Lithologic and Process Controls on Meander Wavelengths and Lateral Migration Rates of Rivers Meandering in Bedrock
Abstract
The presence of bedrock meander valleys, and the paired bluffs and slip-off slopes therein, is emblematic of vertical and lateral bedrock incision proceeding in tandem and implies lateral migration rates that outpace lowering of the surrounding landscape. The influence of lateral migration of bedrock meandering rivers on landscapes, e.g., as a source of transience in drainage network development, is recently receiving more attention. However, much is still unknown about the lithologic and process controls, e.g., the dominance of abrasion versus plucking, on the sizes of meanders and the rates of lateral migration, and few direct measurements of lateral migration rates in bedrock rivers exist. We hypothesize that, where strata are thickly-bedded and scree grain sizes are large, abrasion is the dominant process, whereas, where strata are thinly-bedded and scree grain sizes are small, plucking dominates. These hypotheses predict that lateral migration and meander wavelengths will be 1) slower and smaller where abrasion, which is scaled to the flows that mobilize bed sediments, is dominant, and 2) faster and larger where plucking, which is more efficient but requires larger flows, is dominant. These predictions are modulated by the volume of scree, and by extension bluff height, that shields the bluff and may retard lateral migration. We test hypotheses with field data from the Buffalo National River, Arkansas. Data include measurements of meander wavelengths, bluff and scree characteristics, and lateral migration rates calculated from weathering rind thicknesses and 10Be concentrations in chert-gravel sourced from limestone strata exposed in river bluffs on the outsides of meander bends. Results broadly support the hypotheses. The Buffalo River migrates slower and has shorter wavelengths where bluffs comprise massively-bedded boulder-producing sandstone strata and migrates faster and has larger wavelengths where bluffs are shorter in height and comprise thinly-bedded limestone strata with smaller scree pile volume and grain sizes. The lateral migration rates are on the order of 102 m/Ma, which is greater than the catchment-averaged lowering rates of 13 to 26 m/Ma and consistent with the presence of river bluffs. Lateral rates imply timescales of bend development on the Buffalo on the order of 106 years.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMEP41A1830Z
- Keywords:
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- 1815 Erosion;
- HYDROLOGY;
- 1825 Geomorphology: fluvial;
- HYDROLOGY;
- 1862 Sediment transport;
- HYDROLOGY;
- 5419 Hydrology and fluvial processes;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS