slope of alluvial fans in humid regions of japan, taiwan and the philippines

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Geomorphology 70 (

Slope of alluvial fans in humid regions of Japan, Taiwan and

the Philippines

Kyoji Saitoa, Takashi Oguchib,*

aFaculty of Education, Saitama University, Shimo-Okubo, Sakura-ku, Saitama 338-8570, JapanbCenter for Spatial Information Science, University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa 277-8568, Japan

Received 18 May 2004; received in revised form 15 April 2005; accepted 25 April 2005

Available online 26 May 2005

Abstract

This study constructed a database for 690 alluvial fans in humid regions of Japan, Taiwan and the Philippines, and analyzed

their slopes and other morphometric parameters such as the area and relief ratio of the source area. This type of comprehensive

geomorphological research on alluvial fans in humid regions has been limited, although numerous studies have dealt with fans

in arid and semi-arid regions. Semi-conical depositional landforms larger than 2 km2 and steeper than 0.118 (0.002 m/m) were

selected as alluvial fans. About 60% of the fans formed during the Holocene and the rest (40%) formed during the Pleistocene.

Mean fan slopes for both all the Quaternary fans and Holocene fans follow a lognormal frequency distribution. The distribution

also fits slopes of fans belonging to a certain areal range. The area and relief ratio of the source area, which correlate well with

the fan slope, also follow a lognormal distribution, indicating the dimensions of fan/basin systems vary gradually rather than

abruptly. These findings contrast with a previous notion that depositional landforms rarely have slopes of 0.58 (0.009) to 1.58(0.026) and hence only semi-conical landforms steeper than 1.58 should be called alluvial fans.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Alluvial fans; Source areas; Humid regions; Fan slope; Frequency distribution

1. Introduction

Semi-conical depositional landforms along moun-

tain piedmonts are generally referred to as alluvial

fans. The strict definition of alluvial fans, however,

is still an open question. A possible factor affecting

0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.geomorph.2005.04.006

* Corresponding author.

E-mail address: [email protected] (T. Oguchi).

the definition is surface inclination, because very flat

or very steep depositional landforms are usually called

floodplains or talus rather than alluvial fans. There-

fore, the slope or gradient of semi-conical depositional

landforms needs to be analyzed to establish a common

definition of alluvial fans.

Studies in arid regions especially in the American

Southwest during the 1950s and 1960s suggested

that alluvial fans are relatively steep, since fans

formed by debris flows were the main research

2005) 147–162

K. Saito, T. Oguchi / Geomorphology 70 (2005) 147–162148

targets. Blissenbach (1954) classified alluvial fans in

semiarid regions into steep (N58=0.087 m/m), gentle

(28 to 58=0.035 to 0.087) and flat (b28=0.035)types, and implied that landforms gentler than

18(0.017) are not alluvial fans but floodplains.

Anstey (1965) examined data for more than 4,000

alluvial fans in the arid American Southwest and

Pakistan, and pointed out that most fans have slopes

between 18(0.017) and 58(0.087). Hooke (1968a)

suggested that the slope of alluvial fans ranges

from 28(0.035) to 128(0.206).In humid regions, however, gentler depositional

landforms have been identified as alluvial fans.

Tomita (1951) compiled morphometric data for 29

alluvial fans in eastern Taiwan whose lowest slope

is 0.58(0.009). Toya et al. (1971) noted that 153

alluvial fans in Japan have slopes lower than

0.578(0.01). Such gentle fans have been formed by

fluvial processes rather than debris flows (Kadomura,

1971). In Alaska and Iceland, semi-conical deposi-

tional landforms gentler than 18(0.017) have also been

called fans (Boothroyd, 1972; Boothroyd and Num-

medal, 1978). Evans (1991) suggested that slopes of

alluvial fans in humid tropics range from

0.0578(0.001) to 0.578(0.010).Much gentler depositional landforms in humid

regions have also been regarded as alluvial fans.

According to Davis (1898), a very gentle (0.0148=0.0002) depositional landform formed by the Yellow

River in China is one of the largest alluvial fans in the

world. Another large and gentle landform formed by

the Kosi River at the foot of Himalayas (0.0198=0.0003; Stanistreet and McCarthy, 1993) has been

called a wet fan (Schumm, 1977), mega-cone (Par-

kash et al., 1980), large flat fan (Wells and Dorr,

1987), and megafan (Gohain and Parkash, 1990).

Such gentle fans with many distributary channels,

which have been sedimentologically called fluvial

distributary systems (Nicols and Hirst, 1998), also

occur in the other Himalayan piedmonts and along

the Andes (Singh et al., 1993; Iriondo, 1993; Sinha

and Friend, 1994; DeCelles and Cavazza, 1999). One

of the flattest alluvial fans so far reported is the

Okavango fan in northern Botswana, with a mean

slope of 0.0138(0.0002; McCarthy et al., 1991). Sta-

nistreet and McCarthy (1993) named it blosimean fanQ(low sinuosity/meandering fan) because the river on it

is freely winding.

The observations of such gentle alluvial fans were

challenged by Blair and McPherson (1994) based on

data for the 237 piedmont depositional landforms

previously classified as alluvial fans. They indicated

that alluvial fans have average slopes between

1.58(0.026) and 258(0.411), whereas rivers in sedi-

mentary basins are significantly gentler, rarely ex-

ceeding 0.48(0.007). They noted that this clas-

sification is validated from morphology, sedimentary

facies, hydraulics and sedimentary processes, and as-

sumed alluvial fans are constructed mainly by cata-

strophic fluid gravity flows or sediment gravity flows.

