What is megalopolis? Where does the megalopolis appear? Jean Gottmann, a famous geographer, gave a good answer in his significant works published in l96l: "Megapolis: The Urbanized Northeastern Seaboard of the United States". He had earlier introduced the term "megalopolis" to describe the major urbanized areas of northeastern seaboard of the United States. According to Gottmann Jean's megalopolis theory, megalopolis means a large city system, which possesses super-door location (such as international sea ports or river mouth ports), is distinctly separated from other units by less urbanized areas, and plays an important international exchange function for trade, technology and banking in the areas it is located. Cities in the system are distributing and gathering along the development axes (such as highways, railways, waterways, pipelines and telephone lines, etc.), and have functional interrelationships. Nowadays, all of megalopolis in the world appear in the high urbanized countries, for example, the north-eastern seaboard of the United States; the Five Great lakes areas from Chicago east to Detroit and the southern shores of Lake Erie; the Tokaido area of Japan centered on Tokyo--Yokohama; the megalopolis of north--western Europe, considered to be emerging in the area of Amsterdam, Paris and the Ruhr. As facts showing, the megalopolis is the trends of urbanization. With the policy opening to the world and the marketing economy developments, urbanization in China will speed up greatly and the megalopolis will come true.

I. LOCATION MODEL OF MEGALOPOLIS IN CHINA

By analyzing the industrial development axes (mainly along the railway lines, the Yangtse River and coastal line) in China, we can see the megalopolis location models in the future are as the follows (see Fig. l):

Longitudinal development axes which includ:

    (l) The Coastal line, and the King--Hu railway line (Peking--Shanghai) and the Hu--Hang railway line (Shanghai--Hangzhou--Ningbo);
    (2) The King---Guang railway line(Peking--Guangzhou);
    (3) The Ha--Da railway line(Harbin--Dalian);
    (4) The Jiao--Liu railway line(Jiaozuo--Liuzhou).

Latitudinal development axes which includ:

II. FACTORS TO DEVELOPE MEGALOPOLIS ALONG THE MIDDLE AND LOWER REACHES OF YANGTSE RIVER

According to Gottmann's megalopolis theory, there are four key factors to develop megalopolis:

    (1) The door location, i.e. international seaport or river mouth port as the door to the world;
    (2) High population density;
    (3) Developed economy;
    (4) Comprehensive transport networks.

Along the middle and lower reaches of the Yangtse River, there are 32 large or middle scale cities. By analyzing these cities, we can see that these. four factors to develop the megalopolis have been possessed or are being possessed(see Fig.2. ).

2.l Door Location of the Yangtse River

From the view of the world, the Yangtse River is the third biggest river and an important passageway between the Pacific Ocean and the Europe--Asian continent. Shanghai, locating in the mouth of Yangtse River, is the biggest city of Chinese cities and has the double functions of sea port and riverport. As the famous port city, Shanghai is playing a hinge role in international trade. In addition to these, the Yangtse River inter-weaving with the middle of the Chinese coastal line passes through the Chinese eastern, middle and western economic zones and combines with coastal line to form the Chinese "?" strategical development pattern. The orientation of the Yangtse River is identical with the direction of Chinese economic development strategy (from east, to middle and west). The door location is the key factor for industrial belts to develop into megalopolis along the middle and lower reaches of Yangtse River.

2.2 Ideal Physical Geographical Environment to Population Gathering

The natural environment is an important factor to influence the population distribution and sometimes plays a decisive role in population density. Along the middle and lower reaches of Yangtse River, there are three major natural factors to attract population:

    (l) The third biggest plain--the Jiang--Han plain, only secondary to the Central-North plain and North-East plain in China;
    (2) Attributing to subtropical climate which is warm and humid;
    (3) The longest river in China and the third longest river in the world the Yangtse River.

According to Prof. Hu's "Chinese Population Geography", the average population density in the south-eastern side of Aihui--Tenchong line is 235.9 persons per kilometer squares. The average population density along the middle and lower reaches of Yangtse River surpasses this number, especially for along the lower reaches it is up to 800 persons per kilometer squares.

