Geoscience Reference
In-Depth Information
1982
; Avni,
2005
; Avni et al.,
2006
), as well as in semi-arid Australia (Wasson,
1976
;
Williams et al.,
1991
b; Dunkerley,
1992
; Tooth,
1999
; Tooth,
2000
; Tooth and Nanson,
2000
; Williams et al.,
2001
; Tooth and Nanson,
2004
; Dunkerley,
2008c
; Dunkerley,
2010
; Glasby et al.,
2010
; Haberlah et al.,
2010a
; Haberlah et al.,
2010b
).
Bull (
1997
) drew a distinction between gullies and arroyos. He defined an arroyo
as a 'continuously entrenched stream channel in cohesive valley-floor alluvium' (op.
cit., p. 228). The length varied from 5 to 200 km, and the channel cross-section was
typically a flat floor and near vertical banks. Unlike gullies, which are relatively small
and exist for only a few years, arroyos may persist for more than a century. Bull
also noted that 'ephemeral streamflow is typical of many arroyos, but intermittent
(occasional ground-water inflow) or perennial flow is common in the arroyos of large
drainage basins' (op. cit., p. 228).
One of the key questions relating to arroyos is why they sometimes deposit sediment
along their beds and sometimes cut down. Many hypotheses have been advanced
to account for arroyo incision, including climatic change and human activity. In
fact, the reality is far more complex, as Cooke and Reeves (
1976
) have rigorously
demonstrated. Their conceptual model of arroyo formation (op. cit.,
fig. 1.2
)showsthat
more than thirty variables may be involved, so a search for single-cause explanations
will generally prove fruitless.
Bull (
1997
) concurred with this conclusion and proposed a model based on changes
in the balance between streampower and resistance to erosion. Bagnold (
1966
)defined
stream power (
w
) as the rate of energy loss per unit length of stream, expressed as the
product of tractive force (
r
) and velocity (
v
) per unit width of channel:
w
=
rv
(10.3)
Tractive force is the product of hydraulic radius (
R
) (i.e., the submerged channel cross
section area divided by the wetted perimeter), slope (
S
) and the specific weight of the
fluid (
y
):
r
=
yRS
(10.4)
Both stream power and sediment transport rate are proportional to stream velocity
cubed (Schumm,
1977
). Once stream power falls below a limiting threshold value,
bank erosion and sediment transport will diminish, leading to local sedimentation
(i.e., aggradation) within the ephemeral stream channel. Bull (
1997
) emphasised
the importance of plant cover in minimising erosion and in promoting sedimenta-
tion within the ephemeral stream network, a conclusion confirmed by the work of
Tucker et al. (
2006
) in the semi-arid rangelands of the Colorado High plains. Once
incision occurred, the surface soil mantle would become drier, plants would die
and run-off would be concentrated towards the arroyo headwalls, further enhancing
channel entrenchment. Bull's analysis of radiocarbon-dated Holocene arroyo bank
