Even if you don't spend a lot of time outdoors, you've probably noticed that rivers move downhill. But unless they're moving extremely fast, they don't move directly downhill. They tend to take their time and wander or meander a bit, taking a serpentine course that often moves more in a lateral direction than in a straight line. The degree to which they move this way and that has to do with several factors such as the overall tilt (slope) of the land their traversing and the type of underlying rock structure. Hard, resistant rocks will more likely keep a river or stream confined to a channel that's fairly straight. Soft rocks and sediment, which are easily eroded by flowing water, allow rivers to have more latitude as to where they can move.

The San Juan River in southwestern Colorado is one of the most crooked large rivers in the US. It meanders wildly, creating series of goosenecks that in places nearly loop back on each other. In places, it travels a distance of about 6 miles where the flight of a crow would only be about 1 1/2 miles.

In a nutshell, the greater the curve of the river bend, the faster the water moves around it. The fast moving water collides with the river bank and then is deflected toward the opposite bank, where upon it then moves to the opposite bank again and begins to carve another bend. Erosion usually occurs along the outer bank of a river loop, where the water is moving faster. Sand and gravel scoured from the outer banks are deposited along the inner banks, where the water is moving somewhat slower. As a result, inner banks tend to be more shallow and outer banks steeper.

A river can be loopy almost anywhere. It makes no difference if the river has its source near the Equator or near the Arctic Circle, but a particular feature of rivers in high latitudes is partially attributable to the Coriolis Force, which is more noticeable as one moves north or south away from the Equator.

The Coriolis force results from Earth's west-to-east rotation. As Earth rotates, points on the surface are moving eastward (from west to east) past a fixed point in space at a given speed. Points on the equator are moving at approximately 1,000 miles per hour, but the poles are not moving at all, they're merely pivoting. The points somewhere between the Equator and the poles are moving at speeds between 1,000 and zero miles per hour, depending upon their latitude.

The effects of the Coriolis Force must be considered for things that move relative to the Earth, especially at the higher latitudes of both hemispheres. For example, in World War I, during a naval battle near the Falkland Islands (off the east coast of South America, about 52 degrees south latitude) between the German and British Navy, British gunners were surprised to see their salvos falling 100 yards to the left of the German ships. The engineers who designed the sighting mechanisms were well aware of the Coriolis deflection and had carefully considered it, however, they neglected the fact that not all sea battles occur in the Northern Hemisphere. Thus, during the engagement, the initial British shots fell at a distance from the targets equal to twice the Coriolis deflection.

Knowledge of the Coriolis Force is extremely important in understanding basic concepts in meteorology and oceanography since it plays a major role in deflecting winds and ocean currents. In the Northern Hemisphere, primary wind and ocean currents are deflected toward the right, and in the Southern Hemisphere they're deflected toward the left. In addition, geographers have known for some time that many north flowing Arctic rivers cut faster into their right banks (from the perspective of looking upstream) than their left ones. Albert Einstein even addressed this in 1926 in the journal Die Naturwissenschaften. Many of you probably subscribe to this journal - well maybe not. We usually associate Einstein's with relativity and particle physics, but he was a man of many interests.

It seems that although other forces, such as the velocity of the current and the mass of the water, are much more significant in determining how and where rivers move, the Corilois Force can't be ignored at high latitudes. At low and mid latitudes, the effect of the Coriolis Force on rivers is hardly perceptible, and even in Arctic areas where the land has moderate relief, it's not a factor. However, in Arctic regions having little relief, rivers carve steep cliffs on their western banks while their eastern banks are noticeably lower. This is frequently observed on a number of the great north flowing rivers in Russia, such a as the Ob, Lena and Yenisey.

Yet. there is still debate as to whether this is primarily a result of the Coriolis Effect. Remember that the Coriolis Force does not operate over very short distances. It has no effect on the movement of water in a wash tub or wind in your back yard. Perhaps river meanders are also too short for the Coriolis Force to come into play. It may be that the prevailing wind, over thousands of years, redistributed soil particles, contributing to the perched nature of some of the river banks in the Arctic. Nevertheless, in general, erosion at higher latitudes in the Northern Hemisphere is observed to be more severe on the right river bank than on the left.