Geomorphology 37 Ž2001. 93–104
www.elsevier.nlrlocatergeomorph
Channel avulsion on alluvial fans in southern Arizona
John Field )
Green Mountain College, One College Circle, Poultney, VT 05764, USA
Received 6 January 1999; received in revised form 1 September 2000; accepted 4 September 2000
Abstract
Historical aerial photographs and field observations on five fluvially dominated alluvial fans in southern Arizona
demonstrate that channel avulsion invariably occurs where bank heights are low and often at channel bends. Channel
abandonment occurs through stream capture when overland flow from the main channel accelerates and directs headward
erosion of smaller channels heading on the fan surface. Five distinct channel morphologies observed on the fans are related
to different stages of the avulsion process and can be used to identify areas on a fan surface that are prone to avulsion. A
descriptive model of channel avulsion illustrates how the morphology of a single channel reach will evolve through time as
it captures the main flow path and is itself eventually abandoned. Immediately following avulsions, small preexisting
channels that capture flow from the main channel will typically experience three fold or greater increases in channel width.
Subsequent large floods can be stably conveyed through these high-capacity reaches. An uninterrupted sequence of
sediment-charged small flows, however, will eventually begin to back-fill the wide channels as vegetation growth stabilizes
the banks. The stabilized and back-filled channels are now prone to abandonment during large floods because the decrease in
the channel’s capacity leads to the generation of overland flow beyond the margins of the shallowed channels. The action of
the small aggrading floods is critical in the avulsion process since the greatest amount of overland flow is generated where
bank heights are lowest. As a result, both small and large floods are effective agents of landscape change on the fans.
Channel avulsions on the five fans are not completely random events in space and time because their occurrence is
controlled by the relative positioning of low banks along the main channel and smaller channels draining the fan surface.
Consequently, the location and timing of future channel avulsions can potentially be anticipated in an effort to improve flood
hazard assessment on fluvial fans in the rapidly urbanizing southwestern United States. q 2001 Elsevier Science B.V. All
rights reserved.
Keywords: avulsion; alluvial fan; Arizona; geomorphic effectiveness
1. Introduction
The importance of channel migration on alluvialfan development has long been recognized ŽDrew,
1873.. Over long time periods Ž10 3 to 10 4 years.,
)
Tel.: q1-802-287-8270.
E-mail address: [email protected] ŽJ. Field..
channels must migrate over the entire surface to
ensure the establishment and maintenance of fan
form. When considering the long-term aggradation
of alluvial fans, the exact location of channels through
time is indeterminate and channel migration can be
modeled as a stochastic process ŽPrice, 1974; Hooke
and Rohrer, 1979.. However, short-term processes of
channel migration on alluvial fans are of greater
0169-555Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 9 - 5 5 5 X Ž 0 0 . 0 0 0 6 4 - 7
J. Field r Geomorphology 37 (2001) 93–104
94
Fig. 1. Map of Arizona showing major cities and the location of
ŽA. Ruelas, Wild Burro, and Cottonwood Fans on the Tortolita
Mountains piedmont; ŽB. White Tank Fan; and ŽC. Tiger Wash
Fan. See Table 1 for UTM coordinates of fan apices.
interest to engineers and geologists studying flood
hazards and the effect of individual floods on fan
morphology. Mathematical models used by the Federal Emergency Management Agency to establish
flood hazard zones on alluvial fans assume randomness in the location of channel avulsions during a
single flood ŽFederal Emergency Management
Agency ŽFEMA., 1995.. However, a recent report by
the National Research Council ŽNational Research
Council ŽNRC., 1996. calls into question whether
the position of alluvial-fan channels during a single
event should be considered indeterminate. Previous
studies have examined short-term channel avulsion
processes on debris-flow fans ŽBeaty, 1963; Whipple
and Dunne, 1992. and fluvial fans alike ŽKesel and
Lowe, 1987; Wells and Dorr, 1987., but many ques-
tions related to the National Research Council report
ŽNRC, 1996. remain unanswered. What role do
floods of different magnitude play in the process of
channel avulsion? How do pre-existing fan morphology and drainage patterns control the avulsion process? In addition, should the locations of future
channels on shorter time scales Ž10s to 100s of
years. be considered indeterminate as they are on
longer time scales? This paper addresses these questions by examining the influence of both large- and
small-scale floods on channel morphology and channel avulsion processes on five fluvial fans in southern Arizona ŽFig. 1; Table 1.. Documentation of
recent channel changes on the five fans is used to
construct a model of channel avulsion that links
channel morphology to different stages of channel
development and the avulsion process. Channel morphology can thus be used to identify areas on a fan
surface prone to avulsion and target areas for more
quantitative hydraulic analysis of flood hazards.
