Undoubtedly the most common types of noticeable changes are those affecting river courses. Major and minor changes in channel, the construction and destruction of natural levees, the creation and siltation of backwaters, the seaward growth of deltas, and the diminution or swelling of stream flow in re- sponse to changes in climate are all abundantly, though not adequately, documented.

The diagrams below illustrate some changes in river courses over time. Figure 1.1 shows some of the major fluctuations in the historically important zone of confluence of the Ghaghara and Son Rivers with the Ganga, and figure 1.2 provides a fuller conspectus, over a shorter time period, of the progressive west- ward shift of the Kosi River between the point where it de- bouches from the Mahabharat Range of Nepal and its juncture with the Ganga. Of all the rivers of South Asia, the rampant Kosi perhaps offers the most extreme example of the variabil- ity of river courses. Figures 1.3–1.6, all drawn at a much smaller scale than 1.1 and 1.2, show a reconstruction of the general alignments of the major rivers of the Punjab at a few selected periods of history.

Changes in coastline, of course, are also constantly taking place in response to the opposing forces of deposition and ero- sion. A number of settlements known to have once been flour- ishing seaports (e.g., ancient Tamralipti, or modern Tamluk, in West Bengal; or Kayal, in Tamil Nadu, visited by Marco Polo in 1292) are now well inland. We have already men- tioned the growth of deltas, which on certain main rivers like the Ganga and the Indus has surely been considerable. In con- sidering the growth of deltas it is tempting to assume a uniform rate of seaward progression, but there is little to support such a view. For one thing, unpredictable cataclysmic events—espe- cially tidal waves, which are not infrequent in the northern reaches of the Bay of Bengal—can sweep away in hours the accumulation of decades of alluvial deposition. More impor- tant, the rate of deltaic deposition is a function of the rate of upstream erosion, which in turn is largely a consequence of man's deforestation of the highland regions where South Asia's rivers originate. On the whole, it appears, given the recent ac- celeration of population growth and the mounting pressure of population on land, that the rate of alluvial deposition should be more rapid today than in most previous periods of history. Similar arguments may be made for the greater instability of river courses today; figure 1.2 provides a striking case.

Numerous coastal changes have undoubtedly been brought about by diastrophic movements. Earthquakes, such as the ma- jor convulsion of 1810, have repeatedly brought about pro- nounced alterations in the configuration of the shallow Rann of Kutch and have also markedly affected the Makran coastal region to the northwest. Some of the postulated folk wander- ings and historically known movements of peoples and armies from Sind to Gujarat, which today seem rather improbable, may be accounted for if one allows for easier passage between the two areas in past times. For example, the drift of the Indus Civilization into Gujarat as it declined in its core area may have been achieved more easily than one might suppose on the basis of today's physiography. If one goes well back into the prehistoric period, very substantial changes in coastline would inevitably have resulted from the global changes in sea level attending the glaciation and deglaciation of Pleistocene times (see graph on plate II.1).

As for changes in the surface configuration of land forms in the interior portions of South Asia, there have probably been no more than a few in recent millennia that have had a very significant effect on the course of human history. The raising of a limestone sill across the Indus at Sukkur as a result of an earthquake has been suggested as a cause of upstream flooding that contributed to the decline of Indus Civilization. The fact that the major Nara distributary of the Indus takes off from Sukkur and that the present Nara itself appears to have been the major channel of the Indus in previous times underlines the importance of the aforementioned sill and its possible his- torical role. To the east, desiccation and the consequent loss or degradation of vegetative cover have contributed to the expan- sion of the Rajasthan Desert and the formation of large, drift- ing sand dunes that appear to have engulfed and buried a number of settlements of the Indus Civilization (for a dis- cussion of climatic change, see the text for plate I.C.1). Of course, a physiographic change of major proportions that does not appear to play a significant historical role may still have a marked effect in changing the landscape. The 1897 Assam earthquake, for instance, which produced a shock felt over an area of 1.2 million square miles, caused visually perceptible alterations in the elevations and even in the relative positions of hills over a zone of 10,000 square miles.