Thus, 18 piedmont depositional landforms gentler

than 0.48(0.007), formerly classified as alluvial fans

or fan-deltas, were renamed rivers or river deltas. For

example, gentle fans in Alaska and Iceland are gravel-

bedded rivers; the Kosi River fan is a sand-bedded

river; and the Okavango fan is a mud-dominated

river.

Blair and McPherson (1994) also concluded that

depositional slopes of 0.58–1.58 (0.009–0.026) are

uncommon in aggrading alluvial basins, and called

this interruption bthe natural depositional slope gapQ.McCarthy and Cadle (1995) as well as Kim (1995)

cast doubt on the occurrence of the gap, and ascribed

it to the incomplete choice of data by Blair and

McPherson (1994). In reply to such criticisms, Blair

and McPherson (1995a,b) further insisted that their

inferences are valid, because they thought that the

additional data presented by Kim (1995) and

McCarthy and Cadle (1995) are inappropriate.

Saito et al. (2003) surveyed 123 alluvial fans in

Death Valley, California, based on 1 :24,000 topo-

graphic maps and satellite images and revealed that

almost all the fans there are steeper than 1.58, whiletheir surrounding floodplains and playas are generally

gentler than 0.58. This finding suggests that, if data

are taken only from Death Valley, Blair and McPher-

son’s (1994) depositional slope gap can be detected.

However, Blair and McPherson (1994) suggest that

their slope gap is universal and applicable to any

climatic regions.

The presence or absence of a universal depositional

slope gap is crucial for not only understanding the

nature of fluvial sedimentation, but also the proper

definition of alluvial fans. If the gap can be recog-

nized in many places, it seems natural to allocate

different terms to steeper and gentler piedmont depo-

K. Saito, T. Oguchi / Geomorphology 70 (2005) 147–162 149

sitional landforms. In contrast, if the gradient of semi-

conical depositional landforms tends to vary gradual-

ly, all of them can be called alluvial fans. To discuss

this issue on a global scale, geomorphological infor-

mation from humid regions is indispensable, because

previous studies rarely dealt with a large dataset of

alluvial fans in humid regions. Although Blair and

McPherson (1994) analyzed data for both arid and

humid regions, the number of the fans from humid

regions examined seems to be small.

This paper constructs a database for 690 alluvial

fans in Japan (annual precipitation is ca. 1,000–3,000

mm), Taiwan (ca. 2,000–3,000 mm) and the Philip-

pines (ca. 1,000–4,000 mm), to analyze fan slopes and

other morphometric properties in humid regions based

on a large data set. Attention is directed toward

whether the depositional slope gap exists, to test the

validity of Blair and McPherson (1994) from a geo-

morphological viewpoint. We also collected morpho-

metric data for the source areas of alluvial fans and

analyzed them in relation to the fan slope, because

many previous studies noted the effects of source-area

properties on the form of alluvial fans.

2. Data collection

Alluvial fans in the three countries were identified

using the complete sets of topographic maps:

1 :25,000 maps for Japan were published by the Geo-

graphical Survey Institute with a contour interval of 5

or 10 m; the 1 :50,000 maps for Taiwan were com-

plied by Gakuseisha (1982) with a contour interval of

10 or 20 m; and the 1 :50,000 maps for the Philippines

were published by the Board of Technical Surveys

and Maps with a contour interval of 10 or 20 m.

We identified semi-conical depositional landforms

in piedmont areas larger than 2 km2 and steeper than

0.118(0.002). Accurate derivation of morphometric

parameters for smaller and/or gentler landforms is

difficult, because they tend to contain only a small

number of contour lines. Defining the smallest fan

size upon data collection is not unusual; previous

alluvial-fan studies also dealt with data for fans larger

than a certain size because the number of very small

semi-conical depositional landforms is numerous and

their complete sampling is practically impossible. Al-

though we used both the 1 :25,000 and 1 :50,000

maps to measure fan slope, the difference in the

obtained slope due to the different map scales was

found to be negligible, since we dealt with only

relatively steep and large landforms.

Fig. 1 shows the distribution of the identified

alluvial fans in the three countries. The number of

fans is 490 for Japan, 71 for Taiwan, and 129 for the

Philippines. All the fans formed in the Quaternary

(Bureau of Mines and Geo-Sciences, 1982; Saito,

1988; Chang, 1997). Some fans exhibit varying

extents of terraced surfaces formed by fluvial erosion.

In such cases only data for the lowest surface were

collected, because tectonic movement may have sig-

nificantly altered the slope of higher surfaces since

their formation.

We measured the mean slope of each alluvial fan,

which is defined as height difference between the fan

apex and the fan toe divided by the fan length (Saito,

1985). The range of the slope was 0.118(0.002) to

4.08(0.070) for Japan, 0.118(0.002) to 3.48(0.059) forTaiwan, and 0.158(0.003) to 6.38(0.110) for the Phi-

lippines. If the concavities of fan surfaces are signif-

icantly different, fans with similar values of the

overall fan slope may not be regarded as having

similar slope characteristics. Thus, the mean slope of

the highest one third of each fan was adopted as an

additional slope parameter representing steepness near

the fan apex. We applied this parameter only to Jap-

anese fans, since the 1 :25,000 topographic maps for

Japan permitted detailed topographic measurement.

The source basins of the alluvial fans in the three

countries were delineated on the topographic maps.

We measured their areas and the relief ratio, basin

height divided by basin length (Schumm, 1956), to

describe their sizes and general inclination. Some

more properties of these fans and source areas, such

as dominant bedrock geology, are listed in Saito

(2003).

3. Examples of alluvial fans

Unlike alluvial fans in some arid regions such as

the American Southwest, fans in Japan, Taiwan and

the Philippines have not been well known to interna-

tional researchers. Therefore, this section introduces

several typical examples of alluvial fans in the three

countries with contour maps. As this paper focuses on

Fig. 1. Distribution of 490 alluvial fans in Japan, 71 fans in Taiwan, and 129 fans in the Philippines. Fans formed in the Holocene and Pleistocene are shown with different symbols.

