M2 Fluvial Regime and Channel Morphodynamics

Topic

A river's flow regime describes the typical pattern of seasonal and year-to-year variation in its discharge—a pattern determined by the basin's climate (precipitation and snowmelt), geology, and vegetation cover.

Channel morphodynamics examines how the river channel itself—its shape, size, sinuosity, and gradient—adjusts to the flow and sediment regimes constantly passing through it; the channel evolves toward a state of dynamic equilibrium, wherein the "dominant discharge"—the flow that fills the channel to bankfull level, typically occurring once every year and a half—becomes the primary shaper of the channel.

Lateral meander migration is the most visible morphodynamic process: in medium-sized rivers, meanders typically migrate between one and ten meters per year, though in exceptional episodes, they can shift between fifty and one hundred meters annually. The mechanism driving this migration is the higher water velocity on the outer curve, where the bank is eroded, contrasted with the lower velocity on the inner curve, where sediment is deposited to form a point bar.

The sediment connectivity linking the basin, the river, and the coast determines how much sediment eroded from hillslopes actually reaches the coast; in large basins, only ten to twenty percent makes it through, as much of the remainder is stored in the floodplains the river traverses along its course.

Avulsions are abrupt changes in the channel's path; they typically occur when the gradient of an alternative channel exceeds that of the current channel by roughly tenfold. They are common on the floodplains of highly sediment-laden rivers, such as those with a braided pattern. The expected behavior of this entire river system involves constant morphological adjustment in response to changes: an increase in sediment supply causes the channel to become wider and shallower, whereas the construction of a dam causes that same channel to narrow as it loses the dominant discharge that previously maintained its form. Thus, from the river regime with which this document begins to the morphological adjustment with which it concludes—encompassing dominant discharge, meander migration, sediment connectivity, and avulsions—the river regime and channel morphodynamics ensure that a river's form is never truly fixed, but rather a constantly shifting equilibrium between the flow and the sediment the river itself transports.

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