Chapter 3. Geomorphology
In response to significant long-term changes in watershed conditions, a river
channel will alter its dimensions to accommodate changes in the delivery of
sediment and water to the river channel. A typical response is for the channel
gradient to change, which can lead to significant changes in the channel bed
elevation. Watershed perturbations that result in a decrease in sediment supply
will lead to bed lowering, leaving the previous channel and floodplain isolated
above the newly established channel level. These abandoned floodplain and
channel surfaces are referred to as terraces that formed under a previous hydro-
logic regime (Fig. 11).
Figure 11. River terraces along Wadi Mujib, Jordan. The river terraces are
the flat surfaces high above the well-vegetated channel bottom.
The effects of repeated climate changes during the Quaternary are manifested
along many rivers and piedmonts in the southwestern U.S. as a series of terraces,
the youngest of which formed during the Holocene or at the Pleistocene-
Holocene transition about 10,000 years ago (Basel and Royse 1972, Bull 1991,
Reneau 2000). Tectonism can also create a succession of terraces with hundreds
of feet of total relief (Schumm et al. 2000).
While many terraces are high above the active channel, others exhibit little
relief above the current channel so are sometimes difficult to distinguish from the