Fans with names correspond to Figs. 2, 3 and 4.

K.Saito

,T.Oguchi/Geomorphology70(2005)147–162

150


fan slopes, the examples are divided into three cate-

gories based on their slopes.

3.1. Steep fans

Small and steep alluvial fans with slopes more than

1.58 occur in some areas of all the three countries. A

Fig. 2. Examples of steep alluvial fans. (A) Ashima River fan at the foot of

interval is 10 m. (B) San Vicente fan, Luzon Island, Philippines. Mean fa

typical example of such fans is the Ashima River fan

at the foot of the Northern Japanese Alps, central

Japan (Fig. 2A). It is 5.5 km2 in area, 4.1 km in

length, and 3.48(0.059) in slope. The thickness of

Holocene gravel in the middle fan is ca. 40 m (Ogu-

chi, 1997), reflecting abundant post-glacial sediment

supply from the upstream area due to frequent heavy

the Northern Japanese Alps. Mean fan slope is 3.48(0.059). Countern slope is 2.48(0.042). Contour interval is 20 m.

Fig. 3. Examples of medium slope alluvial fans. (A) Kurobe River fan in central Japan, facing Japan Sea. Mean fan slope is 0.618(0.011).Contour interval is 5 m. Grain size data are from Ouchi (1979). (B) Shoufeng River fan in East Taiwan. Mean fan slope is 0.968(0.017). Contourinterval is 10 m.


Fig. 4. Examples of gentle alluvial fans. (A) Kiso River fan in central Japan. Mean fan slope is 0.148(0.002). Contour interval is 5 m. Grain size

data (in F) are from Yatsu (1955a). Crosses with numbers show sites of grain-size measurement. (B) Choshui River fan in West Taiwan, facing

Taiwan Straight. Mean fan slope is 0.188(0.003). Contour interval is 5 m.



storms and slope failure (Oguchi, 1996a,b). The river

on the fan is braided, and large boulders transported

by debris flows often occur on the riverbed. The

source area of the fan, composed of Cretaceous gra-

nitic rocks, has an area of 11.0 km2 and a relief ratio

of 0.253.

The San Vicente alluvial fan in central Luzon

Island, the Philippines (Fig. 2B), is 3.9 km2 in area,

2.2 km in length, and 2.48(0.042) in slope. It is locatedat the southern foot of the Sierra Madre Mountains,

underlain by pre-Jurassic sedimentary and metamor-

phic rocks (Bureau of Mines and Geo-Sciences,

1982). The sharp boundary between the mountains

and the adjacent lowland may reflect active tectonics

in the area. For instance, the 1990 Luzon earthquake

led to a new fault scarp near the San Vicente fan, with

lateral and vertical slips of 5.3 m and 1.9 m, respec-

tively (Nakata et al., 1996). The source area of the fan

is 16.2 km2 in area and 0.197 in the relief ratio.

3.2. Medium slope fans

Some alluvial fans studied have medium slopes

(0.58 to 1.58), which correspond to the natural depo-

sitional slope gap of Blair and McPherson (1994). The

alluvial fan of the Kurobe River (Fig. 3A) has been

regarded as a typical alluvial fan in Japan (Fukai,

1966; Nakayama, 1981). It is 69.2 km2 in area, 11.8

km in length, and 0.618(0.011) in slope. The fan faces

the Japan Sea. The river on the fan is clearly braided

reflecting abundant supply of clastic sediment from

high mountains behind (Oguchi et al., 2001). The

maximum grain size attains 40 cm (Ouchi, 1979).

The fan slope is nearly constant from the fan apex

to the toe. Most of the fan surface has been forming in

the Holocene (Fukai, 1966). The source area of the

fan, underlain mainly by granitic rocks, has an area of

655 km2 and a relief ratio of 0.056.

The alluvial fan of the Shoufeng River, Huatung

Longitudinal Valley, East Taiwan (Fig. 3B), is 53.4

km2 in area, 9.0 km in length, and 0.968(0.017) in

mean slope. It is located at the eastern foot of the

Taiwan Central Range. The fan has been forming in

the Holocene (Shih et al., 1983; Chang, 1997). The

river on the fan is braided and associated with pebble-

size gravel (Chang et al., 1994). The source area of the

fan consists of Paleozoic to Mesozoic schist rocks. It

has an area of 204 km2 and a relief ratio of 0.135.

3.3. Gentle fans

Some alluvial fans in the three countries are less

than 0.58 in mean slope, which corresponds to Blair

and McPherson’s (1994) brivers or river deltas ques-tionably classified as alluvial fansQ. The alluvial fan of

the Kiso River, central Japan (Fig. 4A), has a mean

slope of 0.148(0.002), an area of 94.4 km2, and a

length of 13.9 km. The depositional slope and the

modal grain size change abruptly at the fan toe: ca.

0.068(0.001) to 0.028(0.0003) and 64 mm to 1 mm

(Yatsu, 1955a,b). The fan has been forming in the

Holocene. Iseki (1981) suggested that the thickness of

the late Holocene fan gravel is ca. 3 m. The river on

the fan is braided, although the branching of the

channels is not very frequent. The source area is

4,850 km2 in area and 0.028 in the relief ratio. It

consists of various rock types.

The Choshui alluvial fan is the largest fan in

Taiwan (Fig. 4B), having an area of 472 km2, a length

of 29.7 km, and a mean slope of 0.188(0.003). The fanslope at the apex (ca. 100 m a.s.l.) is 0.288(0.005) but0.068(0.001) at the toe (ca. 20 m a.s.l., Chang, 1985).

