Bedforms of the Tana River, Norway

Abstract

The greater part of the water and sediment discharge in the Tana River takes place in the weeks following the ice break-up and flood. During this time the river discharge changes rapidly with an overall falling tendency. Sand is transported as large, ripple-like bedforms, linguoid bars, though a whole hierarchy of bedforms can be recognised. These range from permanent islands and banks through side bars, linguoid bars, dunes and ripples. It is possible that there is a continuous size gradation between linguoid bars and dunes. Super- imposition of dunes on the backs of linguoid bars, which has previously been used as a criterion for their separation into distinct classes of structure, is only found on exposed or shallowly submerged bars and is rarer on deeply submerged bars, suggesting that the dunes are a response to the lower water stage. During the falling stages of the river, the bedforms are modified by a combination of falling stage currents, waves and wind activity. Flow is concentrated between bars which may be eroded or may grow by lateral accretion. Wave activity tends to lower the slip face angles of the linguoid bars, to concentrate heavy minerals and to build small beach features. Excavation shows that ripples and dunes produce small and large scale trough cross-bedding respectively while the frontal parts of linguoid bars show tabular planar cross-bedded sets interbedded with ripple cross-laminated units. Ripple morphology may be preserved undisturbed below linguoid bar foresets, this pattern probably representing an early falling stage modification when the bars were still avtice but the separation eddies in their lees were much reduced. Lateral accretion on the sides of linguoid bars, wave erosion surfaces and reactivation structures due to a rise in water stage can all be recognised in the internal structures.