Editing Stream gradient

{{Short description|Surface slope along a watercourse}}
{{More citations needed|date=March 2025}}
'''Stream gradient''' (or '''stream slope''') is the [[grade (slope)|grade]] (or slope) of a [[stream]]. It is measured by the ratio of drop in [[elevation]] and horizontal distance.<ref>{{Cite book|url=https://books.google.com/books?id=7VriHqlCehQC&q=Stream+gradient+measured+by+ratio+of+drop+in+elevation+of+a+stream+per+horizontal+distance|title=A Study of the Effects of Stream Channelization and Bank Stabilization on Warm Water Sport Fish in Iowa: the effects of long-reach channelization on habitat and invertebrate drift in some Iowa streams|last1=Zimmer|first1=David William|last2=Bachmann|first2=Roger W.|date=1976|publisher=Iowa Cooperative Fishery Research Unit, Iowa State University|language=en}}</ref> It is a [[dimensionless quantity]], usually expressed in [[unit of measurement|units]] of [[metre|meter]]s per [[kilometre|kilometer]] (m/km) or [[Foot (length)|feet]] per [[mile]] (ft/mi); it may also be expressed in [[percent]] (%).
The world average [[river reach]] slope is 2.6 m/km or 0.26%;<ref name="Cohen2018">{{cite journal | last1=Cohen | first1=Sagy | last2=Wan | first2=Tong | last3=Islam | first3=Md Tazmul | last4=Syvitski | first4=J.P.M. | title=Global river slope: A new geospatial dataset and global-scale analysis | journal=Journal of Hydrology | publisher=Elsevier BV | volume=563 | year=2018 | issn=0022-1694 | doi=10.1016/j.jhydrol.2018.06.066 | pages=1057–1067| doi-access=free | bibcode=2018JHyd..563.1057C }}</ref>
a slope smaller than 1% and greater than 4% is considered gentle and steep, respectively.<ref name="SH2019">{{cite web | title=Classifying Your Stream Slope | website=streamhandbook.org | date=2019-01-20 | url=https://streamhandbook.org/evaluating-your-property/classification/stream-slope/ | access-date=2023-10-23}}</ref>

Stream gradient may change along the stream course.
An average gradient can be defined, known as the '''relief ratio''', which gives the average drop in elevation per unit length of river.<ref name="Streams II">{{cite book
 | last = Shaw
 | first = Lewis C.
 | others = Prepared in Cooperation with the United States Department of the Interior Geological Survey
 | title = Pennsylvania Gazetteer of Streams Part II (Water Resources Bulletin No. 16)
 | edition = 1st
 | publisher = Commonwealth of Pennsylvania, Department of Environmental Resources (no ISBN)
 | location = Harrisburg, PA
}}</ref>
The calculation is the difference in elevation between the river's [[source (river or stream)|source]] and the [[river terminus]] ([[confluence (geography)|confluence]] or [[river mouth|mouth]]) divided by the total [[length]] of the river or stream.

==Hydrology and geology==
A high gradient indicates a steep slope and rapid [[volumetric flow rate|flow]] of [[water]] (i.e. more ability to erode); where as a low gradient indicates a more nearly level [[stream bed]] and sluggishly moving water, that may be able to carry only small amounts of very fine [[sediment]].  High gradient streams tend to have steep, narrow V-shaped [[valley]]s, and are referred to as young streams. Low gradient streams have wider and less rugged [[valley]]s, with a tendency for the stream to [[meander]]. Many rivers involve, to some extent, a flattening of the river gradient as approach the terminus at sea level.

==Fluvial erosion==
A stream that flows upon a uniformly [[erosion|erodible]] substrate will tend to have a steep gradient near its source, and a low gradient nearing zero as it reaches its [[base level]]. Of course, a uniform substrate would be rare in nature; hard layers of [[rock (geology)|rock]] along the way may establish a temporary base level, followed by a high gradient, or even a [[waterfall]], as softer materials are encountered below the hard layer.

Human [[dam]]s, [[glaciation]], changes in [[sea level]], and many other factors can also change the "normal" or ''natural'' gradient pattern.

==Topographic mapping==
{{further|Topographic profile}}

On [[topographic map]]s, stream [[Grade (slope)|gradient]] can be easily approximated if the scale of the map and the contour intervals are known. [[Contour lines#Elevation and depth|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.

==See also==
*[[Channel types]]
*[[Discharge (hydrology)]]
*[[Hydraulic gradient]], concept used for aquifers
*[[Manning equation]]
*[[Rapids]]
*[[Thalweg]]
*[[waterfall#Types|Types of waterfall]]

==References==
{{reflist}}

{{Rivers, streams and springs}}

[[Category:Limnology]]
[[Category:Geomorphology]]
[[Category:Rivers]]
[[Category:Water streams]]
[[Category:Water and the environment]]
[[Category:Ratios]]