The river on the fan is braided, and gravel deposits

near the fan apex are 60 to 80 m thick (Chang, 1985).

The source area is 2,870 km2 in area and 0.048 in the

relief ratio. It contains the summit of Yushan (3,997

m), the highest mountain in Taiwan.

4. Frequency distributions of fan slope and source

area properties

The fan slope data for Japan, Taiwan, and the

Philippines were arranged to examine their frequency

distribution, one of the most basic statistical properties

of a numerical dataset. Fig. 5A shows the histogram

of the slopes of all the 690 alluvial fans with a 0.18(partly 18) slope bin. The negatively skewed histo-

gram shows that slopes between 0.28(0.003) and

0.88(0.014) are the most frequent. For larger slopes,

the frequency of fans gradually decreases with an

increasing slope. The histogram has no distinct gaps,

in contrast with Blair and McPherson’s (1994) pro-

posal of the natural depositional slope gap at 0.58–1.58. As Blair and McPherson (1994) identified the

gap from data for modern alluvial fans, we con-

structed the slope histogram for the 418 Holocene

Fig. 5. Histograms of mean alluvial fan slopes in Japan, Taiwan and the Philippines. (A) Quatenary fans. (B) Holocene fans. Slope bin interval is

0.18 for slopes smaller than 28, and is 18 for larger slopes.


fans (Fig. 5B). The shape of the histogram is similar to

that for all the Quaternary fans, confirming the ab-

sence of the gap. Blair and McPherson (1995a) noted

that each fan examined by Blair and McPherson

(1994) basically has a straight longitudinal profile,

although they used data for rather concave landforms

such as the Trollheim fan in Deep Springs Valley in

California (Blair and McPherson, 1992) and the Reno

River fan in Italy (Ori, 1982). We selected 90 nearly

straight Holocene fans in Japan whose slope near the

fan apex is 0.8 to 1.2 times larger than the mean fan

slope. The slope histogram for such fans has a shape

similar to that for all the Quaternary fans (Fig. 6A).

Moreover, the data of slope near the apex for the 262

Holocene fans in Japan have also yielded a similar

histogram without gaps (Fig. 6B).

Fig. 6. (A) Histogram of slopes of Holocene fans with straight longitudinal profiles. Slope bin interval is 0.18. (B) Histogram of slopes near

apices of Holocene fans. Slope bin interval is 0.18 for slopes smaller than 28, and is 18 for larger slopes.


The histograms of logarithmic fan slopes for both

all the Quaternary fans and the Holocene are almost

symmetric (Fig. 7A and B), indicating the slope data

follow a lognormal distribution, which applies to

many other geomorphological and hydrological phe-

nomena such as grain-size distribution (Krumbein and

James, 1965; Smith, 1992; Kosugi, 1996; Istanbulluo-

glu et al., 2002). Fig. 7C and D are the histograms of

the logarithmic values of tangent fan gradients instead

of slopes in degrees, also showing nearly symmetric

shapes. The D’Agostino–Pearson K2 test for normal-

ity (Zar, 1998) was applied to the original logarithmic

fan slope/gradient data used for creating Fig. 7A to D.

The result yielded the p values larger than 0.05 for all

the cases, which statistically confirms the applicability

of the lognormal distribution.

We also investigated the frequency distribution of

logarithmic fan slopes for fans belonging to a certain

areal range, based on their histograms and the

D’Agostino–Pearson K2 test. The areal ranges were

operationally classified using a log 0.5 bin: b3.2 km2,

3.2–10 km2, 10–32 km2, 32–100 km2 and N100 km2.

The fans larger than 100 km2 were not used for the

analysis since their number is small. The results show

that the fan slopes for each areal range also follow a

lognormal distribution, implying the broad applicabil-

ity of the lognormal distribution to fan slope data.

The fan slope tends to decrease with increasing fan

area (Fig. 8A), which have also been found elsewhere

with various climatic conditions (e.g., Drew, 1873;

Eckis, 1928; Tomita, 1951; Bull, 1964; Melton,

1965; Hooke, 1968b; Beaumont, 1972). The fan

Fig. 7. Histograms of logarithmic fan slopes/gradients in Japan, Taiwan and the Philippines. (A) Quaternary fans, in degrees. (B) Holocene fans,

in degrees. (C) Quaternary fans, tangent. (D) Holocene fans, tangent.


slopes also correlate negatively with the source basin

area (Fig. 8B), but positively with the basin relief ratio

(Fig. 8C). Similar relationships have been observed in

the American Southwest, England, Greece and Spain

(Bull, 1964; Hooke, 1968b; Harvey, 1984, 1987,

1992, 2002; Silva et al., 1992). We analyzed the

frequency distributions of the area and the relief

ratio of the source basins, based on their histograms

and the D’Agostino–Pearson K2 test. The results sta-

tistically show that these properties also follow the

lognormal distribution without distinct gaps.

5. Discussion

The 690 alluvial fans in Japan, Taiwan, and the

Philippines cannot be divided logically based on their

slopes, because the slopes follow the lognormal fre-

quency distribution without distinct gaps. The lognor-

mal distribution also applies to the area and relief ratio

of the source basin, indicating that the shape of the

fan/basin systems varies gradually rather than abrupt-

ly. Such geomorphological continuity casts doubt on

Blair and McPherson’s (1994) concept of the natural

depositional slope gap at least for humid regions.

Milana and Ruzycki (1999) and Harvey (2002) also

note that the lack of specific gaps in the drainage basin

area should lead to gradual change in the fan slope. As

shown in Figs. 2–4, the sampled fans have semi-

conical shapes and occur in piedmont areas. There is

no reason to give them a name other than balluvialfanQ.