2.3 Leading status of Economic and Urban Development

Industry in the Yangtse River valley is one of the origins of Chinese recent and modern industry. Especially along the middle and lower reaches of Yangtse River, the industrial bases are solid and the industrial technology .is more advanced than other areas. There are 3 of l0 biggest large-scale steel industrial bases of China (Shanghai, Maanshan, and Wuhan), 2 large-scale oil-chemical industrial bases (Shanghai, Nanjing), and 7 large-scale oil-refined plants (Shanghai, Nanjing, Anqing, Jiujiang, Wuhan, Yueyang, Jinmen). By comparing the urban targets along the middle and lower reaches of Yangtse River with the others, we can easily find out the average levels of urban developments along the middle and lower reaches are in the leading status. From the map of city distribution, we can see that cities mainly concentrate on the lower reaches of Yangtse River, especially from Nanjing to Shanghai, the average interval distance of cities is about 65 kilometers, including the Chinese largest city -- Shanghai (7.5 million population), a super large city (>l million population) � Nanjing (2.09 million population), 3 large cities (>0.5 million population) -- Shuzhou (0.76 million population), Wuxi (0.83 million population) and Changzhou (0.53 million population). Many peasants leave their lands to for non-agricultural work due to the rural township enterprises, so the percentage of town population along the middle and lower reaches will surpass 25 (data from statistics).

2.4 Good Trends of Transport Networks

There are five transport models along the middle and lower reaches of Yangtse River: railway, road, waterway, airline and pipeline. The Yangtse River, which is called "The Golden Waterway' in China, is a major passage from west to east and vice versa, and open to navigation at all seasons. The ten-thousand-ton rank ships can get to Nanjing and the thousand-ton rank ships to Wuhan. There are three national longitudinal railway lines cross the Yangtse River: King--Hu line (Peking--Shanghai), King--Guang line (Peking--Guangzhou) and Jiao--Liu line (Jiaozou--Liuzhou), and two local railway line paralleling the river: Hu--Ning--Tong line (Shanghai--Nanjing--Tongling) and Han--Jiu line (Wuhan--Jiujiang). Further morel the Ning--Qi line (Nanjing--Qidong ), Ning--Xiang line (Nanjing--Xiangfan) and Wuhan--Yichang--Changing line will be built up. These railway lines will enhance the connections among cities. Along the lower reaches, the roads are developed into networks, especially for the cities on southern side, the road density is very high. Nowadays, the highway from Shanghai to Nanjing is to be constructed. With the building and developments of parallel railways and highways, the transport networks along the middle and lower reaches will get more perfect and city system will be toward the megalopolis.

III. STATUS OF CITY DEVELOPMENTS

Until the late l970s, Chinese investment focal points had been toward the inland and the industrial layout along the middle and lower reaches of Yangtse River was ignorant. The city developments were limited and the advantage of the Yangtse River couldn't give full play.

3.l The City Distribution Zones along the Middle and Lower Reaches

Industry developments are in supports of city developments and vice versa. Industrial zones or manufacture zones are identical with urban zones such as in the United States of America, they are mainly concentrated on the north-eastern seaboard and the Five Big Lakes areas. Along the middle and lower reaches of the Yangtse River, especially on the southern side, the bases of the mechanical engineering industry, oil chemical industry and metallurgical industry are more solid than other areas. The social-economic and physical conditions are also ideal to develop and layout cities, so the city distribution zones are obvious. From Nanjing to Shanghai segment, the average distance among cities is about 65 kilometers. From Wuhan to Jiujiang segment and from Anqing to Nanjing segment, the average distance of cities is about 67 kilometers and 75 kilometers, respectively. It is known that the length of development axes of megalopolis in the North-eastern Seaboard of the United States is about 750 kilometers and the average distance among more than 20 cities is about 39 kilometers; the average distance among more than l7 cities in Japanese megalopolis (Tokaido areas) is about 40 kilometers. By comparing the average distance among cities along the middle and lower reaches of Yangtse River with these two megalopolis, we can conclude that the megalopolis along the Yangtse River is only in the primary state.

3.2 Unbalanced Transport Networks

Along the both sides of Mississippi and Rhine River, parallel railways and roads have been constructed. As facts shown, the parallel transportation along the river can propel the industry and city developments. However, along the southern side of the middle and lower reaches of Yangtse River there are only 760 kilometers parallel railway, which is 42% of total mainstream length of the middle and lower reaches, and there is no railway along the northern side. Road density along the lower reaches and the southern side is higher than that along the middle reaches and the northern side. So city developments along the middle reaches and the northern side are blocked heavily by the unbalanced transport networks.

3.3 The Differences of City Distribution between Both Sides

Along the middle and lower reaches of Yangtse River, the core of city developments is obviously on the southern side. There are 32 cities on the both sides, among which there are 2l cities on the southern side, and only l0 cities on the northern side (except a cross-river city: Wuhan). From the table of differences of city distribution on both side (see Tab. l), we can see that:

(l) The percentage of city population on the southern side is 74% of total population, and only 11% on the northern side;
(2) The percentage of super large city population (>2.0 million) is 62% of total city population, but small city population (<0.2 million) is only 8.0, i.e. small cities develop slowly;
(3) On the northern side, there isn't any large city (>0.5 million), and on the southern side there is no super large city (l-2 million), i.e. city system is short of secondary centers (see Fig.3)

3.4 Dominant Status of Heavy Industry and Backward of Tertiary Industry

At the view of the world, the tertiary and manufacture industries of cities in megalopolis are much developed, and the employment proportion of both are more than 90%. The comprehensive function of central cities is due to the developed tertiary.

