Dr. Kyoji Saito

Work that overturned the “conventional wisdom” in alluvial fan research

I love traveling and have been interested in landforms (ground surface features) that are often part of a scenic view. This is why I chose to specialize in geomorphology. This field investigates how various landforms were created and how they will change in the future.

Topographical changes can progress gradually over a long period of time or they can occur abruptly, triggered by some rare natural phenomenon. The latter case includes geomorphic changes caused by volcanic activity, crustal movement, and large-scale flooding that occur once in a hundred to thousand years. As such changes occur over a period of thousands to hundreds of thousands of years, landforms change drastically.Dr. Kyoji Saito

Historical documents recording topographical changes are rare, so understanding these changes requires analysis of the landforms themselves or the sediments that constitute them. Such analysis reveals past phenomena and thus enables us to make predictions about phenomena that may occur in the future.

As stated above, volcanic activity, crustal movement, and large-scale flooding can cause topographical changes. Research on the process of such changes can significantly contribute to human life by enabling prediction of natural disasters and suggesting the possibility of an unprecedented natural disaster.

Also, as the public better understands landforms, people become able to judge what kind of land is safe or dangerous when a natural disaster occurs.

In this context, the learning goal in the physical geography course offered by the Faculty of Education is that the students, who will become teachers in the future, become able to discuss the process of land formation in connection with disaster prevention.

Question the “conventional wisdom” in alluvial fan research!

~Alluvial fans can be formed by rivers as well as by debris flows~

Proving to the world the correctness of the prevailing view in Japan on alluvial fans that are discussed in textbooks used in the Japanese junior high schools.

Research Process

  • Identification of the issue
    We found a paper arguing that alluvial fans cannot be formed by rivers.
  • Data collection
    To make a counterargument we collected depositional slope data from topographical maps and confirmed the accuracy of the data through field studies.
  • Data processing
    We prepared a data set which was then used to reject the basis of the paper.
  • Rebuttal
    We presented a counterargument to the assertion of the paper at academic conferences and in our academic publications and gained support.
  • Tackling a new issue
    We work on the issue of whether megafans are really alluvial fans.


The beginning of the research

The amount of available data on alluvial fans is small compared with other landforms that make up plains. Therefore, we did not conduct detailed investigations of individual alluvial fans, but instead considered alluvial fans located all over Japan and tried to get a general sense of where they are formed and how they will change.

River-flow alluvial fan

River-flow alluvial fan: Saigawa Alluvial Fan (southern part of Nagano City)


Definition of an alluvial fan

Alluvial fan and alluvial cone (Source: Suzuki (2000)

Alluvial fan and alluvial cone (Source: Suzuki (2000)

Alluvial cone (Taiwan’s Central mountains)

Alluvial cone (Taiwan’s Central mountains)


Conventional wisdom in Japan

Alluvial fans are fan-shaped landforms resulting from deposition of gravel carried by rivers. Six of the seven textbooks used at the junior high schools in Japan state that alluvial fans are formed by rivers. Relatively small alluvial cones that are composed of gravel and have a steep slope have conventionally been regarded as alluvial fans formed by rivers.

Conventional wisdom around the world

The alluvial fans are fan-shaped landforms formed by either debris flows or river flows. (They are mainly debris-flow alluvial fans, but river-flow alluvial fans are also included.)

Paper challenging the conventional wisdom held around the world, especially the view held in Japan

Alluvial fans are formed by debris flows not river flows. (Denial of river-flow alluvial fans) (Blair and McPherson, 1994) Need for redefinition; prediction of rare sediment-related disaster and floods enabled by landform classification

A difficulty in the research

Distinction between alluvial fans formed by rivers and those formed by debris flows is generally made by conducting field studies and making judgements based on observation of deposits. However, it is difficult to find outcrops containing such deposits.

Research aims and expected results

The goal is to provide an answer to the question of where alluvial fans are formed and how they will change. Areas with an alluvial fan and areas without one differ in how floods occur. Also, alluvial fans formed by debris flows and those formed by rivers differ in how sediment-related disaster occur. Therefore, as a by-product of the research, we will be able to make predictions about future natural disasters and create hazard maps.

