Drainage area and incised valley fills in Texas rivers: A potential explanation

The largest rivers draining to the Gulf of Mexico in Texas are at more advanced stages of valley filling than rivers with smaller drainage areas, with corresponding differences in alluvial morphology and architecture, and avulsion style. This seems to imply that the larger fluvial systems either produce more sediment relative to the accommodation space in the alluvial valleys, or that they have undergone longer periods of valley aggradation. However, it is not clear that either of these is the case. This paper presents a theoretical analysis showing that fluvial systems with larger drainage areas can produce more advanced stages of infilling than those with smaller drainage areas (but with identical histories and sediment production per unit area) due to the nature of the scaling relationships between drainage basin area and incised valley size, and between basin area and sediment yield. Data for rivers in the region shows a strong linear relationship between valley width and drainage area. The nonlinear relationship between drainage area and proportion of the incised valley filled is somewhat weaker, but still significant, and major deviations are readily explained in terms of local inherited morphology or tectonic features. Results show that major differences in the degree or stage of valley filling could occur due to differences in drainage basin size alone. This indicates caution in interpreting incised valley fills of drainage basins of different sizes in terms of sediment supply.

► Extrabasinal rivers at more advanced stage of valley fill than smaller rivers.
► No evidence of greater sediment loading relative to drainage area (A).
► Model based on relationships between A, sediment yield, incised valley dimensions.
► Differences in valley filling could occur due to differences in drainage area alone.
► Indicates caution in comparisons of valley fills of basins of different sizes.