Stream gradient (or stream slope) is the
grade (or slope) of a
stream measured by the ratio of drop in elevation per unit horizontal distance,
usually expressed as
meter
The metre ( British spelling) or meter ( American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its pr ...
s per
kilometer
The kilometre ( SI symbol: km; or ), spelt kilometer in American English, is a unit of length in the International System of Units (SI), equal to one thousand metres ( kilo- being the SI prefix for ). It is now the measurement unit used for ...
or
feet per
mile
The mile, sometimes the international mile or statute mile to distinguish it from other miles, is a British imperial unit and United States customary unit of distance; both are based on the older English unit of length equal to 5,280 Engli ...
.
Hydrology and geology
A high gradient indicates a steep slope and rapid
flow of
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
(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
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
. High gradient streams tend to have steep, narrow V-shaped
valley
A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over ...
s, and are referred to as young streams. Low gradient streams have wider and less rugged
valley
A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over ...
s, with a tendency for the stream to
meander
A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave bank ( cut bank) and deposits sediments on an inner, convex ban ...
. 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
erodible substrate will tend to have a steep gradient near its source, and a low gradient nearing zero as it reaches its
base level
In geology and geomorphology a base level is the lower limit for an erosion process. The modern term was introduced by John Wesley Powell in 1875. The term was subsequently appropriated by William Morris Davis who used it in his cycle of erosi ...
. Of course, a uniform substrate would be rare in nature; hard layers of
rock along the way may establish a temporary base level, followed by a high gradient, or even a
waterfall
A waterfall is a point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf.
Waterfalls can be formed in severa ...
, as softer materials are encountered below the hard layer.
Human
dams,
glaciation
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate bet ...
, changes in
sea level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...
, and many other factors can also change the "normal" or ''natural'' gradient pattern.
Topographic mapping
On
topographic map
In modern mapping, a topographic map or topographic sheet is a type of map characterized by large-scale detail and quantitative representation of relief features, usually using contour lines (connecting points of equal elevation), but histori ...
s, stream
gradient
In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...
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.
See also
*
Channel types A wide variety of river and stream channel types exist in limnology, the study of inland waters. All these can be divided into two groups by using the water-flow gradient as either ''low gradient channels'' for streams or rivers with less than two ...
*
Discharge (hydrology)
In hydrology, discharge is the volumetric flow rate of water that is transported through a given cross-sectional area. It includes any suspended solids (e.g. sediment), dissolved chemicals (e.g. CaCO3(aq)), or biologic material (e.g. diatoms) in ad ...
*
Hydraulic gradient, concept used for aquifers
*
Manning equation
*
Relief ratio
The relief ratio is a number calculated to describe the grade of a river or stream.
The calculation is the difference in elevation between the river's source and the river's confluence or mouth divided by the total length of the river or strea ...
*
Rapid
Rapids are sections of a river where the river bed has a relatively steep gradient, causing an increase in water velocity and turbulence.
Rapids are hydrological features between a ''run'' (a smoothly flowing part of a stream) and a ''cascade ...
s
*
Types of waterfall
References
Limnology
Geomorphology
Rivers
Water streams
Water and the environment
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