2. Site description
The five alluvial fans are located in a tectonically
inactive portion of the Basin-and-Range province in
southern Arizona with source areas largely composed of felsic intrusive rocks ŽTable 1.. The climate
is semi-arid with mean annual rainfall ranging from
15 to 28 cm, increasing to the southeast. The five
fans are active secondary fans ŽBlissenbach, 1954.
forming at the downstream termini of fanhead
trenches passing through abandoned Pleistocene surfaces higher on the respective piedmonts. Fan de-
Table 1
Selected drainage-basin characteristics for the five southern Arizona study fans
Alluvial fan
Location of fan apex
X
USGS 7.5 Quad UTM Coordinates
Ruelas Fan
Wild Burro Fan
Cottonwood Fan
White Tank Fan
Tiger Wash Fan
Ruelas Canyon
Ruelas Canyon
Marana
White Tank Mts.
Weldon Hill
35
91 200r4 92 200
91 250r4 90 500
35
92 600r4 82 700
37
09 750r3 50 400
37
27 000r2 85 050
35
Total drainage
area Žkm2 .
Distance fan apex
to mtn front Žkm.
Dominant
lithology
Average annual
rainfall Žcm.
9.3
18.5
34.5
14.6
249.6
1.8
4.3
10.4
4
6.5
granite, granodiorite
granite, granodiorite
granite, granodiorite
granite, gneiss
Mix
28
28
28
20
15
J. Field r Geomorphology 37 (2001) 93–104
95
Fig. 2. Schematic plan view and longitudinal profile of a discontinuous ephemeral-stream system showing relationship between sheetflood
zones, depositional and erosional channels, overland flow zones, and bank heights. Not to scale.
posits are predominately fine grained Žsand through
clay fraction. and boulders are uncommon.
Detailed studies of surficial processes have been
completed on all five fans ŽHouse et al., 1991, 1992;
Maricopa County, 1992; Field, 1994.. Secondary
fans throughout southern Arizona are dominated by
fluvial processes associated with discontinuous
ephemeral streams, a distinctive stream pattern characterized by alternating erosional and depositional
reaches ŽFig. 2; Schumm and Hadley, 1957; Bull,
1997.. Overland flow, a critical component of the
channel avulsion process described below, emanates
from the margins of the sheetflood zones and aggrading downstream ends of channels ŽFig. 2.. Channel
backfilling caused by the headward migration of
aggradational reaches can transform a deep channel
into an area of sheetflooding over periods of tens to
hundreds of years ŽBull, 1997.. The five alluvial fans
studied in this report show no evidence of debris-flow
activity; modern rates of weathering in the mountain
ranges of southern Arizona are insufficient to produce debris flows with enough sediment to reach
distant fan apices ŽTable 1; Melton, 1965; Bull,
1991..
3. Evidence of past channel avulsions on the alluvial fans
Historical aerial photographs extending over 60
years were used to establish the timing of recent
channel avulsions on the five alluvial fans by noting
changes in flow paths between pairs of aerial photographs taken on different years ŽFig. 3.. In addition, aerial photographs and field reconnaissance
were used to document changes in channel morphology associated with each avulsion and to identify
channel reaches abandoned prior to the earliest photographs. Avulsion is defined here as the diversion of
a majority of flow from one channel into another,
leading to a total or partial abandonment of the
previous flow path. Avulsions that occurred at or
near the fan apex and which appeared to have diverted more than 50% of the fan’s total discharge
96
J. Field r Geomorphology 37 (2001) 93–104
Fig. 3. Aerial photographs of Ruelas Fan: Ža. 1936 and Žb. 1988 showing the avulsion that occurred between 1949 and 1956. The arrow, in
the same position on both photos, highlights the channel reach into which flow was diverted. The letter AaB, indicating the fan apex, is in the
same position on both photos. Note the bend created in the main channel when flow was diverted into the narrow preexisting channel.
J. Field r Geomorphology 37 (2001) 93–104
were termed major avulsions; all others were considered minor avulsions. All five fans show evidence of
at least one minor avulsion with evidence of a major
avulsion on four of the fans ŽTable 2.. Channels
abandoned by major avulsions prior to the photographic record are identified by vegetation growth in
and minor incision of the former channel bottoms.
The following observations are critical for understanding the avulsion mechanisms on the five alluvial fans.
