CHINA) deserts this mature valley to turn abruptly southwards and in a very rapid and obviously immature course rushes through a long trench cut into the very border of the loess-filled basin of North Shensi. At the south end of this north-south course, it receives first the Fen-ho on its left bank and then the Wei-ho on its right bank, which together gather up the drainage of a string of fertile loess-basins in south Shansi and central Shensi. Their combined waters then flow due east, as though the Wei-ho were the master-stream, through the Tungkwan gorge, where the river is restricted by the approach of the ridges of south Shansi towards the Tsin-ling, and enter on the funnel-shaped valley which leads across Honan to the north China plain. The Tungkwan gorge forms the historic gateway into the north China plain from the Wei-ho valley and beyond from Kansu and the Tarim basin. About Kaifeng the valley gives way to the plain and here the character of the river changes also. With the neighbourhood of Kaifeng as a pivot, the Hwang-ho, after breaking its dykes during flood, has several times shifted bodily the whole of its lower course through the plain. For over five centuries before 1852 the Yellow river entered the Yellow sea well to the south of the Shantung uplands, but since 1852 its mouth has lain to the north of them, involving a change of over 25o miles.
The most irregular feature in this long course of the Hwang-ho is the entrenched torrent along the Shansi-Shensi border in the very middle of its course and between two mature valley stretches, that across the Ordos and that through Honan. This disposition suggests that the torrent stretch has comparatively recently joined two formerly independent drainage systems. In its course across the Ordos the Hwang-ho is heading for the ridge and trough country of the "grill of Peking." The troughs are broad enough to take the river and an enormous deltaic fan to the north-west of Peking, too big to be produced by any existing stream of the neighbourhood, may represent its outf all. In its valley course through Honan the Hwang-ho simply continues the line of the broad Wei-ho valley and these together, though broken by the Tungkwan gorge, may represent a second formerly inde pendent river. The head-streams of the Wei-ho valley lie near to the point where the upper Hwang-ho emerges from the Kuen-Lun ranges. The upper Hwang-ho may be related therefore to either the Ordos or the Wei-ho system. If the "grill of Peking" were tilted, up in the north and down in the south, which the decreas ing elevation of its ridges to the south would indicate has actually taken place, then the Ordos river would be compelled to turn southwards until it could escape to the east. This it could not do until it came across the great Ta-hwa fault along which the grill system is broken against the Tsin-ling. By this means it would encounter the southernmost of the once independent rivers, that flow along the northern foot of the Tsin-ling ranges.
In its torrential stretches the current is too swift and in its course through the plain the channel is too wide and shallow for the Hwang-ho to be of much use for navigation. The Ordos loop is navigable from Chungwei to Hokow save during low water in winter and flood in summer, but the region through which it flows is unproductive. Further down, there is some local traffic along the trench between Hokow and Tungkwan, but it is nearly all downstream because of the swift current. In its lower course only the last 25 miles are really suitable for navigation.
The economic significance of the Hwang-ho is not as an artery of commerce but as the source of many of the floods which from time to time devastate the north China plain and which have earned it the names of "China's Sorrow," "The Ungovernable," "The Scourge of the Sons of Han." The plain has only a very gentle slope towards the sea and is to some degree the deltaic accumulation of the Hwang-ho, the Hwai-ho and the Chihli rivers. In its course over it the channel of the Hwang-ho becomes ex ceedingly broad and shallow. With the agricultural reclamation of the plain this broad channel has become confined by dyke con struction and the river silt formerly spread over a wide area has become concentrated on the river bed itself so that the river is now flowing on the top of the plain as much as within it. The river surface of the Hwang-ho is at low water 15 feet above the general level of the plain, at high water as much as 3o feet. It is estimated that it is raising its bed, mainly when the current is slackening after the summer floods, at the rate of one foot in every hundred years, but this represents only about 1% of the total amount of silt brought down within such a period. In this lower course the dykes are sufficiently far apart for the river to have considerable swing, which not only lengthens the river course but also facilitates the breaching of the dykes by an aggressive meander. The International Famine Relief Commission propose to control the river by straightening its channel which would increase the current and thereby the scour of the river bed and would prevent the breaching of dykes by meandering. By this means it is hoped not only to prevent flood but also to add to the land reclaimed for agriculture.