Avulsion regimes in southeast Texas rivers

Abstract

Avulsions -relatively sudden changes in course, or establishment of new anabranches -are an important process in alluvial rivers. Their key role in floodplain construction and alluvial architecture, and the general conditions favouring avulsions, are well known. However, avulsion processes and evolution, and the factors controlling avulsion regimes, are poorly understood. In the southeast Texas coastal plain, where avulsions are common features of the river valleys, avulsions were studied on the lower Brazos, Navasota, Trinity, Neches and Sabine rivers using a combination of aerial imagery, digital elevation models and field surveys. Avulsions have important influences on the surface morphology and contemporary processes in all five rivers. Features associated with avulsions are active and distinct throughout the study area, and all the rivers have experienced geologically (if not historically) recent avulsions. However, no two of the study rivers have the same contemporary avulsion regime. First-order differences in avulsion style are controlled by the stage of valley filling, and within the three rivers characterized by an unfilled incised valley, antecedent morphology associated with late Quaternary and Holocene coastal and fluvial-deltaic processes accounts for the major differences. In the Navasota (27 avulsions in 185 km) and Neches (21 in 340 km) rivers, subchannels associated with avulsions exist in all stages of development from active to infilled, and some have occurred in recent decades. The other rivers have fewer avulsions, but both the Sabine and Trinity have experienced historic channel shifts. Only the Brazos River has experienced no avulsions within the past c. 300 years. Results show that even within a region of similar environmental controls and geological history local variations in inherited morphology can result in different avulsion regimes.

Landscape evolution space and the relative importance of geomorphic processes and controls

“…Recent reviews of debates about the Quaternary geologic framework and sea level history of the region are provided by Blum et al (2002), Rodriguez et al (2004), andOtvos (2005). More detailed information on the lower Brazos River area is given by Sylvia and Galloway (2006), Phillips (2007aPhillips ( , 2009a and Taha and Anderson (2008).…”

“…The coastal plain of Texas is no exception. Notwithstanding debates and uncertainties about the pace and timing of sea level changes, and of Quaternary climate variations, several recent efforts have been made to untangle the influence of climate and sea-level on observed features of alluvial valleys in the region (e.g., Morton et al, 1996;Blum and Price, 1998;Otvos, 2005;Blum and Aslan, 2006;Sylvia and Galloway, 2006;Phillips and Slattery, 2008;Taha and Anderson, 2008;Phillips, 2009a).…”

Universal and local controls of avulsions in southeast Texas Rivers

“…However, work on a number of river systems shows that while the setup factors are necessary conditions for avulsions to occur, they are not sufficient, and do not allow for prediction of where and when avulsions occur (e.g. Jones and Schumm, 1999;Makaske et al, 2002;Aslan et al, 2005;Taha and Anderson, 2008;Phillips, 2009). …”

“…In rivers of southeast Texas, Phillips (2009) found that valley aggradation state was a first-order control of differences in avulsion in regimes, but that antecedent, inherited morphology was an important second-order control, and local, contingent trigger factors a thirdorder control. Thus five rivers with the same general set of environmental controls showed five different avulsion regimes.…”

Genesis of textural contrasts in subsurface soil horizons in the Northern Pantanal-Brazil

“…Overbank flows and avulsion processes may directly affect the genesis of several soil properties, such as: a) the formation of a textural gradient, where fine sediments of floodplains are overlaid with sediments deposited in the avulsion process (Smith et al, 1989;Farrell, 2001); b) textural contrast in deeper layers formed due to the burial of avulsion sediments (Phillips, 2009); c) alternate horizons with different textures (fluvic characteristic -Embrapa, 2006;FAO, 2006), formed from overbank flows caused by floods (Cazanacli & Smith, 1998), among other combinations of sediment deposition.…”