River regime can describe one of two characteristics of a reach of an alluvial river:
- The variability in its discharge throughout the course of a year in response to precipitation, temperature, evapotranspiration, and drainage basin characteristics (Beckinsale, 1969)
- A series of characteristic power-law relationships between discharge and width, depth, and slope
The latter is described by the fact that the discharge through a river of an approximate rectangular cross-section must, through conservation of mass, equal
where is the volumetric discharge, is the mean flow velocity, is the channel width (breadth) and is the channel depth.
Because of this relationship, as discharge increases, depth, width, and/or mean velocity must increase as well.
Empirically-derived relationships between depth, slope, and velocity are:
refers to a "dominant discharge" or "channel-forming discharge", which is typically the 1–2 year flood, though there is a large amount of scatter around this mean. This is the event that causes significant erosion and deposition and determines the channel morphology.
The variability in discharge over the course of a year is commonly represented by a hydrograph with mean monthly discharge variations plotted over the annual time scale. When interpreting such records of discharge, it is important to factor in the time scale over which the average monthly values were calculated. It is particularly difficult to establish a typical annual river regime for rivers with high interannual variability in monthly discharge and/or significant changes in the catchment's characteristics (e.g. tectonic influences or the introduction of water management practices).
There are three basic types of regimes (Pardé, 1955):
- simple regime - one maximum and one minimum per year
- mixed regime/double regime - two maximums and two minimums per year
- complex mode - several extrema
Simple regimes can be nival, pluvial or glacial, depending on the origin of the water.
- Glacial regime
The glacial regime is characterised by:
- Very high discharge in summer after the ice melt
- Very low discharge from the end of autumn to early spring
- Amplitude of monthly variation of discharge greater than 25
- Very high daily variability in discharge during the year
- High flow (several hundred l/s/km2)
The nival regime is similar to the glacial, but attenuated and the maximum takes place earlier, in June. It can be mountain or plain nival. The characteristics of the plain nival (example: Simme at Oberwi) are:
- Short and violent flood in April–May following massive spring thawing ofwinter snows
- Great daily variability
- Very great variability over the course of the year
- Great inter-annual variability
- Significant flow
The pluvial regime is characterized by:
- high water in winter and spring
- low discharge in summer
- great inter-annual variability
- flow is generally rather weak
It is typical of rivers at low to moderate altitude (500 to 1,000 metres or 1,600 to 3,300 feet). Example: Seine.
- Tropical pluvial
The tropical pluvial regime is characterized by:
- very low discharge in the cold season and abundant rainfall in the warm season
- minimum can reach very low values
- great variability of discharge during the year
- Relatively regular from one year to another
Mixed régimes/double regime
- only one true maximum, which occurs in the late spring or the early summer (from May to July in the case of the Northern hemisphere)
- relatively high diurnal variations during the hot season
- significant yearly variation, but less than in the snow regime
- significant flow
- two maximums, the first occurring in the spring and the other in autumn
- a main low-water in October and a secondary low-water in January
- significant inter-annual variations
- a period of rainfall in late autumn due to abundant rainfall, followed by a light increase due to snow melt in early spring
- the single minimum occurs in autumn
- low amplitude
The complex regime is characteristic of large rivers, the flow of which is diverly influenced by numerous tributaries from different altitudes, climates etc. The influences diminish extreme discharges and increase the regularity of the mean monthly discharge from upstream to downstream.