River-flow alluvial fan

River-flow alluvial fan: Azusagawa Alluvial Fan (western part of Matsumoto City)

The riverbed (above) of the Matsukawa River (an upper stream of the Mogamigawa River) in the Yonezawa Basin in Yamagata Prefecture and deposits at an alluvial fan (below). It looks like a river-flow alluvial fan, but is actually a debris-flow alluvial fan.

Rounded river-flow deposits

Rounded river-flow deposits


Angular debris-flow deposits

Angular debris-flow deposits

Denial of river-flow alluvial fans by Blair and McPherson (1994)

Depositional slopes of 0.5-1.5° are not found. Alluvial fans are steep fan-shaped landforms with a slope of 1.5° or more (Blair and McPherson 1994).


Their result had been forming the “conventional wisdom” around the world.

Many gently sloped, fan-shaped landforms in Japan that were formed by rivers are not alluvial fans.

Leave some room for thinking
When asked a question by children, it is important to give an answer to only what is asked and leave some room for their thinking. It is also important to talk to them in a way to calm them and lead them to the next word.


The conventional wisdom in Japan had been losing supports around the world.

Frequency distribution of depositional slopes

Frequency distribution of depositional slopes

Depositional slope

Depositional slope


Counterargument made by Saito and Oguchi (2005)

As depositional slopes of 0.5-1.5° are found, alluvial fans with slopes less than 1.5° must be in humid climate regions (Japan, Taiwan, and the Philippines). Therefore, the basis for rejecting the fact that gently sloped, fan-shaped landforms created by rivers are alluvial fans was not correct.


We defended the conventional wisdom held in Japan.

Frequency distribution of depositional slopes

Frequency distribution of depositional slopes
Saito and Oguchi (2005)

Hanaupah Canyon alluvial fan

Hanaupah Canyon alluvial fan
Death Valley (United States)
Typical debris-flow alluvial fan

Indo-Gangetic Plain

Megafan of the Kosi River
Indo-Gangetic Plain (India, Nepal)
It is half the size of the Shikoku Island.
Large gravel are seen at the fanhead.


Okavango fan

Okavango “fan”
Botswana (southern Africa)
Wetlands extend over “fan” that is larger
than Shikoku Island.

Megafan of the Rio Grande

Megafan of the Kosi River
Indo-Gangetic Plain (India, Nepal)
Megafan of the Rio Grande
Eastern part of Bolivia (South America)
Riverbed deposits consist of sand not gravel.



Kyoji Saito

Social Studies Education

Education and professional experienceEducation
Bachelor’s degree in Earth science (University of Tokyo, Faculty of Science, 1975)
Master’s degree in geography (University of Tokyo, Graduate School of Science, 1977)
Doctoral coursework in geography completed (University of Tokyo, Graduate School of Science, 1982)
Doctor of Science (University of Tokyo, 1985)

Academic positions
Lecturer, Hokkai-Gakuen University (1984)
Assistant Professor, Hokkai-Gakuen University (1985)
Professor, Hokkai-Gakuen University (1989)
Assistant Professor, Saitama University, Faculty of Education (1989)
Professor, Saitama University, Faculty of Education (1999 to 2014)

Other positions
Councilor, Saitama University (2000-2004)
Dean, Saitama University, Faculty of Education (2012 to 2014)

Academic societies
Director, Association of Japanese Geographers (2006-2010)
Chief Editor, Association of Japanese Geographers (2008-2010)
Director, Tokyo Geographical Society (2009 to 2011)
Liaison, Tokyo Geographical Society (2009 to 2011)
Committee member, Japanese Geomorphological Union (1999 to 2015)
Chief Editor, Japanese Geomorphological Union (2009 to 2013)

*Alluvial Fans in Japan* (1988, Kokon Shoin, in Japanese, single author)
*Alluvial Fans in the World* (2006, Kokon Shoin, in Japanese, single author)
*The Topography of Japan, Part 1: An Overview* (2001, University of Tokyo Press, in Japanese, coauthor)
*The Geography of Japan: An Overview, Part 1 (Natural Features)* (2005, Asakura Publishing, in Japanese, coauthor)