River Avulsions and Their Deposits

Abstract

Avulsion is the natural process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain. Avulsions are primarily features of aggrading floodplains. Their recurrence interval varies widely among the few modern rivers for which such data exist, ranging from as low as 28 years for the Kosi River (India) to up to 1400 years for the Mississippi. Avulsions cause loss of life, property damage, destabilization of shipping and irrigation channels, and even coastal erosion as sediment is temporarily sequestered on the floodplain. They are also the main process that builds alluvial stratigraphy. Their causes remain relatively unknown, but stability analyses of bifurcating channels suggest that thresholds in the relative energy slope and Shields parameter of the bifurcating channel system are key factors.

A minimum time for the formation of Holden Northeast fan, Mars

“…31, L21701, doi:10.1029/2004GL021326, 2004 Copyright 2004 by the American Geophysical Union. 0094-8276/04/2004GL021326 occupying different sites on the fan via relatively abrupt lateral shifts, or avulsions [Mohrig et al, 2000;Rannie, 1990;Slingerland and Smith, 2004]. A previously identified meander 'cutoff' [Malin and Edgett, 2003;Moore et al, 2003] is more likely an avulsed channel that reoccupied a former channel course (Figure 2b), as is frequently seen in terrestrial outcrops where old channels may persist as topographic lows [Mohrig et al, 2000].…”

Cosmogenic nuclide budgeting of floodplain sediment transfer

“…26 Al/ 10 The important result of this analysis is that in all likely cases of active floodplains, cosmogenic Be ratios account for minimum burial depths of 5 to 10 m for a duration of > 5 My. 10 Be and 26 Al concentrations remain virtually unchanged over the interval sediment usually spends in the basin. Thus, spatially-averaged denudation rates of the sediment-producing area can be inferred throughout the entire basin, provided that nuclide production rates are scaled for the altitudes of the sediment-producing area only, because floodplain storage does not modify nuclide concentrations introduced from the sediment source area.…”

“…A change in nuclide concentration resulting from storage in a floodplain is potentially important in large drainage basins in which sediment is prone to repeated burial and remobilization as a river migrates through its floodplain. We have modeled depth-and time-dependent cosmogenic nuclide concentration changes for 10 Be, The cosmogenic nuclide composition of old deposits in currently inactive floodplains that have been isolated for periods of millions of years from the river that once deposited them are predicted to either increase or decrease in C, nuclide concentrations modeled for the in situ-produced variety of this nuclide are, however, sensitive to floodplain storage on residence times of < 20 ky. 10 Be and 26 Al concentration, depending on the depositional depth. These conditions can be evaluated using the 26 Al/ 10 We illustrate these models with examples from the Amazon basin.…”

“…Floodplain storage time, estimated from channel migration rates, ranges from 0.4 ky for the Beni River basin (Bolivia) to 7 ky for the Amazon River basin, while floodplain storage depth, estimated from channel depth, ranges from 1 to 25 m. 10 Be and 26 Al show neither significant increase in nuclide concentration from irradiation nor decrease from decay.…”

“…We predict a hypothetical response time after which changes in 10 Be or 26 Al concentrations become analytically resolvable. This interval ranges from 0.07 to 2 My and exceeds in all cases the typical residence time of sediment in a floodplain.…”