Fundamentals of Fluvial Geomorphology and Channel Processes
Location-for-time substitution was used to generate a five-reach type, incised channel evolution
sequence for stream of the Yazoo Basin (Schumm et al., 1984), as shown in Figure 3.14. In each reach
of an idealized channel, Types I and V occur in series and, at a given location, will occur in the channel
through time. The channel evolution model describes the systematic response of a channel to base level
lowering, and encompasses conditions that range from disequilibrium to a new state of dynamic equilibrium.
The following paragraphs characterize the conceptual types. It should be recognized that these categories
are only conceptual and variation may be encountered in the field.
Type I reaches are characterized by: a sediment transport capacity that exceeds sediment supply,
bank height (h) that is less than the critical bank height (hc), a U-shaped cross section, small precursor
knickpoints in the bed of the channel providing that the bed material is sufficiently cohesive, and little or no
bed material deposited. Width-depth ratios at bankfull stage are highly variable.
Type II reaches are located immediately downstream of the primary knickpoint and are
characterized by: a sediment transport capacity that exceeds sediment supply, a bank height that is greater
than the critical bank height (h > hc), little or no bed sediment deposits, a lower bed slope than the Type
I reach, and a lower width-depth ratio value than the Type I reach because the depth has increased but the
banks are not failing.
Type III reaches are located downstream of Type II reaches and are characterized by: a sediment
transport capacity that is highly variable with respect to the sediment supply, a bank height that is greater
than the critical bank height (h > hc), erosion that is due primarily to slab failure (Bradford and Piest,
1980), bank loss rates that are at a maximum, bed sediment accumulation that is generally less than two
feet, but can locally be greater due to local erosion sources, and channel depth that is somewhat less than
in Type II. The channel is widening due to bank failure.
Type IV reaches are downstream of Type III reaches and are characterized by: a sediment supply
that exceeds sediment transport capacity resulting in aggradation of the channel bed, a bank height that
approaches the critical bank height with a rate of bank failure lower than Type III reaches, a nearly
trapezoidal cross-section shape, and a width-depth ratio higher than the Type II reaches. The Type IV
reach is aggradational and has a reduced bank height. Bank failure has increased channel width, and in
some reaches the beginnings of berms along the margins of an effective discharge channel can be observed.
These berms are the initiation of natural levee deposits that form in aggraded reaches that were over-
widened during earlier degradational phases. Bradford and Piest (1980) observed that in the later phases
of evolution, the mode of bank failure changes from circular arc to slab-type failures.
Type V reaches are located downstream of Type IV reaches and are characterized by: a dynamic
balance between sediment transport capacity and sediment supply for the effective discharge channel, a
bank height that is less than the critical bank height for the existing bank angle, colonization by riparian
vegetation, an accumulated bed sediment depth that generally exceeds 3 feet, a width-depth ratio that
exceeds the Type IV reach, and generally a compound channel formed within a newly formed floodplain.
The channel is in dynamic equilibrium. Bank angles have been reduced