The lower Birch River alluvial valley displays morphologic features of a once active laterally accreting meandering river system which includes scroll bar plains and oxbow lakes however, the most recent of these features do not conform to previously documented lateral accretionary deposits and flu vial style. The older scroll bar plains display typical ridge and swale topography, whereas the distal sequences (away from the river) are partially or wholly emersed in water producing small, shallow lakes, which suggests valley aggradation of the alluvial fill. The morphology and stratigraphy of the Birch River alluvial valley and delta is hypothesized to have been and is presently influenced by a lake transgression (Lake Claire) induced by differential postglacial isostatic rebound. This has caused valley and deltaic aggradation and land loss due to rising lake level. Eighty kilometres northeast of the delta, the Riviere des Rochers, the regional hydrologic outlet, is rebounding at a faster rate than the Lake Claire basin, with the net result of pooling and expansion of water in the Lake Claire basin. Lake level rise has resulted in: 1) erosion and drowning of most of the delta front, 2) a decrease of the Birch River gradient producing a change in fluvial morphology and stratigraphy from a laterally to a diagonally to a near-stable meandering river with localized drowned oxbows and point bars, 3) vertical aggradation of the Birch delta and the Birch River alluvial valley. Analysis of subsurface sediments from vibracores combined with six radiocarbon dates on buried wood and peat in the Birch River alluvial valley and delta were used to determine the rates of vertical sedimentation, lateral accretion, and transgression. Calculated rates of lake level rise are 1.0 mm/yr at the northeast margin of the delta and 1.4 mm/yr at the proximal delta. On the basis of radiocarbon dates collected from the proximal delta, Lake Claire may not have existed prior to 3,000 BP and may double in size to about 3000 km2 in 3000 years from now, at which time it will become the largest lake in Alberta, exceeding Lake Athabasca.
Bibliography: p. 97-100.