Fluvial terrace

Fluvial terrace

Fluvial terrace is an abandoned floodplain with a long, narrow stairs that is formed when the river flowed at a higher level than at present. Fluvial terrace consists of two parts; a tread, which is the flat surface at the upper side of fluvial terrace, and a scarp, which is the steep slope connecting the tread to the downside. Fluvial terrace is divided into Rock terrace and Gravel terrace according to structure materials, and it is also classified as Tectonic terrace and Climate terrace according to causes of formation.

Classification

Fluvial terraces are either called erosional or depositional.

Erosional terrace

Cross section of Fluvial terrace

Erosional terrace is a terrace that the tread has been formed primarily by lateral erosion. During low water level stage, fine sediment is deposited and during normal flow, coarse sediment is deposited at the end of a high water event. When the river reaches its highest level, it entrains all the channel sediment and scours the underlying bedrock before coarse detritus is deposited again on the channel floor.

Depositional terrace

Depositional terrace is a terrace that the tread represents the surface of a valley fill. Valley filling occurs when the amount of sediment produced in a basin exceeds the amount that the river system can carry away. It is usually triggered by glacial outwash, climate change, or change in base level, slope, or load caused by rising water level. Depositional terrace is climatically controlled, whereas erosional terrace is tectonically controlled.

Fluvial Terrace Media

• 

  Eroded alluvial fill 60 feet (18 m) thick at Kanab Creek, Kane County, Utah. In 1884 the stream ran at top of the terrace. 1939 photo by United States Geological Survey.

• 

  Hypothetical valley cross-section illustrating a complex sequence of aggradational (fill) and degradational (cut and strath) terraces. Note ct = cut terrace, ft = fill terrace, ft(b) = buried fill terrace, fp = active floodplain, and st = strath terrace.

• 

  Unpaired fluvial terraces on the South Fork of the Shoshone River, Park County, Wyoming, 1923. The river at left has encountered a formation of erosion-resistant volcanic breccia, causing it to downcut more rapidly on the right, leaving terraces of different elevations.