Although Blair and McPherson (1994) suggest that

catastrophic fluid/sediment gravity flows create allu-

vial fans steeper than 1.58, while fluvial processes

provide much flatter landforms, the distinction of

the two types of processes is often difficult without

direct observation of sediment exposures. Sediment

transport processes whose characteristics are interme-

diate between fluvial processes and debris flows have

Fig. 8. Relationship between alluvial fan slope/gradient and other morphometric properties for Japan, Taiwan and the Philippines. (A) Logarithmic

mean fan gradient versus logarithmic fan area, showing negative correlation. (B) Logarithmic mean fan gradient versus logarithmic source basin

area, showing negative correlation. (C) Logarithmic mean fan gradient versus logarithmic basin relief ratio, showing positive correlation.



been observed on alluvial fans: transitional debris

flows or hyperconcentrated flow (Wells and Harvey,

1987; Orme, 1989; Harvey, 1990), intermediate flow

associated with sieve deposition (Hooke, 1967; Was-

son, 1974; Bull, 1977), and sediment sheet flow

(Takayama et al., 2002). Various types of fan deposits

are often observed within a single alluvial fan or

adjacent fans (e.g., Blissenbach, 1954; Harvey 1984;

Mack and Rasmussen, 1984; Kamp et al., 2004; May

and Gresswell, 2004; Oguchi and Oguchi, 2004),

reflecting spatial and temporal variations in dominant

geomorphic processes. Moreover, not mass flow but

fluvial processes play a major role in forming some

gentle fans (e.g., Hooke, 1968b; Kadomura, 1971;

Bull, 1977; Nakayama and Takagi, 1987). In other

words, an oversimplified correlation between alluvial-

fan occurrence and a few specific sedimentary pro-

cesses should be avoided.

The range of applicability of a scientific term is

variable, and most geoscientists have been using the

term balluvial fanQ in a broad sense. Therefore, semi-

conical depositional landforms with different sizes,

slopes and deposits have been regarded as alluvial

fans. Blair and McPherson (1994) criticized this

practice based on the depositional slope gap at 0.58to 1.58, but as shown in the present study, the occur-

rence of the gap does not look universal. Therefore,

we recommend keeping the broad applicability of the

term balluvial fanQ, and propose to define the term in

such a flexible way. A reasonable geomorphological

distinction between alluvial fans and other deposi-

tional landforms such as floodplains is whether they

have semi-conical shapes. In this sense, very gentle

semi-conical depositional landforms such as those

formed by the Kosi River in India, the Kiso River

in Japan (Fig. 4A), the Choshui River in Taiwan (Fig.

4B), and the Reno River in Italy can be called

alluvial fans.

Some previous studies in arid regions also sug-

gested the absence of the depositional slope gap at

0.58–1.58. Bull (1964, Table 1) reported 20 alluvial

fans whose mean slopes of 0.208(0.003) to

1.668(0.029) from Fresno County, California. Anstey

(1965, Table 2) indicates that 91 out of 317 large

alluvial fans in Pakistan and 72 out of 588 large fans

in the United States have slopes smaller than 1.58.However, as noted before, almost all the fans in

Death Valley are steeper than 1.58, while their sur-

rounding floodplains and playas are generally gentler

than 0.58 (Saito et al., 2003). Therefore, if data are

taken only from Death Valley, a slope gap compara-

ble to Blair and McPherson’s (1994) can be detected,

although this is not the case in many other regions.

Some local characteristics of Death Valley, such as

tectonic activity and similarities in the size and

gradient of upstream areas, may have provided

only steep fans and gentle playas throughout the

area.

The slope gap in Death Valley is associated with

a distinct sedimentological contrast between gravel

on alluvial fans and sandy deposits on surrounding

flatter areas (Denny, 1965; Hooke, 1972). Such an

abrupt change in the depositional slope, associated

with a sudden change in riverbed material from

gravel to sand, also occurs at the toes of some

gentle Japanese fans (Yatsu, 1955a,b; Ohmori,

1991; Inoue, 1992), as is the case with the Kiso

River fan (Fig. 4A). The slope of the lowest part of

such Japanese fans is ca. 0.068(0.001), which is

considerably smaller than the slope gap in Death

Valley. Abrupt gravel to sand transitions along rivers

with rapid change in river gradients have been

observed in many places and their formative pro-

cesses have been discussed (Yatsu, 1957; Wolcott,

1988; Kodama, 1994; Sambrook Smith and Fergu-

son, 1995; Wilcock, 1998; Ferguson, 2003), but

such segmented rivers have various gradients. Har-

vey (2002) also indicates that gradient differences

between alluvial fans and rivers without fans tend to

vary with the catchment area. All these observations

cast doubt on Blair and McPhersonTs (1994) univer-

sal depositional slope gap at a fixed slope range of

0.58–1.58.

Acknowledgements

We thank Thad Wasklewicz for reviewing an early

draft of this paper and correcting English, and Adrian

Harvey and Martin Stokes for their useful review

comments. We also thank Gary Weissmann and Justin

Wilkinson for encouraging us to write this paper. This

study was supported by the Grants-in-Aid for Scien-

tific Research, from the Ministry of Education, Cul-

ture, Sports, Science and Technology (Exploratory

Research # 15650188).


References

Anstey, R.L., 1965. Physical characteristics of alluvial fans. U.S.

Army Natick Laboratories, Technical Report ES20, 109.

Beaumont, P., 1972. Alluvial fans along the foothills of the Elburz

Mountains, Iran. Palaeogeography, Palaeoclimatology, Palaeoe-

cology 12, 251–273.

Blair, T.C., McPherson, J.G., 1992. The Trollheim alluvial fan and

facies model revisited. Geological Society of America Bulletin

104, 762–769.

Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural

distinction from rivers based on morphology, hydraulic process-

es, sedimentary processes, and facies assemblages. Journal of

Sedimentary Research. Section A, Sedimentary Petrology and

Processes 64, 450–489.

Blair, T.C., McPherson, J.G., 1995a. Reply: alluvial fans and their

natural distinction from rivers based on morphology, hydraulic

processes, sedimentary processes, and facies assemblages. Jour-

nal of Sedimentary Research. Section A, Sedimentary Petrology

and Processes 65, 583–586.

Blair, T.C., McPherson, J.G., 1995b. Reply: alluvial fans and their

natural distinction from rivers based on morphology, hydraulic

processes, sedimentary processes, and facies assemblages. Jour-

nal of Sedimentary Research. Section A, Sedimentary Petrology

and Processes 65, 708–711.

Blissenbach, E., 1954. Geology of alluvial fans in semiarid regions.

Geological Society of America Bulletin 65, 175–189.

Boothroyd, J.C., 1972. Coarse-grained sedimentation on a braided

outwash fan, northeast Gulf of Alaska. University of South

Carolina Coastal Research Division Technical Report 6, 127.

Boothroyd, J.C., Nummedal, D., 1978. Proglacial braided outwash:

a model for humid alluvial-fan deposits. In: Miall, A.D. (Ed.),

Fluvial Sedimentology, Canadian Society of Petroleum Geolo-

gists, Memoir, vol. 5, pp. 641–668.

Bull, W.B., 1964. Geomorphology of segmented alluvial fans in

western Fresno County California. U.S. Geological Survey

Professional Paper 352E, 89–129.

Bull, W.B., 1977. The alluvial-fan environment. Progress in Phys-

ical Geography 1, 222–270.

Bureau, of Mines and Geo-Sciences, Ministry of Natural Resources,

Philippines, 1982. Maps of Geology and Mineral Resources of

the Philippines, Millionth scale.

Chang, J.C., 1985. Topographical analysis and landform changes of

Choshui Plain. Geographical Research, the Institute of Geogra-

phy, National Taiwan Normal University 11, 199-228 (in Chi-

nese with English abstract).

Chang, J.C., 1997. Current geomorphological studies in Taiwan.

Chiri (Geography) 42 (8), 53–63 (in Japanese).

Chang, J.C., Shih, T.T., Yang, S.C., Lin, Y.F., Chen, H.L., 1994. A

geomorphological study of alluvial fan in Huatung Longitudinal

Valley. Geographical Research, the Institute of Geography, Na-

tional Taiwan Normal University 21, 43-74 (in Chinese with

English abstract).

Davis, W.M., 1898. Physical Geography. Ginn and Company, Bos-

ton, p. 432.

DeCelles, P.G., Cavazza, W., 1999. A comparison of fluvial mega-

fans in the Cordilleran (Upper Cretaceous) and modern Hima-

layan foreland basin systems. Geological Society of America

Bulletin 111, 1315–1334.

Denny, C.S., 1965. Alluvial fans in the Death Valley region, Cali-

fornia and Nevada U.S. Geological Survey Professional Paper

466, 62.

Drew, F., 1873. Alluvial and lacustrine deposits and glacial records

of the upper-Indus Basin. Quarterly Journal of the Geological

Society of London 29, 441–471.

Eckis, R., 1928. Alluvial fans of the Cucamonga district, southern

California. Journal of Geology 36, 224–247.

Evans, J.E., 1991. Facies relationships, alluvial architecture, and

paleohydrology of a Paleogene, humid-tropical alluvial-fan sys-

tem: chumstick formation Washington state, U.S.A. Journal of

Sedimentary Petrology 61, 732–755.

Ferguson, R.I., 2003. Emergence of abrupt gravel to sand transitions

along rivers through sorting processes. Geology 31, 159–162.

Fukai, S., 1966. Microform of the Kurobe River alluvial fan and its

formation. In: Academic Research Group of Toyama University,

(Ed.), The Kurobe River. Kokon Shoin, Tokyo, pp. 121–138 (in

Japanese).

Gakuseisha, 1982. Collection of 1 :50,000 Topographic Maps of

Taiwan. Gakuseisha, Tokyo (112 sheets).

Gohain, K., Parkash, B., 1990. Morphology of the Kosi megafan.

In: Rachocki, A.H., Church, M. (Eds.), Alluvial Fans: A Field

Approach. John Wiley & Sons, Chichester, pp. 151–178.

Harvey, A.M., 1984. Debris flows and fluvial deposits in Spanish

Quaternary alluvial fans: implications for fan morphology. In:

Koster, E.H., Steel, R.J. (Eds.), Sedimentology of Gravels and

Conglomerates, Canadian Society of Petroleum Geologists

Memoirs, 10, pp. 123–132.

Harvey, A.M., 1987. Alluvial fan dissection: relationships between

morphology and sedimentation. In: Frostick, L., Reid, I. (Eds.),

Desert Sediments: Ancient and Modern, Geological Society

Special Publication, vol. 35, pp. 87–103.

Harvey, A.M., 1990. Factors influencing Quaternary alluvial fan

development in Southeast Spain. In: Rachocki, A.H., Church,

M. (Eds.), Alluvial Fans: A Field Approach. John Wiley &

Sons, Chichester, pp. 247–269.

Harvey, A.M., 1992. The influence of sedimentary style on the

morphology and development of alluvial fans. Israel Journal

of Earth-Sciences 41, 123–137.

Harvey, A.M., 2002. The relationships between alluvial fans and fan

channels within Mediterranean mountain fluvial systems. In:

Bull, L.J., Kirkby, M.J. (Eds.), Dryland Rivers: Hydrology

and Geomorphology of Semi-Arid Channels. John Wiley &

Sons, Chichester, pp. 205–226.

