Grant Hoag, age 13, of Oakland, California, for his question:

How fast does water flow downhill?

This is a tantalizing topic for students who like to talk around and about tricky problems. Such discussions lead far afield and before you know it, you are coming to grips with the basic laws of nature. Today's topic could lead to careers in irrigation, flood control and other fields of geology.

One takes it for granted that you know that this tricky question has a million different answers. Some people might dodge the problem by saying ent answers. Some people might dodge the problem by saying„ "It all depends." But naturally you want to know the underlying laws of nature that make water flow downhill at different speeds. Our working examples are streams, flowing down slopes and inclined planes on the bumpy surface of the earth. There must be basic laws that make some streams dash along, others dawdle at a few feet per second and the plunging Niagara reach the foot of its 150 foot fall at 50 miles per hour.

All streams, of course, are pulled down their inclined planes by the force of the earth's gravity. The force of gravity acts most directly on a cataract, as it plunges vertically downward in free fall. The cataract falls at the same rate as any freely falling body: it increases by 32 feet per second with every second of falling time. This law of standard acceleration of falling bodies gives us a starting point in determining a. the speed of a flowing stream. But many factors work to retard streams from flowing down hill at the rate of stones falling through the air, or cataracts roaring over a cliff.

The downhill flow is delayed by the friction of stones and muddy river banks, by the angle of the plains and slopes it crosses and by swooping curves in the river bed. These conditions oppose the pull of gravity and make streams flow slowly. In clear streams, flowing down steep, straight rocky paths, the obstacles to the pull of gravity have less force.

Strong pressure froth behind may speed up a flowing river. In March of 1945, the velocity of our great Columbia river was measured at a point 170 miles from the sea. Its average rate of flow was 1.9 feet per second. In May, swollen streams from its far flung water shed pushed the river's velocity to 11 feet per second.

The earth's slopes slant at different angles, steep and gentle, the muddiness and quantity of flowing water varies with the seasons. Along with all these variables, there is another modifying factor called turbidity flow. This occurs when the sloping land finally leads a great river to its rendezvous with the sea. The salty ocean may slow its speed, forcing it to dump its suspended debris in a silty delta. An earthquake or swollen streams, however, may speed the river current down the continental shelf far out to sea.

Every stream presents a unique problem of complicated factors. But the problem can be solved, at least in theory. With no opposing factors, water would plunge like a stone  32 feet the first second, 64 feet the next and so on. The velocity of a stream can, in theory, be computed by deducting the force of each opposing factor from this standard acceleration of falling bodies.

Discussing these problems, solving and trying to solve them can be very rewarding. The mental exercise of observing the laws of nature in action is fascinating. What's more, it is useful and also necessary. It will add to your enjoyment and appreciation of the earth and perhaps help you towards a useful career. Irrigation and flood control, the demand and supply of water, are problems of worldwide concern. All these problems must reckon with the downhill rate of drainage and, goodness knows, we need to figure out all we can about it.