Abstract
AbstractClimate change is decreasing glacier cover and increasing the frequency and magnitude of precipitation‐driven high flows and floods in many regions of the world. Precipitation may become the dominant water source for river systems in recently deglaciated catchments, with major rainfall events driving significant changes in river channel morphology. Few studies, however, have examined river channel response to repeated precipitation‐driven high flows. In this study, we measured the geomorphological condition of four low‐order rivers in recently deglaciated catchments (70–210 years ice free) before and after a series of repeated precipitation‐driven high flows during summer 2014. High flows drove substantial initial morphological change, with up to 75% change in baseflow channel planform position and active channel form change from pre‐ to post‐high flow. Post‐high flow years were associated with increased instream wood and geomorphological complexity at all but the youngest river. Channel changes were part of an active relaxation stage at all rivers, where channels continued to migrate, and complexity varied through time. Overall, these measurements permit us to propose a conceptual model of the role of geomorphologically effective high flows in the context of paraglacial adjustment theory. Specifically, we suggest that older rivers in recently deglaciated catchments can undergo a short‐term (<10 years) increase in the rate of geomorphological development as a result of the recruitment of instream wood and channel migration during and following repeated precipitation‐driven high flows. Enhancing our knowledge of these geomorphological and paraglacial processes in response to high flows is important for the effective management of riverine water and ecosystem resources in rapidly changing environments.
Highlights
The highest pre-high flow persistence score was recorded at the oldest site (Rush Point Creek – 65.8%), whilst the lowest score was recorded at the intermediate age (146 years) site (Ice Valley Stream – 51.5%)
Our findings offer some support for H3 that repeated high flow driven fluvial sediment transport represent an important element of ongoing paraglacial adjustment (Miller, 1990; Darby et al, 2007; Guerra et al, 2017)
Repeated high flows caused significant change in baseflow channel planform persistence, adjusted active channel form, recruited large amounts of instream wood at three rivers and removed instream wood from the youngest river, with years following the high flows generally being associated with increased geomorphological complexity
Summary
Glacier Bay National Park and Preserve in south-east Alaska (58100–59150 N; 135150–138100 W) is dominated by a tidal fjord, 150 km long and 20 km wide (Figure 1). During the summer of 2014, recurrent intense precipitation events occurred from June to September, which resulted in the wettest summer period in a 30 year record (Milner et al, 2018), causing repeated flooding and high flows across the region of south-east Alaska and within Glacier Bay. Additional information regarding weather patterns and regional river flows are reported in Supplementary Information 1 (Supplementary Figures S1, S2 and Table 1). Principal component analysis (PCA) was used to explore relationships in summary planform metrics, proportional contribution of channel geomorphological unit types and geomorphological complexity (SHDI) between rivers through the study period including the 2014 high flows. Cross sections established in the 1980s by Sidle & Milner (1989) in a reach representative of the wider river network were resurveyed in a number of years pre- and post-high flow (years varied with sites, see Figure 4). The change in total sediment area (%) and the persistence of cross section position (%) was calculated for pairs of years
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