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Stream gradient (orstream slope) is thegrade (or slope) of astream. It is measured by the ratio of drop inelevation and horizontal distance.[1] It is adimensionless quantity, usually expressed inunits ofmeters perkilometer (m/km) orfeet permile (ft/mi); it may also be expressed inpercent (%).The world averageriver reach slope is 2.6 m/km or 0.26%;[2]a slope smaller than 1% and greater than 4% is considered gentle and steep, respectively.[3]
Stream gradient may change along the stream course.An average gradient can be defined, known as therelief ratio, which gives the average drop in elevation per unit length of river.[4]The calculation is the difference in elevation between the river'ssource and theriver terminus (confluence ormouth) divided by the totallength of the river or stream.
A high gradient indicates a steep slope and rapidflow ofwater (i.e. more ability to erode); where as a low gradient indicates a more nearly levelstream bed and sluggishly moving water, that may be able to carry only small amounts of very finesediment. High gradient streams tend to have steep, narrow V-shapedvalleys, and are referred to as young streams. Low gradient streams have wider and less ruggedvalleys, with a tendency for the stream tomeander. Many rivers involve, to some extent, a flattening of the river gradient as approach the terminus at sea level.
A stream that flows upon a uniformlyerodible substrate will tend to have a steep gradient near its source, and a low gradient nearing zero as it reaches itsbase level. Of course, a uniform substrate would be rare in nature; hard layers ofrock along the way may establish a temporary base level, followed by a high gradient, or even awaterfall, as softer materials are encountered below the hard layer.
Humandams,glaciation, changes insea level, and many other factors can also change the "normal" ornatural gradient pattern.
Ontopographic maps, streamgradient can be easily approximated if the scale of the map and the contour intervals are known.Contour lines form a V-shape on the map, pointing upstream. By counting the number of lines that cross a certain segment of a stream, multiplying this by the contour interval, and dividing that quantity by the length of the stream segment, one obtains an approximation to the stream gradient.
Because stream gradient is customarily given in feet per 1000 feet, one should then measure the amount a stream segment rises and the length of the stream segment in feet, then multiply feet per foot gradient by 1000. For example, if one measures a scale mile along the stream length, and counts three contour lines crossed on a map with ten-foot contours, the gradient is approximately 5.7 feet per 1000 feet, a fairly steep gradient.