Ži. All of the avulsions, major or minor, happened
along aggrading channel reaches or sheetflood zones
ŽFig. 2; Table 2.. The location of the avulsions along
these discontinuous ephemeral stream systems corresponds to areas where channel banks were low or
nonexistent ŽTable 2..
Žii. Channel avulsions preferentially occurred on
the outside bends of channels ŽFig. 4; Table 2., but
97
some avulsions actually resulted in the creation of
bends along previously straight reaches ŽFig. 3..
Žiii. Following an avulsion, the new flow paths, in
all cases, followed the preexisting course of a small
tributary channel draining only a small portion of the
fan surface itself ŽFigs. 3 and 4; Table 2..
Živ. The increases in discharge, as the channels
receiving the diverted flow were incorporated into
the main flow path, resulted in greater than three-fold
increases in channel width along some channel
reaches ŽFigs. 3 and 5..
Žv. The bed elevation of the new flow path following the avulsion was generally lower than, but in
a few cases the same as, the bed elevation of the
abandoned reach ŽTable 2..
Žvi. Wide channel reaches abandoned by avulsions sometimes experienced minor incision with a
narrower channel inset into the old channel bed.
Table 2
Characteristics of past avulsions on the five alluvial fans
Avulsion
Year
Avulsion type
Major
Minor
Preexisting
channel?a
Avulsion
at bend? b
Bank height c Žcm.
At avulsion
Max. on fan
Change in bed
elevationd Žcm.
Ruelas fan
RF-1
RF-2
RF-3
1949–1956
Before 1936
Before 1936
X
X
X
Yes
Unknown
Unknown
Yes
Yes
No
8
21
0
123
123
123
6
12
5
X
Unknown
No
19
136
70
X
X
X
X
Yes
Yes
Yes
Yes
Yes
No
No
Yes
Yes
No
32
0
24
0
0
194
194
194
194
194
70
4
40
0
36
White Tank fan
WT-1
1951
X
Yes
Yes
9
54
10
Tiger Wash fan
TF-1
1900Ž?. –1953 e
TF-2
Before 1953
X
Unknown
Unknown
No
No
0
12
142
142
4
91
Wild Burro fan
WB-1
Before 1936
Cottonwood fan
CF-1
1962
CF-2
1949–1956
CF-3
1936–1949
CF-4
1936–1949
CF-5
Before 1936
X
X
a
Indicates whether flow was diverted into an already existing channel. For diversions prior to photographic record, this can be
determined by continuity of new channel with a tributary channel upstream of avulsion site.
b
Indicates whether diversion occurred on the outside of a channel bend.
c
Comparison of bank height at point of avulsion to maximum bank height on fan to illustrate how avulsions occur where bank heights
are low.
d
Taken as the difference in bed elevation between the new channel and abandoned channel at the point of avulsion.
e
Tin cans circa 1900 transported in now abandoned channel.
98
J. Field r Geomorphology 37 (2001) 93–104
Fig. 5. Graph showing the increase in channel width following an
avulsion along channel reaches capturing flow from the main
channel. Points plotting to the left of the 2= and 3= line
represent channels that more than doubled and tripled in width,
respectively. Avulsion names labeled on each point refer to the
avulsions listed in Table 2. Note that a single avulsion can cause
widening of several channel reaches. The CF-1 avulsion did not
cause widening of the reach because flow was diverted into a
large channel already connected to the main flow path.
Fig. 4. Channel network on White Tank Fan in Ža. 1942 and Žb.
1992. Channel patterns were traced from aerial photographs and
the designation of channel types Že.g., active channels. based on
geomorphic maps constructed in the field. Note that the major
avulsion at the fan apex in 1951 occurred on the outside bend of
the channel and activated large portions of the western half of the
fan surface. Arrows point to small tributary channels heading on
the fan surface that were incorporated into the main flow path
following the 1951 avulsion.
Vegetation growing in the abandoned reaches gives
rise to increasing gray tones on subsequent aerial
photographs ŽFig. 3..
Žvii. Five distinctive channel morphologies are
present on the alluvial fans and are associated with
different stages of channel development preceding
and following an avulsion ŽFig. 6..
Except for a short gaging record of the Tiger
Wash Fan drainage basin, no gaged records of discharges exist for the five alluvial fans. Consequently,
the record of floods since the beginning of the
photographic record is spotty and pieced together
from field evidence, eyewitness accounts, and a few
published reports ŽTable 3.. Paleohydrologic analyses in the Wild Burro Fan and White Tank Mountain
Fan drainages and the gaged record from Tiger Wash
Fan provide estimated discharges and recurrence intervals for some of the known floods ŽTable 3..
