29
Many, if not most, changes in river corridors seem to occur in an abrupt, nonlinear manner described via thresholds. A threshold separates distinct forms or modes
of operation of a system. A meandering channel that experiences substantial bank
erosion and widening during a large flood can cross a threshold and become a
braided channel after the flood (Friedman and Lee 2002). External thresholds refer
to change forced by an external factor such as a flood, whereas internal or intrinsic
thresholds describe abrupt changes in a river corridor in the absence of external
change (Schumm 1979). Relatively small ephemeral channels, for example,
undergo episodes of headcut formation and incision that do not necessarily correlate with external changes in water and sediment inputs (Schumm and Hadley
1957). The internal thresholds that trigger headcut formation appear to be caused
by deposition of sediment during brief periods of rainfall and stream flow.
Deposition in the channel during the falling limb of flow creates a local steepening
of streambed gradient that can initiate a headcut during the next period of flow.
Once the headcut forms, it can move upstream and trigger a period of channel incision. Incision in the upstream portion of a channel can result in excess sediment
downstream that causes aggradation, leading to asynchronous channel changes
throughout a single channel or a river network, a phenomenon known as complex
response (Schumm 1973; Trimble 2013).
When change does occur within morphologic features such as channel bars or
floodplain wetlands, the affected morphologic feature can be described in relation
to characteristic form time (Brunsden and Thornes 1979; Bull 1991). A transient
form is created by a disturbance, but then modified to pre-disturbance conditions
before the recurrence of a disturbance of similar magnitude. A flood causes a meandering channel segment to become braided, for example. The braided segment is
transient if it returns to a meandering planform before the recurrence of a flood of
similar magnitude. The braided segment is persistent if it remains braided past the
recurrence interval of the flood. High-magnitude disturbances are more likely to be
primary shapers of river corridors with erosionally resistant channel boundaries. A
very large flood creates bedrock channel morphology and boulder bars, for example, which subsequent smaller flows do not have sufficient hydraulic force to modify (Wohl 1992; Cenderelli and Wohl 2003).
River metamorphosis occurs as an abrupt, sustained change in river form
(Schumm 1969; Wohl 2013). The original example came from western tributaries of
the Mississippi River in the United States. These tributaries crossed the semiarid
steppe of the Great Plains as braided rivers characterized by a late spring snowmelt
peak flow and very low base flow, with minimal woody riparian vegetation.
European agricultural settlement of the region modified flow regime to store the
snowmelt peak for gradual release during the growing season for crops. The resulting less variable flow regime allowed riparian vegetation to encroach along the
channel banks and floodplain. The Great Plains rivers metamorphosed to meandering or anabranching rivers with dense riparian forests (Williams 1978b; Nadler and
Schumm 1981).
2.4 Conceptual Models
Many, if not most, changes in river corridors seem to occur in an abrupt, nonlinear manner described via thresholds. A threshold separates distinct forms or modes
of operation of a system. A meandering channel that experiences substantial bank
erosion and widening during a large flood can cross a threshold and become a
braided channel after the flood (Friedman and Lee 2002). External thresholds refer
to change forced by an external factor such as a flood, whereas internal or intrinsic
thresholds describe abrupt changes in a river corridor in the absence of external
change (Schumm 1979). Relatively small ephemeral channels, for example,
undergo episodes of headcut formation and incision that do not necessarily correlate with external changes in water and sediment inputs (Schumm and Hadley
1957). The internal thresholds that trigger headcut formation appear to be caused
by deposition of sediment during brief periods of rainfall and stream flow.
Deposition in the channel during the falling limb of flow creates a local steepening
of streambed gradient that can initiate a headcut during the next period of flow.
Once the headcut forms, it can move upstream and trigger a period of channel incision. Incision in the upstream portion of a channel can result in excess sediment
downstream that causes aggradation, leading to asynchronous channel changes
throughout a single channel or a river network, a phenomenon known as complex
response (Schumm 1973; Trimble 2013).
When change does occur within morphologic features such as channel bars or
floodplain wetlands, the affected morphologic feature can be described in relation
to characteristic form time (Brunsden and Thornes 1979; Bull 1991). A transient
form is created by a disturbance, but then modified to pre-disturbance conditions
before the recurrence of a disturbance of similar magnitude. A flood causes a meandering channel segment to become braided, for example. The braided segment is
transient if it returns to a meandering planform before the recurrence of a flood of
similar magnitude. The braided segment is persistent if it remains braided past the
recurrence interval of the flood. High-magnitude disturbances are more likely to be
primary shapers of river corridors with erosionally resistant channel boundaries. A
very large flood creates bedrock channel morphology and boulder bars, for example, which subsequent smaller flows do not have sufficient hydraulic force to modify (Wohl 1992; Cenderelli and Wohl 2003).
River metamorphosis occurs as an abrupt, sustained change in river form
(Schumm 1969; Wohl 2013). The original example came from western tributaries of
the Mississippi River in the United States. These tributaries crossed the semiarid
steppe of the Great Plains as braided rivers characterized by a late spring snowmelt
peak flow and very low base flow, with minimal woody riparian vegetation.
European agricultural settlement of the region modified flow regime to store the
snowmelt peak for gradual release during the growing season for crops. The resulting less variable flow regime allowed riparian vegetation to encroach along the
channel banks and floodplain. The Great Plains rivers metamorphosed to meandering or anabranching rivers with dense riparian forests (Williams 1978b; Nadler and
Schumm 1981).
2.4 Conceptual Models