Hooke, R.L., 1967. Processes on arid-region alluvial fans. Journal

of Geology 75, 438–460.

Hooke, R.L., 1968a. Discussion: model geology: prototype and

laboratory streams. Geological Society of America Bulletin

79, 391–394.

Hooke, R.L., 1968b. Steady-state relationships on arid-region

alluvial fans in closed basins. American Journal of Science

266, 609–629.

Hooke, R.L., 1972. Geomorphic evidence for Late-Wisconsin and

Holocene tectonic deformation, Death Valley, California. Geo-

logical Society of America Bulletin 83, 2073–2098.


Inoue, K., 1992. Downstream change in grain size of river bed

sediments and its geomorphological implications in the

Kanto plain, central Japan. Geographical Review of Japan

65B, 75–89.

Iriondo, M., 1993. Geomorphology and late Quaternary of the

Chaco (South America). Geomorphology 7, 289–303.

Iseki, H., 1981. Super-imposed alluvial fan in the Nobi Plain. In:

Sawada, K. (Ed.), Relationships Between Nature and Human

Activity. Kokon Shoin, Tokyo, pp. 11–19 (in Japanese).

Istanbulluoglu, E., Tarboton, D.G., Pack, R.T., 2002. A probabilistic

approach for channel initiation. Water Resources Research 38,

1325–1338.

Kadomura, H., 1971. Microforms on alluvial fans and their forma-

tion processes: focusing on gentle slope fans in the Tokaido

district. In: Yazawa, D., Toya, H., Kaizuka, S. (Eds.), Alluvial

Fans. Kokon Shoin, Tokyo, pp. 55–96 (in Japanese).

Kamp, U., Haserodt, K., Shroder, J.F., 2004. Quaternary landscape

evolution in the eastern Hindu Kush Pakistan. Geomorphology

57, 1–27.

Kim, S.B., 1995. Discussion: alluvial fans and their natural distinc-

tion from rivers based on morphology, hydraulic processes,

sedimentary processes, and facies assemblages. Journal of Sed-

imentary Research. Section A, Sedimentary Petrology and Pro-

cesses 65, 706–708.

Kodama, Y., 1994. Downstream changes in the lithology and grain

size of fluvial gravels in the Watarase River, Japan: evidence of

the role of abrasion in downstream fining. Journal of Sedimen-

tary Research 64A, 68–75.

Kosugi, K., 1996. Lognormal distribution model for unsaturat-

ed soil hydraulic properties. Water Resources Research 32,

2697–2703.

Krumbein, W.C., James, W.R., 1965. A lognormal size distribu-

tion model for estimating stability of beach fill material.

Coastal Engineering Research Center Technical Memorandum

16, 1–17.

Mack, G.H., Rasmussen, K.A., 1984. Alluvial-fan sedimentation of

the Cutler formation (Permo-Pennsylvanian) near Gateway Col-

orado. Geological Society of America Bulletin 95, 109–116.

May, C.L., Gresswell, R.E., 2004. Spatial and temporal patterns of

debris-flow deposition in the Oregon Coast Range USA. Geo-

morphology 57, 135–149.

McCarthy, T.S., Cadle, A.B., 1995. Discussion: alluvial fans and

their natural distinction from rivers based on morphology, hy-

draulic processes, sedimentary processes, and facies assem-

blages.. Journal of Sedimentary Research. Section A,

Sedimentary Petrology and Processes 65, 581–583.

McCarthy, T.S., Stanistreet, I.G., Cairncross, B., 1991. The sedi-

mentary dynamics of active fluvial channels on the Okavango

fan Botswana. Sedimentology 38, 471–487.

Melton, M.A., 1965. The geomorphic and paleoclimatic signifi-

cance of alluvial deposits in southern Arizona. Journal of Ge-

ology 73, 1–38.

Milana, J.P., Ruzycki, L., 1999. Alluvial-fan slope as a function of

sediment transport efficiency. Journal of Sedimentary Research

69, 553–562.

Nakata, T., Tsutsumi, H., Punongbayan, R.S., Rimando, R.E.,

Daligdig, J.A., Daag, A.S., Besana, G.M., 1996. Surface Fault

Ruptures of the 1990 Luzon Earthquake, Philippines. Special

Publication, Research Center for Regional Geography, Hor-

ishima University 25, 86.

Nakayama, M., 1981. Geomorphological study on alluvial fans:

formation of alluvial fans and the Kurobe River alluvial fan.

The Kurobe-gawa Alluvial Fan, Bulletin of the Regional and

Social Research Institute of the Kurobe-gawa Alluvial Fan 5,

22–34 (in Japanese).

Nakayama, M., Takagi, I., 1987. Micro geomorphic analysis of the

Kofu Basin alluvial fans, central Japan. Annals of the Tohoku

Geographical Association 39, 98–112 (in Japanese with English

abstract).

Nichols, G.J., Hirst, J.P., 1998. Alluvial fans and fluvial distrib-

utary systems, Oligo-Miocene, northern Spain: contrasting

processes and products. Journal of Sedimentary Research 68,

879–889.

Oguchi, T., 1996a. Factors affecting the magnitude of post-glacial

hillslope incision in Japanese mountains. Catena 26, 171–186.

Oguchi, T., 1996b. Relaxation time of geomorphic responses to

Pleistocene–Holocene climatic change. Transactions, Japanese

Geomorphological Union 17, 309–321.

Oguchi, T., 1997. Late quaternary sediment budget in alluvial-fan-

source-basin systems in Japan. Journal of Quaternary Science

12, 381–390.

Oguchi, T., Oguchi, C.T., 2004. Late Quaternary rapid talus dissec-

tion and debris-flow deposition on an alluvial fan in Syria.

Catena 55, 125–140.

