Sedimentary record of point bar formation in laterally migrating anabranching and single-channel meandering rivers (The Obra Valley, Poland)
published: Sep 1, 2016
ArtNo. ESP022006003005, Price: 29.00 €
An internal structure of barforms provides important information about processes forming river channels and planforms. Classifications of river patterns distinguish meandering and anabranching patterns. However, processes of lateral channel migration and point bar formation are active both in single-channel meandering and laterally migrating anabranching rivers. This study aims to determine whether these processes differ between the two river types. A field campaign was conducted in the Obra Valley (Poland) by means of geophysical methods (ground-penetrating radar) verified by core data and analyses of granulometric features of sediments and elements of channel geometry. It was demonstrated that a formation of point bars takes place in both river types. However, their sedimentary record differs between single-channel meandering and laterally anabranching channels. Upward-fining sequences and layering pattern oriented towards the river bed are characteristic of barforms in meandering sections. This regular rhythm of sedimentation is disturbed in anabranching sections owing to sediment delivery from upstream. As a result, point bars are built of a uniform material with a weak trend to fine upwards, or are characterized by upward-coarsening sequences. Sequences of point bars are also formed within mid-channel islands in secondary channels, owing to upstream delivery of fines. The study shows that these differences also depend on a curvature of bends situated upstream, channel width to depth ratio and bifurcation angles. However, as sedimentary record produced by single-channel meandering rivers forming chute cutoffs can be similar to the internal structure of point bars in anabranching rivers, identification of a river pattern should be also based on other sources of information, such as historical maps or aerial images.