Detection of paraglacial sediment supply using detrital ¹⁰Be in postglacial landscapes of southwest British Columbia

Abstract The legacy of glaciation persists for tens to hundreds of thousands of years in postglacial landscapes, where transient storage and release of paraglacial sediment masks signals of primary landscape denudation (i.e., bedrock incision). The timescales over which glacial legacies persist are difficult to quantify without detailed information on fluvial sediment load or landscape denudation. Here, we present 33 new detrital 10 Be cosmogenic radionuclide analyses from the Fraser River basin in western Canada. We combine 10 Be concentrations with paraglacial terrace distributions and present a sediment mixing model to assess the extent to which paraglacial terrace sediment contributes to fluvial sediment loads. Estimated basin‐averaged denudation rates are ~0.23 mm yr −1 and largely invariable along the Fraser Canyon, a 375‐km bedrock influenced reach, despite a doubling in drainage area and extensive paraglacial terrace distribution. Denudation rates are poorly correlated with landscape morphometry (slope, channel steepness) and climate. To reproduce patterns in 10 Be concentrations along the canyon, terrace sediment contributions must be limited to ~15% of the total flux. We attribute the low terrace inputs to limited connectivity between the terrace fill and the Fraser River channel, which is now incised into bedrock. Longitudinally invariant specific sediment yields are likely a consequence of limited floodplain storage for sediment that is instead transported through the Fraser Canyon to the delta. Sediment derived from bedrock denudation exceeds sediment inputs from paraglacial terrace deposits in the lower Fraser River. Despite widespread prevalence of paraglacial terrace fill in the landscape, our results indicate that basin‐scale paraglacial effects diminish once channels re‐incise into bedrock and terrace fills become disconnected from lateral river channel erosion. These findings shed new light on the processes controlling the timescales of paraglacial effects on modern river sediment loads and wider postglacial landscape evolution.