Oguchi, T., Saito, K., Kadomura, H., Grossman, M., 2001. Fluvial

geomorphology and paleohydrology in Japan. Geomorphology

39, 3–19.

Ohmori, H., 1991. Change in the mathematical function type de-

scribing the longitudinal profile of a river through an evolution-

ary process. Journal of Geology 99, 97–110.

Ori, G.G., 1982. Braided to meandering channel patterns in humid-

region alluvial fan deposits, River Reno, Po Plain (northern

Italy). Sedimentary Geology 31, 231–248.

Orme, A.R., 1989. The nature and rate of alluvial fan aggradation in

a humid temperate environment, northwest Washington. Physi-

cal Geography 10, 131–146.

Ouchi, S., 1979. A study of minor reliefs and grain size distributions

on alluvial fans in central Japan. Geographical Review of Japan

52, 190–198 (in Japanese with English abstract).

Parkash, B., Sharma, R.P., Roy, A.K., 1980. The Siwalik Group

(molasse): sediments shed by collision of continental plates.

Sedimentary Geology 25, 127–159.

Saito, K., 1985. Comparison between dynamic equilibrium model

and climatic linked model for alluvial fans in Japan. Gakuen

Ronshu (The Journal of Hokkai-Gakuen University) 52, 35–81.

Saito, K., 1988. Alluvial Fans of Japan. Kokon Shoin, Tokyo, p. 280

(in Japanese).

Saito, K., 2003. Model of alluvial fan development based on

channel pattern and gravel size. Report of Research Project,

Grant-in-aid for Scientific Research, 138 (in Japanese).

Saito, K., Yamada, M., Hashimoto, A., Oguchi, T., 2003. Slope of

alluvial fans in Death Valley, California. Occasional Paper of

Department of Geography, Saitama University 23, 29–40 (in

Japanese).


Sambrook Smith, G.H., Ferguson, R.I., 1995. The gravel to sand

transition along river channels. Journal of Sedimentary Research

65A, 423–430.

Schumm, S.A., 1956. Evolution of drainage systems and slopes in

badlands at Perth Amboy New Jersey. Geological Society of

America Bulletin 67, 597–646.

Schumm, S.A., 1977. The Fluvial System. John Wiley & Sons, New

York, p. 338.

Shih, T.T., Chang, J.C., Hwang, C.E., Shih, C.D., Yang, G.S.,

Sunlin, Y.M., 1983. A geomorphological study of active fault

in northern and eastern Taiwan. Geographical Research, the

Institute of Geography, National Taiwan Normal University 9,

20-72 (in Chinese with English abstract).

Silva, P.G., Harvey, A.M., Zazo, C., Goy, J.L., 1992. Geomorphol-

ogy, depositional style and morphometric relationships of Qua-

ternary alluvial fans in the Guadalentin Depression (Murcia

Southeast Spain). Zeitschrift fur Geomorphologie, Neue Folge

36, 325–341.

Singh, H., Parkash, B., Gohain, K., 1993. Facies analysis of the

Kosi megafan deposits. Sedimentary Geology 85, 87–113.

Sinha, R., Friend, P.F., 1994. River systems and their sediment flux,

Indo-Gangetic plains, Northern Bihar, India. Sedimentology 41,

825–845.

Smith, J.A., 1992. Representation of basin scale in flood peak

distributions. Water Resources Research 28, 2993–2999.

Stanistreet, I.G., McCarthy, T.S., 1993. The Okavango fan and the

classification of subaerial fan systems. Sedimentary Geology 85,

115–133.

Takayama, J., Fujita, Y., Kondo, Y., Hachiya, K., 2002. A two layer

model unifying debris flow and sediment sheet flow. Interna-

tional Congress INTERPRAEVENT 2002 in the Pacific Rim,

Matsumoto, Japan, Congress Publication, vol. 1, pp. 113–124.

Tomita, Y., 1951. On the geomorphological classification of fans in

Taiwan (Formosa). Journal of Geography (Tokyo) 60, 2–9 (in

Japanese with English abstract).

Toya, H., Machida, H., Naito, H., Hori, N., 1971. Distribution of

alluvial fans in Japan. In: Yazawa, D., Toya, H., Kaizuka, S.

(Eds.), Alluvial Fans. Kokon Shoin, Tokyo, pp. 97–120 (in

Japanese).

Wasson, R.J., 1974. Intersection point deposition on alluvial fans:

an Australian example. Geografiska Annaler 56A, 83–92.

Wells, N.A., Dorr, J.A., 1987. Shifting of the Kosi River, northern

India. Geology 15, 204–207.

Wells, S.G., Harvey, A.M., 1987. Sedimentologic and geomorphic

variations in storm-generated alluvial fans, Howgill Fells,

northwest England. Geological Society of America Bulletin

98, 182–198.

Wilcock, P.R., 1998. Two-fraction model of initial sediment motion

in gravel-bed rivers. Science 280, 410–412.

Wolcott, J., 1988. Non-fluvial control of bimodal grain size distri-

bution in river bed gravels. Journal of Sedimentary Petrology

58, 979–984.

Yatsu, E., 1955. On the formation of the slope discontinuity at the

fan margins. Miscellaneous Reports of the Research Institute for

Natural Resources 36, 57–64.

Yatsu, E., 1955. On the longitudinal profile of the graded river.

Transactions, American Geophysical Union 36, 655–663.

Yatsu, E., 1957. On the discontinuity of grain-size frequency dis-

tribution of fluvial deposits and its geomorphological signifi-

cance. Proceedings of the International Geophysical Union

Regional Conference, Tokyo, Japan, pp. 224–237.

Zar, J.H., 1998. Biostatistical Analysis. Prentice Hall, Upper Saddle

River, p. 663.

slope of alluvial fans in humid regions of japan, taiwan and the philippines

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