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forming one end of a process spectrum, with systems pro-
duced entirely by surface incision at the other end (Nash,
1996). Seepage erosion networks in drylands are often
more readily identified than their temperate counterparts
as there is less likelihood of extensive contemporary sur-
face flow to mask the influence of groundwater processes.
It is, however, possible in such systems that surface flows
have been more prevalent during former periods of wet-
ter climate with the balance between groundwater and
surface erosion processes fluctuating with time (Nash,
Thomas and Shaw, 1994).
As a cautionary note, some canyons that meet all the
morphologic criteria for formation by seepage erosion
may have developed by other mechanisms. Box Canyon,
Idaho, USA, for example, is incised into a basalt plain, has
no drainage network upstream of its valley head and has
groundwater seepage of around 10 m
3
/s from its headwall.
However, sediment transport constraints,
4
He and
14
C
dates, and the presence of plunge pools and scoured rocks
in the valley floor suggest that a megaflood around 45 000
years ago carved the canyon (Lamb
et al.
, 2008). Field ob-
servations and topographic analyses of the amphitheatre-
headed Kohala valleys in Hawaii suggest that these sys-
tems developed by vertical headward waterfall erosion
rather than seepage erosion (Lamb
et al.
, 2007). These
studies may imply that similar features on Mars formed
as a result of surface runoff processes rather than purely
seepage erosion. Recent computer modelling work (Luo
and Howard, 2008) suggests that Martian valley networks
may have developed through the action of
seepage weath-
ering
combined with fluvial runoff, since seepage erosion
alone would be only marginally effective at generating in-
tegrated networks under realistic rates of aquifer recharge.
Overall, while seepage erosion is an important process in
loose or moderately well-cemented sediments, the extent
to which it can operate in more resistant rock types is less
certain (Lamb
et al.
, 2007).
Table 16.3
Prerequisites for the operation of groundwater
seepage erosion in valley development (Howard, Kochel and
Holt, 1988; Baker, 1990).
Hydrogeological and geomorphological prerequisites
A permeable aquifer of a transmissive rock type
A rechargeable groundwater system, preferably of a large
areal extent
A free face at which water can emerge
A structural or lithological inhomogeneity to increase local
hydraulic conductivity
A means of transporting material from the free face
to increase local hydrologic conductivity and a means of
transporting material from the free face.
The operation of seepage erosion processes is affected
by a variety of factors, which vary in significance ac-
cording to scale and time (Figure 16.7). These include
megascale characteristics, such as climate and regional
geology, which may determine whether seepage is peren-
nial, ephemeral or unlikely to occur. Regional water tables
will also affect process operation, as will the gradual de-
velopment of any valley system that will progressively
change the distribution and foci of groundwater flowpaths
by feedback processes. At meso- and microscales, the
scales at which the actual processes of seepage erosion
occur, there are a variety of complex feedback mecha-
nisms, with, for example, the amount of surface water flow
influencing the slope morphology and hence the operation
of seepage erosion processes. Even in the 'textbook' seep-
age erosion valleys of the Colorado Plateau, Lamb
et al.
(2006) note the significant role played by flash-flood dis-
charges upon canyon morphology. Amphitheatre heads
such as Horseshoe Canyon, Utah, that drain moderate to
large surface areas typically have plunge pools below their
headwalls created by waterfalls.
16.2.4
Parameters promoting the operation of
groundwater seepage erosion processes
16.2.5
Groundwater seepage erosion and
environmental change
In addition to many common morphometric character-
istics, there also appear to be a number of parameters
common to valley systems at a variety of scales that influ-
ence the effectiveness of groundwater seepage processes.
Howard, Kochel and Holt (1988) suggest five factors nec-
essary for the operation of seepage erosion (Table 16.3).
These include the need for a permeable aquifer of a trans-
missive rock type, a rechargeable groundwater system
(ideally of a large areal extent), a free face at which wa-
The environmental significance of seepage erosion in dry-
land valley development should not be overlooked (Nash,
Thomas and Shaw, 1994). While theatre-headed valley
forms in some arid regions are relict features, the fact
that groundwater erosion is an ongoing process in many
valleys within the Colorado Plateau suggests that, given
ideal geological conditions (Table 16.3), seepage erosion
may be an extremely important landforming process under
