It depends how far along the river course it is. In theory as you move further down the river's course the bankfull width should increase.

Bankfull width is the maximum width the stream attains and is typically marked by a change in vegetation, topography, or texture of sediment (Herrelson and others 1994).

The dominant channel forming discharge is often assumed to be the bankfull flow.

For example Knighton (1998) argued that "since it seems reasonable to suppose that river channels are adjusted on average to a flow which just fills the available cross-section, dominant discharge has been equated with bankfull flow, thereby giving it additional

morphogenetic significance."

The dominant discharge is supposed to give the same effects as the whole range of

discharges occurring (Brandt, 2000).

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Brandt, S.A., 2000: Classification of geomorphological effects downstream of dams. Catena, vol. 40(4), pp. 375-401. doi:10.1016/S0341-8162(00)00093-X Knighton, D., 1998. Fluvial Forms and Processes: A New Perspective. Arnold, London, xv+383 pp.

Sean M. Lawlor has written:

'Determination of channel-morphology characteristics, bankfull discharge, and various design-peak discharges in western Montana' -- subject(s): River channels, Stream measurements

there is less friction caused by the bottom of the river bed and banks by sediment and rocks etc. velocities are said to be fastest at the surface of the river because of this less amount of friction acting upon it. it is also fastest at the centre of the river where it is the deepest.