While significant floods likely precipitated all of the
avulsions, this cannot be conclusively demonstrated
because of the incomplete record. Of important note,
however, is that no avulsions resulted from an extreme flood on Wild Burro Fan in 1988 or from
moderate floods on Cottonwood Fan in 1990 and
J. Field r Geomorphology 37 (2001) 93–104
99
Fig. 6. Five channel morphologies observed on the five fluvial fans. Note that a single channel reach can go through each phase of channel
development through time, and each stage of development may be observed at different places on a fan at the same time. During the active
channel phase, the channel reach is connected to the upper drainage basin. Aggrading channels are unstable and prone to avulsion while
AnewB channels and adjusted channels are more stable.
Tiger Wash Fan in 1970 ŽTable 3.. The lack of
change resulting from these floods is of particular
interest since small to moderate floods on fluvial
fans in other regions have caused major avulsions
ŽGriffiths and McSaveney, 1986; Kesel and Lowe,
1987; Wells and Dorr, 1987..
4. Processes of channel avulsion
Based on the observations described above, a
model has been developed to illustrate the processes
preceding, accompanying, and following channel diversion on the five fans ŽFig. 7.. The active distributary channels conveying flow from the upper drainage
basin occupy only a small portion of a fan surface at
any one time, so numerous narrow Aon-fanB channels are formed that drain small portions of the
inactive fan surface ŽFig. 6a.; in places, an incipient
dendritic drainage system develops ŽFig. 4a.. Portions of these Aon-fanB channels occasionally approach the larger distributary channels ŽFig. 7a. and
can be incorporated into the main drainage net as the
result of an avulsion ŽFigs. 3, 4, and 7b.. The banks
Table 3
Record of floods on the five alluvial fans
Alluvial fan
Year of flood
Recurrence interval
of flood Žyears.
Associated
avulsion
Type of evidence
for flood occurrence
Refs.
Ruelas
Wild Burro
No record
1988
) 100
none
House Ž1991.
Cottonwood
1962
50–100
CF-1
White Tank
1990
1951
5–15
) 100
none
WT-1
Tiger Wash
1992
1970
10–25
) 10
none
none
flood debrisr
sand sheets
written and
oral accounts
flood debris
aerial photosr
written account
flood debris
stream gage
Rostvedt Ž1968.
Field Ž1994.
Kangieser Ž1969.
Maricopa County Ž1992.
Maricopa County Ž1992.
100
J. Field r Geomorphology 37 (2001) 93–104
Fig. 7. Model of channel avulsion developed from observations on
the five fluvial fans. See text for description. View is looking
upstream. Note relationship with five channel morphologies illustrated in Fig. 6.
of the smaller channel that has captured the flow are
widened and become vertical in response to the
dramatic increase in discharge from the upper
drainage basin ŽFigs. 5 and 6b.. The old and new
segments of the main channel, joined at the point of
avulsion, have contrasting morphologies ŽFig. 7b..
The channel banks along the older upstream segment
are low, rounded, and vegetated, while downstream
of the diversion point the banks are vertical. Widening and perhaps deepening of the newer downstream
channel reach will continue as floods larger than the
diversion event pass through the reach ŽFigs. 6c and
7c.. An avulsion is unlikely in reaches at this stage
of channel development because the channel is adjusted to convey large flows.
As the frequency of floods large enough to continue channel enlargement decreases, smaller floods
will begin to modify the channel’s morphology. In
channels with high width:depth ratios ŽFig. 6c.,
transmission losses are maximized and only the
largest floods can transport the imposed sediment
load through the channel. Ephemeral streams carrying fine-grained sediments characteristically aggrade
during smaller flows because stream discharge infiltrates into the sandy channel bottoms before reaching
the channel mouth ŽBull, 1979, 1997.. Channels with
greater width:depth ratios also have a greater hydraulic roughness, further promoting channel aggradation. In the absence of a flood large enough to
flush accumulating sediments through the channel
reach, a succession of small floods will eventually
reduce the height of the channel banks through a rise
in the bed elevation ŽFigs. 6d and 7d.. The carrying
capacity of the channel is further reduced by the
slumping, rounding, and revegetation of the increasingly stabilized channel banks. Floods at this stage of
channel development begin to overtop the channel
banks, generating overland flow. With continued
aggradation, sheetflood zones may ultimately develop; and the magnitude of the smallest flood needed
to produce overland flow decreases.
Overland flow is a critical component of the
channel avulsion process on the five study fans.
