Characterizing large river history with shallow geophysics: Middle Yukon River, Yukon Territory and Alaska

Changes in longitudinal valley-fill thickness can provide inferences about fluvial behaviour over millennia. Increased thickness of channel deposits may reveal climate-induced accelerated sediment supply, basin subsidence, tectonic displacements, or rising downstream base level. Decreases in thickness, expressed as terraces, may suggest active uplift, decreased sediment flux, or falling base level within a reach. To date, methods to characterize valley fills have depended largely on borehole data. However, recent developments in shallow geophysics make such investigations possible and provide a practical methodology to investigate alluvial rivers. We carried out a longitudinal geophysical survey, coupled with floodplain chronologies to characterize a 1000 km reach of the middle Yukon River between western Yukon Territory and central Alaska. penetrating radar (GPR) and electrical resistivity ground imaging (ERGI) profiles along the middle Yukon River between central Yukon and Alaska, coupled with channel-bottom echo profiles, demonstrates that there is little variation in thickness of gravel fill and depth of active scour holes. GPR surveys near the White River, the largest sediment source in the upper Yukon, show no significant longitudinal variation in gravel thickness (8–16 m) up to near Circle, Alaska (530 km of wandering channels). Geophysical profiles across the Yukon Flats sedimentary basin (430 km of braided and wandering channels) show an increase in gravel thickness (25–30 m), but this is consistent with compaction of underlying lacustrine sediment and maximum depths of contemporary scour holes. From the western margin of the Yukon Flats to the Dalton Highway bridge (30 km, confined valley with no floodplain or braid bars), borehole data at the bridge indicates 6–10 m of gravel on bedrock beneath 3–6 m of water. lain stratigraphy indicates there is little evidence of fluvial aggradation of the middle Yukon during the Holocene. Radiocarbon ages from the gravel–overbank silt contact near present river level demonstrate a general vertical stability of the river with a period of rapid eolian accumulation during the late Pleistocene and early Holocene. nsistency of gravel-fill and scour hole depth, and lack of Holocene terraces along the 1000 km reach of the Yukon, indicate that the river has been in a state of mass-balance equilibrium of sediment transport (input=output) over the Holocene. Despite massive sediment influxes from the glacial-fed White River and crossing major faults, variation in valley-fill depth is not significant. And further, the Yukon Flats reach shows no evidence of bed aggradation during the Pleistocene and has probably existed in equilibrium since the late Pliocene.