Tributary response to local base level lowering below a dam

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Abstract

Tributaries located immediately downstream from a dam responded to lowered local base level by incising vertically, widening, and expanding headwardly. The eight tributaries examined, join the Osage River along a 17 km reach directly below the Bagnell Dam in central Missouri. These tributaries flow through unconsolidated alluvium, and respond rapidly to base level lowering.

Local base level lowering below Bagnell Dam is believed to result from a combination of factors, including an increase in channel cross-sectional area caused by degradation and channel widening, and hydrological desynchronization of the trunk river and the tributaries during periods of high discharge. Of these, degradation appears to be the most important cause of tributary incision. Age dates from cores of trees which project cross-channel roots indicate that tributary entrenchment has occurred after the closure of Bagnell Dam in 1931.

Root-armoured knickpoints, subaerially exposed cross-channel tree roots, broken-off roots, and ‘within channel’ terraces provided the basis for reconstruction of relative pre-entrenchment tributary profiles which, when extended to the Osage River confluence, reveals the magnitude of entrenchment at the tributary mouth. The tributaries incised on average, 2·2 m and widened approximately 2·3 m at their mouths. Root armouring protracts the adjustment period of the tributaries and results in stepped longitudinal profiles. Tributary incision is episodic, and the influxes of high sediment discharge are out of phase from tributary to tributary.

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