|MadSci Network: Earth Sciences|
Quite a few factors affect the direction a river takes. First of all, water runs downhill due to gravity. It may flow northward or southward, to the east, or to the west, but always downhill. Where water is forced to move uphill, for short distances over rocks or small inclines, the force of the flowing water must be sufficient to overcome the gravitational pull downward or the water will stop flowing. The exact course a river or stream takes depends on a combination of many factors.
Topography plays a major role in determining a river's course. Water will always seek the path of least resistance. It will go around or under rather than up and over whenever possible. Confronted with a mountain chain or even a small ridge, the river will turn and flow parallel to the blocking feature unless or until it is able to erode a path across it. The composition of the terrain determines whether or not this is possible. Hard rocks allow little, or only very slow erosion. Sand, gravel and dirt are easily eroded and the river will make its path through these much more easily.
Water in nature never flows very far in a straight line. If you look at a map, you see that rivers, creeks, and streams twist and turn on their way seaward, even where there are no obstacles in their path. This twisting and turning is called meandering and develops not only in response to large obstacles, but also in response to very subtle differences in terrain, even to the the ease of flow over one grain of sand versus another. Slight deviations become magnified and the meander appears. Where natural rivers follow a straight path for any distance, they have been engineered by humans to do so. Rivers meander because it saves work, it is the most efficient use of the energy of flowing water over the ground.
Predicting the time when a river flood will peak, or crest, at a particular location is accomplished by relating the amount of rain which has fallen over the entire river drainage, called the basin, to the amount that will run off, that is, not soak into the ground or be held in storage by lakes, ponds, and the ground itself. When the runoff for each stream and tributary of the river has been determined, the amount of water that will flow into the river and move downstream can be calculated. Travel times for flood waves have been determined for major rivers in the United States and these, along with the volume of water added to the river (the runoff), are used by National Weather Service River Forecast Centers and Service Hydrologists to predict the level of the crest and the time of its occurence at points along the river.
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