Streamflows overtopping the banks of a channel tap
the upper portion of the water column and are thus
relatively clear, devoid of bed load, and capable of
erosion ŽHooke and Rohrer, 1979; Slingerland and
Smith, 1998.. Any sediment carried by the overland
flow is deposited rapidly at the channel and sheetflood zone margins in response to the flow expansion ŽFig. 8.. Although overland flow initially diverges and spreads out over the fan surface, it ultimately recollects into the existing network of small
narrow channels draining the fan surface ŽFigs. 4b
and 8.. Consequently, overland flow enters and en-
J. Field r Geomorphology 37 (2001) 93–104
101
Fig. 8. Simplified geomorphic map of Wild Burro Fan showing channel network, sand sheets, and the position of a sheetflood zone in 1936
and 1988. Map is drawn from 1990 aerial photograph and based on field observations and analysis of historical aerial photographs. Sand
sheets were deposited due to flow expansion during the 1988 flood when the banks along the main channel were overtopped. Note how sand
sheets reconverge downstream as overland flow was captured by small channels draining the fan surface. The upstream migration of the
sheetflood zone between 1936 and 1988 resulted in the widening of Reach A as more overland flow was generated further upstream along
the main channel.
larges these small channels and generates headward
erosion directed towards the aggrading active channel ŽFig. 7d.. When the channel heading on the fan
surface is eroded back to the aggrading reach, stream
capture will occur because the bed of the headward
eroding channel is generally lower than the backfilled channel ŽFig. 7e; Table 2.. The capturing
channel is, in some respects, a passive partner with
overland flow from the main channel dictating where
and how rapidly headward erosion occurs. Since the
bed of the previously abandoned channels remains
lower than the surrounding fan surface, overland
flow often enters these reaches and incises the old
channel bed ŽFigs. 6e and 7d–e.. Previously abandoned channels can, at times, become reactivated
into the main flow path as at the apex of White Tank
Fan ŽFig. 4..
5. Rate of channel avulsion
The number of years required for a single channel
reach to progress through the channel avulsion pro-
cess illustrated in Fig. 7 is dependent on several
factors: Ži. the initial depth of the main channel; Žii.
the magnitude of the largest flood occurring during
the active channel phase ŽFigs. 7b–d.; Žiii. the sequencing of flood magnitudes; and Živ. the location
of the Aon-fanB channels draining the fan surface
with respect to sheetflood zones and aggrading channels along the active channel.
First, the depth to which a new channel incorporated into the main flow path is eroded will determine in part the amount of aggradation and time
required before overland flow is initiated. Second, an
extremely large flood flowing down an AadjustedB
channel ŽFig. 6c. could conceivably overwhelm the
channel’s capacity at any time and produce enough
overland flow in a single event to precipitate a
channel avulsion. However, an aggrading channel or
sheetflood zone elsewhere on the fan ŽFigs. 2 and
6d. during the same flood would be the more likely
site of a diversion since more overland flow would
emanate from these areas.
Third, a series of moderate to large floods during
the early stages of channel development ŽFig. 6b–c.
102
J. Field r Geomorphology 37 (2001) 93–104
may actually prolong the diversion process by periodically flushing sediment out of a channel reach
and hindering the generation of overland flow. In
contrast, considerable aggradation resulting from an
uninterrupted sequence of small sediment-charged
flows will eventually lead to overland flooding during even the smallest discharges. Diversions that
have occurred during small floods on fluvial fans
elsewhere ŽGriffiths and McSaveney, 1986; Kesel
and Lowe, 1987; Wells and Dorr, 1987. fit within
this context and should not be considered anomalous
events. While extended periods of constant discharge
can produce avulsions in experimental studies ŽBryant
et al., 1995., a similar uninterrupted sequence of
small floods is unlikely in arid and semi-arid regions
where record discharges can be hundreds of times
larger than mean annual discharge ŽGraf, 1988.. A
series of small floods on fluvial fans in semi-arid
regions likely hasten the diversion process and increase the effectiveness of extreme events by raising
the channel bed through aggradation.
Finally, overland flow is more likely to result in a
channel avulsion if a nearby channel is receiving the
flow. Headward erosion towards the main channel
will not occur if overland flow does not reconverge
into a small channel draining the fan surface. As
parts of discontinuous ephemeral systems ŽFig. 2.,
aggrading depositional channels on the five alluvial
fans migrate headward; and overland flow is generated at different points along the channel through
time. Eventually the aggrading portion of the main
channel will approach an area where a small channel
draining the fan surface can capture overland flow.
