The Evolution of Floodplain Complexity
2012 CUAHSI Biennial Colloquia
- Thomas Dunne / University of California-Santa Barbara
The floodplains of lowland rivers comprise valuable but hazardous landforms used by hundreds of millions of people for settlement, food production, industrial development, and ecosystem conservation. They are characterized by great complexity and dynamism of form and therefore of inundation regime. The most diverse valley floors are associated with migrating, flooding rivers, but many managed floodplains have been radically simplified. Understanding the origin of this dynamic complexity provides a basis for exploiting the desired services and minimizing risks to people. However, an enduring problem in geomorphology and hydrology is the difficulty of identifying causality and providing explanations of the formation and maintenance of extensive landforms with degrees of rigor attainable in laboratory experiments or at intensively monitored field sites. Developing such understanding requires a combination of measurements of processes at natural scale and mathematical modeling to generalize the necessarily sparse measurements.
The foremost driver of floodplain landform construction is the tendency for rivers to shift laterally through bend migration and also to form new channels through floodplain incision. Most research into
these processes has emphasized the role of channel hydrodynamics on bend growth, neck cutoff, and oxbow lake production. However, the magnitude of sediment supply can also have an important influence on channel migration. The supply of bed material to a reach accelerates point bar growth, which increases the cross-channel acceleration of flow and intensifies bank erosion and bend growth. The vulnerability of floodplain surfaces to chute incision, which depends on point bar growth, floodplain texture, floodplain gradient, and vegetation cover, limits channel sinuosity and alters the average production rate and length of oxbow lakes. The form and rate of sedimentation in oxbow lakes and thus the length of time they survive as open water bodies before filling depends on the initial diversion angle of the cutoff (and therefore, on average, the sinuosity of the channel), and the relative supplies of bed material and wash load. These themes will be illustrated from studies of gravel-bed rivers and their floodplains in the Central Valley of California.