 389 tree cores), Powder (n = 408), and Little Missouri Rivers (n = 643). We used the Regional Curve Standardization approach to reconstruct log‐transformed discharge over the 4 months in early summer that most highly correlated to tree ring growth. The reconstructions explained at least 57% of the variance in historical discharge and extended back to 1742, 1729, and 1643. These are the first flow reconstructions for the Lower Yellowstone and Powder Rivers, and they are the furthest downstream among Rocky Mountain rivers in the Missouri River Basin. Although mostly free‐flowing, the Yellowstone and Powder Rivers experienced a shift from early‐summer to late‐summer flows within the last century. This shift is concurrent with increasing irrigation and reservoir storage, and it corresponds to decreased cottonwood growth. Low‐frequency flow patterns revealed wet conditions from 1870 to 1980, a period that includes the majority of the historical record. The 1816–1823 and 1861–1865 droughts were more severe than any recorded, revealing that drought risks are underestimated when using the instrumental record alone.

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

“…Nonetheless, a decrease in bar thickness of~1.4 m seems ex- Ritchie and McHenry, 1978. Germination elevation as a function of germination year for the six plains cottonwood trees excavated during this study and for eight trees dated by Schook et al (2016Schook et al ( , 2017. Germination elevation as a function of germination year for the six plains cottonwood trees excavated during this study and for eight trees dated by Schook et al (2016Schook et al ( , 2017.…”

“…These trees had previously been dated above-ground level by Schook et al (2016Schook et al ( , 2017 and were selected to span ages of known major flooding events (the 1978 flood, the 1923 flood, and the 1887 flood), and to also include more recently established trees. These trees had previously been dated above-ground level by Schook et al (2016Schook et al ( , 2017 and were selected to span ages of known major flooding events (the 1978 flood, the 1923 flood, and the 1887 flood), and to also include more recently established trees.…”

“…Then, cores and disks were viewed with a dissecting microscope (WILD binocular microscope with a magnification of 6× to 50×) and dated and marked (Speer, 2010). The skeleton plot for Powder River plains cottonwoods was developed by Schook et al (2016Schook et al ( , 2017. For large samples that did not fit on the moveable stage or under the microscope, microscope images were recorded with the handheld microscope (Celestron 'Handheld Digital Microscope Pro' with a magnification of 20× to 200×) and ring widths were digitized via WebPlotDigitizer 4.1 software (Rohatgi, 2018).…”

“…These criteria were validated by observations of the grain sizes of existing bars and recent overbank deposits in the study area, by measuring stratigraphic sections of bar and overbank deposits (identified by grain size and characteristic sedimentary structures) in a variety of exposures along Powder River, and by the observations of Ghinassi et al (2018) in a trench excavated in point bar and Schook et al (2016Schook et al ( , 2017 and Metzger (2018, this study). Sedimentary sequences were then categorized as suspended load (overbank deposits) or bedload (bar deposits).…”

Distributed Plant Hydraulic and Hydrological Modeling to Understand the Susceptibility of Riparian Woodland Trees to Drought‐Induced Mortality

“…This has been attributed to the large storage capacity of the floodplain substrate (Flanagan et al, 2017) and the relatively shallow water table sustained by streamflow (Rood et al, 2013;Scott et al, 1999). Storage could effectively buffer cottonwoods against isolated dry years, although reduced growth has been reported when the drought was severe (Schook et al, 2016).…”