The headward migration of a sheetflood zone on
Wild Burro Fan between 1936 and 1988 coincides
with the widening of Reach A on Wild Burro Fan
ŽFig. 8.. As the sheetflood zone migrated upstream
and backfilled the main channel, more and more
overland flow entered Reach A. The widening may
not have occurred without the upstream shift in the
location of the sheetflood zone.
6. Discussion
If geomorphic effectiveness is defined as the ability of an event to shape or form the landscape
ŽWolman and Gerson, 1978., then channel avulsions
must be considered an effective process on alluvial
fans. Large infrequent floods are widely regarded as
the effective geomorphological agents of change in
arid climates ŽWolman and Miller, 1960; Baker,
1977; Wolman and Gerson, 1978; Kochel, 1988., but
the fine-grained nature of the five study fans means
that small to moderate events can transport sediment
and play an active role in landscape modification.
Although large flows appear to have directly caused
all of the avulsions on the five fans, this should not
imply that small events are ineffective agents of
change. Channel aggradation resulting from small
flows decreases bank heights and increases the
amount of overland flow produced by subsequent
flows, thereby setting the stage for and, to some
extent, dictating the location of future avulsions.
Without the action of small aggrading flows, the
geomorphic effectiveness of large events would be
greatly diminished on the five fans.
Alluvial-fan flooding is of increasing concern in
the rapidly urbanizing southwestern United States,
where over 30% of the landscape is composed of
alluvial fans ŽAntsey, 1965.. The present Federal
Emergency Management Agency method of floodhazard assessment on alluvial fans, based on a
stochastic procedure proposed by Dawdy Ž1979.,
assumes that channel positions move across an alluvial fan surface at random during each flood without
regard to preexisting flow paths ŽFEMA, 1995..
However, channel avulsions on the five study fans
are not completely random events over the short
term, since avulsions invariably occur where channel
banks are low and divert flow into preexisting channels ŽTable 2.. The diversion of flow into preexisting
channels following an avulsion appears to be typical
of fluvial fans in other regions as well ŽKesel, 1985;
Griffiths and McSaveney, 1986; Richards et al., 1987;
Wells and Dorr, 1987; McCarthy et al., 1992.. The
Federal Emergency Management Agency method
also does not account for the fact that large floods on
fluvial fans do not always produce avulsions or
major channel adjustments ŽTable 3; Harvey, 1984;
Ribble, 1988.. While the assumption of randomness
in the Federal Emergency Management Agency
method for assessing flood risk on alluvial fans may
appear safely conservative by assuming the whole
fan surface is equally prone to avulsion, the method
severely underestimates flow depth and velocity
J. Field r Geomorphology 37 (2001) 93–104
along existing channels where flow is most likely to
occur ŽO’Brien and Fullerton, 1990; House et al.,
1992; O’Brien and Fuller, 1993; Field, 1994..
Recognizing the deficiencies in the Federal Emergency Management Agency method, a recent report
by the National Research Council ŽNRC, 1996. recommends site-specific assessment of alluvial-fan
flood hazards. The use of detailed hydraulic modeling along channel reaches identified as probable
avulsion sites on alluvial fans has been suggested
previously ŽGundlach, 1974; Richards et al., 1987.,
and recent advances in quantitative modeling of
avulsions along meandering rivers ŽSlingerland and
Smith, 1998. may also prove useful on alluvial fans.
The model presented in Fig. 7 is useful for identifying those channel reaches most prone to an avulsion
as the channel morphology of a given reach on a
fluvial fan indicates the susceptibility of that reach to
diversion and abandonment ŽFig. 6.. Site-specific
maps of fluvial fans can focus the attention of landuse planners and hydraulic modelers on specific fan
areas such as aggrading reaches with low banks and
nearby small Aon-fanB channels that might capture
overland flow. Through time, a channel’s morphology will evolve as aggrading reaches and sheetflood
zones in a discontinuous ephemeral stream system
migrate headward in response to small sedimentcharged flows ŽFig. 8.. As site-specific studies of
flood hazards on fluvial fans are undertaken, periodic
monitoring will be necessary in order to identify
changes in flood hazards brought about not only by
large floods but also seemingly ineffective small
floods.
7. Conclusions
A descriptive model has been developed that elucidates the processes of channel avulsion on fluvially
dominated alluvial fans in southern Arizona ŽFig. 7..
