Where rivers empty into large bodies of water, such as the great chain of lakes on the northern border of the United States (and these lakes have an outlet connecting one with the other, and finally by a river to the ocean) a constant circulation is being kept up, and the water remains fresh. Owing to the fact, however, of the great evaporating surface that these lakes afford, there is a greater disproportion between the rainfall upon the drainage area tributary to these lakes, and the amount of discharge through the St. Lawrence River, than would be the case with a river that was not connected with a system of lakes. The amount of rainfall upon the area drained by the Mississippi River during one year amounts to about 614 cubic miles of water, while the discharge at the mouth of the Mississippi River is only about 154 cubic miles. The difference between the two figures has been carried up by the process of evaporation or stored in vegetation. These figures vary considerably, however, with different years. The proportion of rainfall to discharge will vary greatly in different rivers from other causes than having a large evaporating surface. This variation is due to the difference in the ability of the soil to retain water after a rainfall. In some drainage areas the ground is more or less impermeable to water, and in this case the water runs readily off, causing a sudden rise in the river; and as suddenly it reaches the low-water mark. In other drainage areas the ground is very permeable to water, so that the rain penetrates to a greater depth into the earth, where it is held, and by a slow process drains into the rivers, while much more of it is carried off by evaporation and into vegetation than is the case in the drainage district before mentioned. The courses of rivers are determined by the topography of the country through which they flow. The sinuous windings, that are found to be a characteristic of nearly all rivers, are caused by the water, through the force of gravity, seeking the lowest level, and avoiding obstructions, which they can flow around more easily than remove. Great rivers often change their courses, especially where they flow through a region of made earth, such as is the case with the lower Mississippi River, and in other great rivers of the world. The loose earth is continually shifted by the current, and where the current is not very strong it will often hold the water back to such an extent of accumulated weight that the flood will break over at some weak point on its banks and make a new course for itself. One of the great rivers of China--the Hwangho--often causes dire destruction to life and property owing to change in its bed from time to time. It is estimated that between the years of 1851-66 this river caused the loss of from 30,000,000 to 40,000,000 lives through drowning and famine by the destruction of crops.