If we analyze the urban economic structure along the middle and lower reaches of Yangtse River, we can find out the heavy industries such as metallurgical industry, mechanical engineering industry and oil chemical industry are in the dominant status and tertiary industry develops slowly, especially along the middle reaches. The employment proportion of the tertiary industry in the central cities is less than 30% (see Tab.2).

As the previous shown, the middle and lower reaches of Yangtse River have the geographical conditions and social-economy bases to develop the megalopolis. According to the forecast, China will go into middle developed countries in 2050&#0;s, we can take the 2050&#0;s as the lower limit of mature stage of the megalopolis developments. Considering the urbanized levels of the developed countries and the Chinese populous characteristics, we can forecast in 2050's the urbanized level along middle and lower reaches of Yangtse River is up to 55% (the ratio of city population to total population), which is 25% at present.

The tentative plan of the megalopolis along the middle and lower reaches of Yangtse River is as follows: the development axes of megalopolis will be developed from Yichang to Shanghai along the river which is about l830 kilometers in length, and from Shanghai to Ningbo along the railway line which is about 400 kilometers. Cities will gather highly along the development axes, i.e. the Yangtse River, parallel railways and roads. From Nanjing to Ningbo (39 kilometers), the average distance among cities is only about 50 kilometers. Because of developed transport networks, cities will have functional interrelationships. Shanghai, Nanjing, Wuhan and Hangzhou will be taken as nuclei cities. Wuxi, Changzhou, Shuzhou, Ningbo, Wuhu, Jiujiang and Yichang will be as the secondary center. The door location of Yangtse River will make the megalopolis a tie between China and other countries or areas.

The megalopolis along the middle and lower reaches of Yangtse River can be divided into three functional segments, each segment has its own function (see Tab. 3).

Because of different natural conditions and historical geographical bases, cities along the middle and lower reaches are in different development stages, some are toward mature and others in the growing stage. According to segment functions, the plans of transport networks and city the roles in city system, we can point out the assumption of city scale ranks (see Tab.4).

As previous mentioned, the spatial structure of megalopolis in the future will mainly inherit contemporary characteristics of city distribution: cities will gather chiefly along the southern side, and the segment from Nanjing to Shanghai on which super large and large cities concentrate will be the core of megalopolis. Because of Chinese large population, I think that the megalopolis along the middle and lower reaches of Yangtse River will become one of the biggest megalopolis in the world.

V. KEY OPPORTUNITIES AND FOCAL POINTS TO DEVELOPE THE MEGALOPOLIS

As previous proved, it is possible and necessary to develop the megalopolis along the middle and lower reaches of Yangtse River. In order to speed up the process of the megalopolis, we must catch the key opportunities and the focal points in different stages. According to the development principle of "S" type logical curve, the megalopolis has three development stages and their characteristics are as follows (see Fig.4):

(l) AB segment----Primary stage: Several super large cities (>l million) enjoy priority to the middle (0.2--0.5 million) and small (<0.2 million) cities whose functions are simple; the regional industrial structure will be that the first industry is prior to the secondary industry, and the secondary industry is prior to the tertiary industry; further morel the heavy industry is dominant in the secondary industry.

(2) BC segment----Speedy developing stage: Super large cities continue to grow and other secondary centers also develop greatly because of improvements of the transportation and industrial layouts; the number of the middle and small cities, increases obviously and the functions of middle cities gradually become comprehensive; the tertiary industry and advanced mechanical engineering industry get more and more attentions though the heavy industry is still in dominant.

(3) CD segment----Mature stage: Industrial structure has gone through qualitative changes because of highly developed economy the tertiary industry is prior to the secondary industry and the later is prior to the first industry; along with the high urbanization (>60%), conurbation appears around the super large cities and city system will become more perfect.

As above shown, the megalopolis along the middle and lower reaches of Yangtse River is in the Primary stage (AB segment), it must go through Speedy developing stage (BC segment) and get to the Mature stage (CD segment).

Professor Lowest, an American economy expert, concluded that it would take about 60 years for a country to get to the mature developed stage from. the beginning of the speedy developing stage. Considering the new situations in the next century such as international relationships, the breakthrough of sciences and technology, etc., I think it will take 40--50 years for the megalopolis along the middle and lower reaches of Yangtse River to go through the speedy developing stage to the mature stage. Therefore, I conclude that there are two key transitions in the process of megalopolis:

(1) The last 20 century to the early 2l century, from the primary stage to the speedy developing stage (AB--BC);
(2) The 2050's, from speedy developing stage to mature stage (BC--CD).