After a period of bank widening along a new channel
reach, aggradation begins due to small floods that are
unable to transport the imposed sediment load
through the stream system. The resulting decrease in
bank height leads to the generation of overland flow,
which accelerates and directs the headward erosion
of small channels heading on the fan surface. Stream
capture ultimately occurs and the channel receiving
103
the diverted flow is rapidly transformed as it is
incorporated into the main channel network. The
model is similar to avulsion and meander neck cutoff
models developed for mega-fans ŽWells and Dorr,
1987; McCarthy et al., 1992. and semi-arid river
systems ŽSchumann, 1989; Gay et al., 1998., suggesting that the avulsion processes outlined here
operate at different scales and in varied environmental settings.
The avulsion process described here demonstrates
how small floods, under certain circumstances, can
cause an avulsion while large floods often do not
precipitate changes. Small floods on the southern
Arizona fans have not directly caused an avulsion,
but they do potentially accelerate the avulsion process culminated by large floods. The importance of
both small and large floods on channel avulsion
underscores the need for geomorphologists to focus
on how different scales of geomorphological events
work in concert to modify the landscape rather than
on which scale of events is the most geomorphologically effective.
Although channel avulsions occur suddenly and
are clearly a hazard on alluvial fans, the location of
channel avulsions on fluvial fans should no longer be
regarded as unpredictable, since new channels almost
invariably follow preexisting flow paths. A careful
analysis of all existing flow paths in relationship to
potentially unstable reaches along the active channel
Ži.e., low channel banks. may help identify the location of future channel positions. The most likely
paths of future channels on the five southern Arizona
fans have been noted elsewhere ŽField, 1994., and
follow-up studies will establish the accuracy of these
predictions. The results of this study suggest that
understanding the interplay between large and
small-scale geomorphic events is critical for anticipating the timing and location of future avulsions on
some, if not most, fluvially dominated alluvial fans.
References
Antsey, R.L., 1965. Physical Characteristics of Alluvial Fans. US
Army Natick Laboratories, Technical Report ES-20, 109 pp.
Baker, V.R., 1977. Stream-channel response to floods, with examples from central Texas. Geol. Soc. Am. Bull. 88, 1057–1071.
Beaty, C.B., 1963. Origin of alluvial fans, White Mountains,
California and Nevada. Ann. Assoc. Am. Geogr. 53, 516–535.
104
J. Field r Geomorphology 37 (2001) 93–104
Blissenbach, E., 1954. Geology of alluvial fans in semi-arid
regions. Geol. Soc. of Am. Bull. 65, 175–190.
Bryant, M., Falk, P., Paola, C., 1995. Experimental study of
avulsion frequency and rate of deposition. Geology 23, 365–
368.
Bull, W.B., 1979. Threshold of critical power in streams. Geol.
Soc. Am. Bull. 90, 453–464.
Bull, W.B., 1991. Geomorphological Response to Climate Change.
Oxford Univ. Press, NY, 326 pp.
Bull, W.B., 1997. Discontinuous ephemeral streams. Geomorphology 19, 227–276.
Dawdy, D.R., 1979. Flood frequency estimates on alluvial fans.
ASCE J. Hydraul. Div. 105, 1407–1413.
Drew, F., 1873. Alluvial and lacustrine deposits and glacial
records of the Upper Indus basin. Q.J. Geol. Soc. London 29,
441–471.
Federal Emergency Management Agency ŽFEMA., 1995. Appendix 5: Studies of alluvial fan flooding. Guidelines and
specifications for study contractors. Document No. 37, Washington DC.
Field, J.J., 1994. Surficial processes, channel change, and geological methods of flood hazard assessment on fluvially dominated
alluvial fans in Arizona, PhD Thesis, Univ. of Arizona, Tucson.
Gay, G.R., Gay, H.H., Gay, W.H., Martinson, H.A., Meade, R.H.,
Moody, J.A., 1998. Evolution of cutoffs across meander necks
in Powder River, Montana, USA. Earth Surf. Processes Landforms 23, 651–662.
Graf, W.L., 1988. Fluvial Processes in Dryland Rivers. SpringerVerlag, New York, 346 pp.
Griffiths, G.A., McSaveney, M.J., 1986. Sedimentation and river
containment on Waitangitaona alluvial fan — South Westland,
New Zealand. Z. Geomorphol. N.F. 30, 215–230.
Gundlach, D., 1974. Flood plain analysis for alluvial fans wabs.x.
Am. Geophys. Union Trans. 56, 1122.
Harvey, A.M., 1984. Geomorphological response to an extreme
flood: a case from southeast Spain. Earth Surf. Processes
Landforms 9, 267–279.
Hooke, R.L., Rohrer, W.L., 1979. Geometry of alluvial fans:
effect of discharge and sediment size. Earth Surf. Processes 4,
147–166.
