CGRG Bibliography of Canadian Geomorphology
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Author : Froese, D.G.; Clement, D.T.; and Smith, G.G.
Date : 2000.
Title : The ‘graded river concept’ tested with shallow geophysics, upper Yukon River, Yukon-Alaska: significance for interpreting ancient fluvial deposits.
Publication : GeoCanada 2000. Calgary, Alberta. May 29-June 2, 2000.
Issue : Abstract
Page(s) :
Abstract
The concept of the graded river, simply stated, is one in a state of mass-balance equilibrium between erosion and deposition in a reach: it is neither incising nor aggrading its floodplain over thousands of years. Despite the practical significance of the concept to interpreting ancient deposits, few datasets from rivers around the world can confirm whether a river is indeed ‘in grade’. In order to test the concept, a longitudinal geophysical survey of the braided upper Yukon River was completed. Ground penetrating radar (GPR) and electrical resistivity ground imaging (ERGI) profiles were collected along mid-channel and side-channel bars from the confluence of the Yukon and White rivers to Dalton Highway, Alaska, a distance of 900 km. These profiles demonstrate little variation in thickness of gravel-fill and equivalent depths of active scour holes across three distinct reaches. GPR surveys near the largest sediment source in the upper Yukon (White River) show no significant variation in gravel thickness (8-12 m) to near Circle, Alaska (530 km). GPR and ERGI profiles across the Yukon Flats sedimentary basin (340 km) show an increase in gravel thickness (25-30 m), but this is consistent with compaction of underlying lacustrine sediment. From the western margin of the Flats sedimentary basin to the Dalton Highway bridge (30 km) borehole data indicates gravel thickness of 6-10 m to bedrock. The remarkable consistency of gravel-fill and scour hole depth and lack of Holocene terraces along the upper Yukon suggests the river is in a state of mass-balance equilibrium or has been ‘in-grade’ over the last several thousand years. Given the fact that this is one of the largest rivers (5th) in North America and has one of the highest gravel bedloads, we think that most rivers today are likely in-grade. Despite their common occurrence, deposits of these in-grade systems are likely under-represented in the geologic record.
Bibliography of Canadian Geomorphology