Regrettably, the data on past changes of the kind we have been discussing are fragmentary and rarely dated, and rela- tively few parts of South Asia have yet been the object of in- tensive research. Accordingly, on the great majority of the maps of this atlas we have elected to portray physical features as they are at present. A few exceptions to this general rule, with respect to river courses, coastlines, and lakeshores, have been made in sections III–V. Specifically, we have depicted former courses of the Indus and its tributaries, partially known and partially inferred, as well as inferred former coastlines along the Indus and Gangetic Deltas (and a few others on maps extending beyond South Asia); the former coastlines of Makran and the Rann of Kutch; and ancient shorelines of the Aral Sea and various lakes in Seistan. For the Gangetic Delta our depiction is little more than guesswork. Wherever ancient river courses and coasts are shown as different from those ex- isting at present, the present features are indicated by a dotted line.

Map Sources for Plate I.B.1

John Bartholomew (1959a), (1959b), (1960), (1964); Great Britain, War Office (1938); India (Republic), National Atlas Organisation (1957, listed under Atlases), plates 3 and 8; United Nations, Economic Commission for Asia and the Far East (1971).

Photo Credits for Plate I.B.2

(a), (b), (d), and (i), courtesy of the United Nations ((b) taken by Toni Hagen); (c), (e), (f), (h), and (m) by Jo- seph E. Schwartzberg; (g) by Stanley Brush, courtesy of John E. Brush; (j) from Norbert Kerbs, Vorderindien und Ceylon, Stuttgart, 1939; (k) by Elie Charlier, courtesy of Ames Li- brary of South Asia, University of Minnesota; and (o) by Benjamin Farmer, from Geographical Review, vol. 40 (1959), courtesy of the American Geographical Society.

Selected Works on Physiographic Changes

Books

J. Abbott (1924); A. Cunningham (1871/1963); H. T. Lam- brick (1964); B. C. Law, ed. (1968). See also sources cited in General Bibliography for section I.

Articles

K. S. Ahmad and A. A. Abbasi (1960); S. P. Chatterjee (1962); H. L. Chhibber (1949); G. Dales (1962a); R. D. Oldham (1874), (1886), (1893); G. Rai (1934); H. G. Rav- erty (1892); Satyakam Sen (1956); H. Wilhelmy (1967); W. H. A. Wood (1924).

General

Complementing the physiographic map and photographs discussed above are a set of maps, graphs, and photographs relating to the climate and vegetation of South Asia. By study- ing all three basic components of the physical geography of South Asia concurrently, one can derive some sense of the rel- ative desirability of various portions of that vast region for human occupation, as well as of the relative ease of moving through a particular area. Considerable areas of South Asia comprising more or less flat plains may appear from the stand- point of topography to be desirable for human settlement; but from an examination of the climatic map we may find that they are not especially attractive, because they are either too dry (e.g., the Thar Desert) or too wet (e.g., parts of northeast India). The dry areas, further, although they posed no topo- graphical obstacles to the movement of armies or, in early days, waves of migrant tribesmen, may well have been avoided as zones of transit because of the scarcity of drinking water. The wet areas presented a different set of problems: dense forest, wild animals, and a high incidence of tropical diseases. On the other hand, mountainous situations were not necessar- ily adverse. Sheltered, more or less isolated, well-watered vales like those of Kashmir, Nepal, and Manipur afforded reason- ably secure havens in which distinctive cultures could develop and flourish in ecologically diversified milieus away from the principal arenas of South Asian history.

I.C.1. Climate

Since the length of the growing season in South Asia is lim- ited by low temperatures in only a few areas, precipitation is the principal climatic element affecting agriculture and hence the sustenance of human populations. The average annual rain- fall is a major determinant not only of what crops can be grown (e.g., rice normally requires around 100 cm), but also of the average yields to be expected. Map (a) shows the marked variability in the distribution of precipitation within South Asia, indicating not only the average annual totals but also the regional pattern with respect to the season of maxi- mum precipitation. The latter type of information is important because the rate of evaporation of surface moisture varies markedly with temperature; thus, a given quantity of rainfall has much greater efficiency for agriculture in the winter months than it would have in summer. Supplementing the map infor- mation on amount and season of precipitation are the climo- graphs in the margin of plate I.C.1, showing precipitation and temperature for fifteen regionally representative weather sta- tions. It will be noted from these climographs that over most of South Asia rainfall is highly concentrated during a few months of the year. Inevitably, much of this concentrated pre- cipitation is wasted as runoff from already saturated soil. The same quantity distributed more or less evenly throughout the year (as it is in much of Western Europe, for example) would be far more beneficial for agriculture. As it is, in the absence of irrigation most of South Asia is limited, despite year-round warmth, to one crop a year.

As a general rule, the percentage of variability in the amount of precipitation increases as the annual total decreases, whereas