Editing Stream gradient (section)

{{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.