House, P.K., 1991. Paleoflood hydrology of the principal canyons
of the Southern Tortolita Mountains, southeastern Arizona.
MS Thesis Prepublication Manuscript, Univ. of Arizona, Tucson.
House, P.K., Pearthree, P.A., Vincent, K.R., 1991. Flow patterns,
flow hydraulics, and flood-hazard implications of a recent
extreme alluvial-fan flood in southern Arizona wabs.x. Geol.
Soc. Am. Abs. Programs 23, A121.
House, P.K., Pearthree, P.A., Vincent, K.R., 1992. Geological
insights into deficiencies in federal procedures for flood hazard assessment on alluvial fans wabs.x. Geol. Soc. Am. Abs.
Programs 24, A284.
Kangieser, P.C., 1969. Major rainstorms and snowstorms in Arizona 1897–1969. Unpublished Report, Arizona Weather Bureau, Tempe, AZ, 25 pp.
Kesel, R.H., 1985. Alluvial fan systems in a wet-tropical environment, Costa Rica. Natl. Geogr. Res. 1, 450–469.
Kesel, R.H., Lowe, D.R., 1987. Geomorphology and sedimentology of the Toro Amarillo alluvial fan in a humid tropical
environment, Costa Rica. Geografis. Ann. 69A, 85–99.
Kochel, R.C., 1988. Geomorphological impact on large floods:
review and new perspectives on magnitude and frequency. In:
Baker, V.R., Kochel, R.C., Patton, P.C. ŽEds.., Flood Geomorphology. Wiley, New York, pp. 169–188.
Maricopa County, 1992. Maricopa County alluvial fan data collection and monitoring study, 1992. Flood Control District of
Maricopa County, Phoenix, AZ.
McCarthy, T.S., Ellery, W.N., Stanistreet, I.G., 1992. Avulsion
mechanisms on the Okavango fan, Botswana: the control of a
fluvial system by vegetation. Sedimenttology 39, 779–795.
Melton, M.A., 1965. The geomorphological and paleoclimatic
significance of alluvial deposits in southern Arizona. J. Geol.
73, 1–38.
National Research Council ŽNRC., 1996. Alluvial Fan Flooding.
National Academy Press, Washington, DC, 172 pp.
O’Brien, J.S., Fuller, J.E., 1993. Discussion on APreferred directions of flow on alluvial fansB. J. Hydraul. Eng. 119, 1316–
1318.
O’Brien, J.S., Fullerton, W.T., 1990. Two dimensional modeling
of alluvial fan flows. In: French, R.H. ŽEd.., HydraulicsrHydrology of Arid Lands. American Society of Civil Engineers,
New York, pp. 262–267.
Price Jr., W.E., 1974. Simulation of alluvial fan deposition by a
random walk model. Water Resour. Res. 10, 263–274.
Ribble, G.E., 1988. Scodie Canyon flash floods of 1984. In: Abt,
S.R., Gessler, J. ŽEds.., Hydraulic Engineering. American
Society of Civil Engineers, New York, pp. 258–263.
Richards, K.S., Brunsden, D., Jones, D.K.C., McCraig, M., 1987.
Applied fluvial geomorphology: river engineering project appraisal in its geomorphological context. In: Richards, K. ŽEd..,
River Channels: Environment and Process. Blackwell, Basil,
Switzerland, pp. 352–382.
Rostvedt, J.O., 1968. Summary of floods in the United States
during 1962. US Geological Survey Water-Supply Paper 1820.
Schumann, R.R., 1989. Morphology of Red Creek Wyoming, an
arid-region anastomosing channel system. Earth Surf. Processes Landforms 14, 277–288.
Schumm, S.A., Hadley, R.F., 1957. Arroyos and the semiarid
cycle of erosion. Am. J. Sci. 255, 161–174.
Slingerland, S., Smith, N.D., 1998. Necessary conditions for a
meandering-river avulsion. Geology 26, 435–438.
Wells, N.A., Dorr Jr., J.A., 1987. Shifting of the Kosi River,
northern India. Geology 15, 204–207.
Whipple, K.X., Dunne, T., 1992. The influence of debris-flow
rheology on fan morphology, Owens Valley, California. Geol.
Soc. Am. Bull. 104, 887–900.
Wolman, M.G., Gerson, R., 1978. Relative scales of time and
effectiveness of climate in watershed geomorphology. Earth
Surf. Processes 3, 189–208.
Wolman, M.G., Miller, J.P., 1960. Magnitude and frequency of
forces in geomorphological processes. J. Geol. 68, 54–74.