5.l Primary Stage to Speedy Developing Stage (AB &#0; BC)

The last 20 century to the early 2l century is the key transition to speed up the process of megalopolis along the middle and lower reaches of Yangtse River. The focal points will enhance the industrial layouts develop. the heavy industries along the Yangtse River, and explore the water resources, water power and water transportation.

(l) Construction of the heavy industrial belts from Yichang to Nanjing

The present bases of mechanical engineering, metallurgical and oil-chemical industries are good. The sector from Wuhan to Jiujiang whose good. The sector from Wuhan to Jiujiang whose mainstream is about 200 kilometers long includes Wuhan, Daye, Huangshi, Ezhou and Jiujiang, and the other sector from Anqing to Nanjing whose mainstream is about 252 kilometers long includes Anqing, Tongling, Wuhu, Maanshan and Nanjing. There are parallel railways along the southern side of these two sectors. According to the characteristics of the primary stage, it primary stage, it is important and reasonable to continue to develop the black and color p the black and color metallurgical industries, oil-refined and oil-chemical industries and mechanical engineering industry in the last 20 century and early 2l century. Consequently, there will be two heavy industrial belts along the parallel railways on the southern side of these two sectors in the near future.

(2) Developments of the secondary centers and parallel transportation

It is necessary to improve the transport location and increase city scales of the secondary centers, including Wuhu, Jiujiang and Yichang. First, more local industry layouts and national focal investments will be attracted in these cities such as power industry and material industry because of The Three Gorges Project. Secondly, interrelationships among cities will be enhanced by building parallel transportation as follows:

            Railway planning:   Wuhan--Daye--Jiujiang railway line
                                            Jiujiang--Tongling railway line;
                                            Nanjing--Wuhan--Xiangfan railway line;
                                            Nanjing--Qidong railway line.

            Highway planning: Shanghai--Nanjing highway; Nanjing--Wuhu highway.

            Bridge planning:     Jiujiang bridge and Wuhu bridge.

(3) Developments of comprehensive ports

On the basis of the water depth and water transport location, Wuhan, Jiujiang, Wuhu, Nanjing, Zhenjiang and Zhangjiagang will be developed into comprehensive ports and some will become exports.

5.2 Speedy Developing Stage to Mature Stage (BC to CD)

The 2050s is the key transition for the megalopolis to go through the speedy developing stage to the mature stage. The major tasks are to change the industrial structure, improve the city system and build up the developed transport networks.

(l) Qualitative changes of economic structure

According to the characteristics of the mature stage, the industrial structure in Shanghai, Wuhan, Nanjing and the two industrial belts (Wuhan---Jiujiang and Anqing----Nanjing) will be changed from the heavy industries, such as metallurgical, oil-chemical and material industries, to more advanced technology and high process industries.

(2) Developments of tertiary industry

In order to pay full plays to the central cities and sub-central cities, territory industries such as banking, trade, service and advice industries should be developed greatly. So the functions of these cities----Shanghai, Wuhan, Nanjing, Wuhu, Jiujiang, Yichang will be more comprehensive and city radiate forces be increased.

(3) Building of port system and transport networks

Because the development of ports and the transport networks is essential to the megalopolis along the Yangtse River, Shanghai, Wuhan, Nanjing, Wuhu, Jiujiang and other comprehensive ports will be taken as the central ports, the middle and small ports will be developed greatly and port system will be build up. The transport networks along the middle and lower reaches are important to port system and city system. Cities and ports will be connected directly by developed transport networks.

(4) Paying attention to sciences and technology

As facts shown, sciences and technology play an important role in economic growth in U.S.A. and Japan, accounting for 7l% and 65% respectively, but only l9% now in China (data from "Newspaper on World Economy Directions" Jan.2, l989). Wuhan and the sector from Shanghai to Nanjing, having solid intelligence bases, will make the most use of intelligence re source s.

VI. POSSIBLE PROBLEMS

The research on the megalopolis along the middle and lower reaches of Yangtse River includes natural, social, economic and political fields. This paper chiefly concentrates on the possibility and the future model of the megalopolis. I think that the following four major possible problems should be studied further:

(l) The influences of megalopolis along the middle and lower reaches of Yangtse River on unbalanced developments of cities and industries in China.
(2) The environmental problems, such as transport and settlement congestion, water and air pollution because of assembling developments.
(3) The demand and supply of land, water and power resources in the process of megalopolis.
(4) The influences of increasing water freight volume on the river mouth port.

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