Since flow is faster around the outside of a bend, meanders tend to shift sideways by eroding their outer bank (faster flow -> erosion) and at the same time depositing sediment on their inner bank (slower flow -> deposition).

The resulting plain is the FLOODPLAIN, because it is also the area that is flooded by the river when it overflows its banks.

Flooding adds to the deposition of alluvium, because finer suspended sediment (silt and clay) is carried in over-bank flows and then deposited on the surface of the floodplain. So floodplains are built from point bars and flood deposits. Floodplains are one of the major storages for sediment within the fluvial system. As noted last week, the deposition of sediment in the floodplain is a form of aggradation, but it is possible for the river to be in a state of dynamic equilibrium, if the amount of deposition is balanced by the amount of erosion, so there is no net gain or loss and sediment continues to be transported through the system.

River Terraces

River terraces are benches on the valley side formed when a river cuts into a former floodplain. Terraces can be formed in bedrock, but most terraces result from AGGRADATION followed by INCISION of the river valley - i.e. the formation of a VALLEY FILL (deposition of sediment raising the valley floor), followed by downcutting (caused by environmental changes).


Terraces in New Zealand.

These terraces consist of alluvial deposits (sand, mud) and may contain relict floodplain features (e.g. meander scars).

Valley fills form when the input of sediment to the river system is too large to be transported - for example, a climate change that results in more weathering and erosion.

In this area, large scale aggradation followed by incision and terrace formation occurred widely during the Pleistocene epoch (the last ice age, beginning 3 million, ending 10,000 years ago). Thus, many Quaternary (= Pleistocene + Holocene epochs) fluvial deposits are in the form of terraces. As climates fluctuated during the Quaternary, the balance between sediment supply and river discharges changed: generally speaking, periglacial (marginal glacial areas) climates produced large amounts of sediment (by frost action, for example) - often more than could be transported, causing aggradation. During interglacials, sediment supplies were reduced and rivers incised their valley fills, creating terraces.

In this area the Hickory Creek Terrace (recently named by Reid Ferring) is one of the best defined. Probably formed 30-76 thousand years ago, the Hickory Creek Terrace represents slow aggradation over a long period, resulting in a very wide floodplain with a gradient less than that of the present-day floodplain.

The Hickory Creek terrace is responsible for discontinuous, relatively flat-topped benches, standing above the regular floodplain of local streams e.g. in the vicinity of Green Valley adjacent to the Elm Fork.

Another consequence of climate and river discharge changes is that many streams can be "underfit" or "misfit", meaning that they occupy valleys created by larger rivers in the past. Although the controls on valley width are still uncertain, generally speaking, the larger the river, the wider it's valley tends to be; and, most valleys are not much wider than the width of the river's meander belt. Many streams is this area are apparently underfit, because their valleys were formed by larger rivers in the past, which were also responsible for forming larger floodplains - the remnants of which form the present-day terraces.

Example Questions:

1. Describe, with the aid of a sketch, river bluffs.

2. Describe, with the aid of a sketch, river terraces.

3. Explain the connection between the Pleistocene epoch and river terraces in the North Texas region.

Back to review index.

Back to Geomorphology Home Page.